Search

Oil and gas accumulations in the sandy conglomerate diagenetic traps that developed in the fan delta of the Triassic Baikouquan Formation of the Mahu sag, Junggar Basin, differ from stratigraphic accumulations controlled by sedimentary facies. This study uses conventional gas chromatography–mass spectrometry measurements, mineralogical and petrological data, fluid inclusion analyses, and geophysical inversion to confirm the trap type and accumulation processes. The results indicate that accumulations in the sandy conglomerate formed in distributary channels of the fan-delta front, with the boundary controlled by reservoir quality. In turn, reservoir quality was controlled by differential diagenesis caused by detrital-feldspar content, paleotemperature, and formation fluids, suggesting that the trap is a diagenetic trap. The diagenetic trap formed from secondary pores developed in the fan-delta front facies with seals forming through a combination of altered sandy beds and mudstone. The critical physical property of the reservoir, based on the ratio of its average capillary radius to the ratio of its seal, is a crucial parameter for describing diagenetic trap boundaries. Here, it effectively distinguishes between reservoirs at different burial depths. A good correlation exists between the formation of the diagenetic traps and hydrocarbon filling. Three hydrocarbon charging and two accumulation stages are identified. The diagenetic traps primarily formed in the Jurassic, and the last charging stage of highly mature oil charge occurred in the Early Cretaceous. Few studies have investigated the conglomerate diagenetic trap studied herein, so this study should improve the understanding of oil and gas traps.

Fan delta conglomerate reservoirs in the Permian Jiamuhe Formation in the Zhongguai area at the northwestern margin of the Junggar Basin, northwestern China, are reservoirs for large accumulations of natural tight gas. The tight conglomerates and sandstones are mainly litharenites with a large amount of texturally and compositionally immature volcanic clastic materials. Core porosities demonstrate the development of anomalously high porosities at depths of 3200–4200 m (10,500–13,800 ft) and 4500–4900 m (14,800–16,100 ft). The Permian reservoirs experienced initial rapid subsidence, uplift, and further subsidence, and diagenetic reactions occurred in these reservoirs involving compaction; precipitation of chlorite clays, calcites, zeolites, iron oxides, kaolinite, and quartz cements; and dissolution of unstable minerals including analcite, laumontite, feldspars, and rock fragments. Low-porosity reservoirs, with extensive compaction and (or) cementation of calcite and heulandite-Ca, consist of only few visible secondary pores formed by dissolution of feldspars and rock fragments. Reservoirs with anomalously high porosity, however, experienced relatively weak compaction and contain significant amounts of secondary pores formed by dissolution of mainly laumontites, some feldspars, and rock fragments. Comprehensive studies of sedimentary features and diagenesis of the reservoirs indicate that these anomalously high porosities originate from chronological coupling of four important geological processes. (1) Sedimentary facies and detrital compositions controlled distribution of various zeolites in reservoirs, with abundant laumonites developed in reservoirs in the fan delta front subfacies. (2) Early precipitation of the laumontites inhibited compaction during deep burial; however, they provided unstable minerals for secondary porosity development. (3) The Permian reservoirs experienced subaerial exposure erosion during the uplift stage, and meteoric freshwater leached laumontite cements and aluminosilicate grains to form secondary pores in reservoirs beneath the unconformity. (4) Hydrocarbon emplacement at relative shallow depth during a second subsidence period preserved the secondary pores in reservoirs by retarding late carbonate cementation.

We thank Zheng et al. (2016) for their comments on our previous work of sedimentary hiatus within the Carboniferous Taiyuan Formation, northeastern Ordos Basin, China (Jiang et al., 2012). Zheng et al. (2016) have different opinions on sedimentary facies and sequence stratigraphy interpretation in the study area. They question our interpretation based on their understanding of sedimentary facies, seismic geometry, orientation of sand body, and sand body thickness. Here, we provide additional evidence to reinforce our interpretation. The structure of this reply follows the order of the questions raised by Zheng et al. (2016). The interpretation of depositional environment for sand bodies in the Upper Pennsylvanian Taiyuan Formation (Ct2) has been controversial for decades. Some previous studies suggested that Ct2 represents a barrier–lagoon–tidal flat system (e.g., Hao et al., 2007; Wang et al., 2007). Zheng et al. (2016) also made such an interpretation. However, northeast–southwest-trending sand bodies in Ct2 were interpreted as fluvial–deltaic channels by many other studies (e.g., Li et al., 1995; Liu, 1998; Fu et al., 2003; Guo et al., 2009; Zhang et al., 2009; Luo et al., 2011). The braided stream fills of the incised valley were documented by extensive erosional surfaces, conglomerate and/or mud clast fluvial lag deposits, and trough cross bedding. Erosional surfaces are one of the most common features observed in cores of Ct2 interval (Figure 1; see also figure 4 in Jiang et al., 2012). The poorly sorted gravels are aligned in various directions, indicating a rapid deposit without significant tidal reworking (Figure 1E). In contrast, the sedimentary structures that Zheng et al. (2016) interpreted as tidal–barrier island signatures, such as mud drapes and cross bedding, can be also found in fluvial deposits. Figure 1. Sedimentary features of fluvial deposit in the …

Major reservoirs in the Songliao Basin (SB) are composed of volcanic rocks below 3000 m (9843 ft) of buried depth. Gas accumulations are mostly found in the buried volcanic highs, which in general correspond to paleovolcanic centers. Porosity in the volcanic rocks depends on both primary and secondary processes. The best porosity is preferentially developed in a proximal facies near the central part of each volcanic edifice. Porosity and permeability decrease with depth of burial for both volcanic and nonvolcanic sections, but their porosity-depth trends differ. Lava and welded ignimbrite slowly lose porosity with burial depth because they solidified by cooling, and their groundmasses are poor in quartz and calcite precipitation, thus preserving porous space. In contrast, the associated sandstone, conglomerate, tuff, and tuffite are more sensitive to overburden pressure, suffering more intense compaction and cementation. As a result, porosity and permeability of lava and ignimbrite exceed that of the other rocks, and thus, they are the best reservoir rocks below burial depths of ca. 3000 m (9843 ft) in the SB. The paleovolcanic domes are rich in both lava rocks and fractures of diverse origin, and the topographic highs provide favorable locations for gas migration and accumulation.

Intrasalt carbonates of the Ediacaran–Cambrian Ara Group constitute a significant reservoir element of the intrasalt “stringer” play in Oman, in which dolomitic carbonates are encased in salt at depths of 3 to 7 km (1.9 to 4.3 mi). These reservoir carbonates have significant microbial influences. Although Ara Group reservoirs are mostly latest Precambrian, the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded, in lacustrine settings where calcified invertebrates are not a significant source of carbonate, or after periods of mass extinction before faunal recovery. A broad range of carbonate facies provides the context in which to understand the origin of the microbialite-dominated reservoirs developed across both ramp and rimmed shelf profiles. Major facies associations include carbonate-evaporite transition zone, deep ramp and slope, subtidal microbialites, clastic-textured carbonates, and restricted peritidal carbonates. Microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties, and that help in predicting the location of reservoir fairways. Microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities (>50%), shallow subtidal pustular laminites with cm-scale variability of lamina morphology, deeper subtidal crinkly laminites that show mm-scale variability of lamina morphology, and intertidal tufted laminates that show mm- to cm-scale tufted textures. Other reservoir facies are more conventional grainy carbonates including ripple cross-stratified grainstone–packstone, hummocky cross-stratified grainstone–packstone, flat pebble conglomerate, ooid and intraclast grainstone–packstone, and Cloudina grainstone–packstone. These facies are almost invariably dolomitized and all have moderate to excellent reservoir quality. These facies comprise carbonate platforms, broken up during salt tectonics, that range up to 160 m (525 ft) in thickness and extended laterally, prior to halokinesis, for tens to over 50 km (31 mi). The distribution of reservoir facies follows sequence stratigraphic predictions, with microbialites occurring in every accommodation profile. Late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build-ups, whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies. Pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods. Crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate.

The Ardath Shale and Scripps Formation exposed along Blacks Beach north of La Jolla, California, record a deep-water channelized slope system of an Eocene forearc basin. The outcrop exposure, which is approximately 100 m (330 ft) high by 1.7 km (1 mi) long, offers insight into reservoir distribution and connectivity within coarse-grained, confined, deep-water channel systems. To use this outcrop as a quantitative subsurface analog, a detailed two-dimensional lithologic model was constructed from measured sections and interpreted photopanels. Elastic rock properties, including compressional-wave velocity, shear-wave velocity, and density typical of shallow offshore west African reservoirs were used to construct an impedance model. This model was convolved with 15-, 25-, and 50-Hz quadrature-phase Ricker wavelets to generate near- and far-angle stack one-dimensional and two-dimensional synthetic seismic reflection models. Because deep-water lithofacies have distinct amplitude-variation-with-offset behaviors and the interpretation of surfaces is intimately coupled with predicting lithofacies, simple bed interface models of conglomerate, sandstone, interbedded sandstone and mudstone, and muddy sandy debrite were used to build a template for successful interpretation. Interpretation of these forward seismic models demonstrates (1) the limits of and uncertainty associated with the interpretation of seismic data at different frequencies commonly encountered in the exploration, development, and production of deep-water reservoirs; and (2) how the combination of near- and far-angle seismic data can be used to interpret channel-fill lithofacies and improve seismic interpretation. Large-scale channel complex set surfaces with significant impedance contrast (e.g., conglomerate overlying interbedded sandstone and mudstone) are readily interpretable at all frequencies with an increasing vertical error of 5 to 30 m (16 to 98 ft) from 50 to 15 Hz, respectively. Channel and channel complex surfaces can only be accurately mapped on the 50-Hz data, albeit with significant uncertainty. Near- to far-angle stack changes enable the identification of upward-fining, amalgamated, and fine-grained channel-fill lithofacies. Far-angle seismic reflections can provide a more detailed image of boundaries defining channel architecture and reservoir facies distribution.

An analysis of 31 whole cores (1600 ft, 490 m) and closely spaced wireline logs (500 wells) penetrating the Lower Cretaceous (Cenomanian) lower Woodbine Group in the mature East Texas field and adjacent areas indicates that depositional origins and complexity of the sandstone-body architecture in the field vary from those inferred from previous studies. Heterogeneity in the lower Woodbine Group is controlled by highstand, fluvial-dominated deltaic depositional architecture, with dip-elongate distributary-channel sandstones pinching out over short distances (typically 500 ft [150 m]) into delta-plain and interdistributary-bay siltstones and mudstones. This highstand section is truncated in the north and west parts of the field by a thick (maximum of 140 ft [43 m]) lowstand, incised-valley-fill succession composed of multistoried, coarse-gravel conglomerate and coarse sandstone beds of bed-load fluvial systems. In some areas of the field, this valley fill directly overlies distal-delta-front deposits, recording a fall in relative sea level of at least 215 ft (65 m). Correlation with the Woodbine succession in the East Texas Basin indicates that these highstand and lowstand deposits occur in the basal three fourth-order sequences of the unit, which comprises a maximum of 14 such cycles. Previous studies of the Woodbine Group have inferred meanderbelt sandstones flanked by coeval flood-plain mudstones and well-connected, laterally continuous sheet sandstones of wave-dominated deltaic and barrier-strand-plain settings. This model is inappropriate, and a full assessment of reservoir compartmentalization, fluid flow, and unswept mobile oil in East Texas field should include the highstand, fluvial-dominated deltaic and lowstand valley-fill sandstone-body architecture.

Garfield field, an old (1948) stratigraphic trap in central Kansas, produced 10 million bbl of oil from Cherokee (Desmoinesian, Pennsylvanian) conglomerate reservoirs and from Mississippian carbonate and sandstone reservoirs. The primary reservoir rock at Garfield conglomerate pool is the Pennsylvanian basal conglomerate (PBC), unique in that it accumulated as an unstratified alluvial-fan deposit, in a subtle structural depression that may be coincident with a microplate boundary. Each of these conditions is uncommon in the northern Mid-continent. The PBC at Garfield pool is composed of pebbles, cobbles, and boulders of reworked Mississippian chert and limestone in a matrix of porous, fine-grained sandstone and sandy shale. Microporosity in the chert cobbles has access to the borehole through the porous sandstone matrix. The sandstone matrix was the product of reworking of the original shaly sand matrix and older sandstones from the substrate during short-term Desmoinesian marine transgressions of the alluvial-fan surface. Ultimately, Marmaton transgressive onlap smoothed and buried the fan surface. A near-modern alluvial fan at Rocky Flats in central Colorado provides an ideal analog for the PBC deposit at Garfield pool. The Rocky Flats fan is nearly identical in scale, areal distribution, and thickness, and its constituents are remarkably similar to conglomerate zones, which produce hydrocarbons at Garfield pool. Future exploration in north-central Kansas and southern Nebraska should focus on the occurrence of conglomerate deposits in similar depositional settings where coincident with structural depressions.

David Martineau is a certified petroleum geologist and received his B.S. degree in geology from the University of Texas in 1960. He joined Pitts in 1972 and is the exploration manager. Pitts has been involved in the Barnett play since its inception. Prior to Pitts, he worked for Prudential Drilling Funds and Coastal States Gas Producing Company (now El Paso). In 2006, the Newark East field was the largest producing gas field in Texas, and according to the Energy Information Administration (EIA) latest reserves estimate (EIA, 2006, tables B2 and B4), Newark East field ranks third in the nation and second in the nation in terms of production. The Newark East field has generated considerable attention because the gas is produced from the Barnett Shale as opposed to conventional sandstone, conglomerate, or carbonate reservoirs. The field was discovered in 1981 by Mitchell Energy Corporation (MEC) in Wise County, Texas, located in the Fort Worth Basin in north central Texas. George Mitchell, founder and chief executive officer of MEC, had the vision and leadership behind MEC's continued exploration and research for the correct technology to extract gas from this unconventional gas reservoir. Dan Steward was the chief geologist for MEC during the critical exploration development of the Barnett Shale and was honored by AAPG with the 2007 Outstanding Explorer Award. The development of the field started slowly, and only 100 wells were completed between 1981 and 1990. In 1998, a major breakthrough in completion technique occurred when water fracturing replaced gel fracturing. From 1997 to 2006, more than 5829 wells were put on production, and hundreds of additional wells were drilled, completed, or waiting on a pipeline. The field has been divided into two areas: (1) the original core area, where the Barnett sits on top of the Viola Limestone, …

A set of regional natural fractures, present in the sandy to conglomeratic, fluvial, channel-fill deposits of the Cedar Mountain Formation (east-central Utah) has a consistent west-northwest strike regardless of the local axial orientations of the sinuous channels. The fracture-producing stresses were not significantly refracted by the mechanical-property contrast between the channel-fill sandstones and the encasing overbank mudstones. In addition, fracture spacing along the sinuous channel axes is relatively constant between one-half and one-third of the bed thickness for both large fractures that cut the full thickness of the channel deposits and for smaller fractures in the thinner, component beds. Fracture spacing was apparently not affected by the variations in stress amplification that commonly result from differently oriented stiff inclusions in a ductile matrix. Therefore, in the absence of other structures, fracture intensity and the orientation of fracture-related maximum horizontal permeability in sinuous elongated reservoirs will be relatively constant regardless of the orientation of the long axis of the reservoir. Whether maximum permeability trends along, oblique, or across such reservoirs, and the relative drainage efficiency of horizontal versus vertical wellbores drilled into them, will vary only with the local trend of the channel axis.

Well-log tomography, a recently developed technique that generates subhorizontal slices through log curves, was used to create a series of amplitude slices from the core permeability, core porosity, and gamma-ray log curves for the wells penetrating the Silurian Brown Niagara reservoir in the Belle River Mills field. Log-curve amplitude slicing and three-dimensional (3-D) imaging of the core permeability and core porosity amplitudes show the detailed anatomy of the pinnacle reef in terms of these important reservoir properties. Although the reef has been dolomitized, the spatial distribution of permeability and porosity amplitudes in the reservoir appears to be controlled by the original pore and depositional fabrics of the wackestone, boundstone, stromatolite, and reef conglomerate rock types.Trends and patterns observed in the normalized gamma-ray log tomography approximate the trends and patterns observed in the core permeability and core porosity tomography in the Belle River Mills field. The similarities in the tomography trends and patterns establishes a potentially significant relationship that can be used to interpret log-curve amplitude patterns in other Niagaran pinnacle reef fields with log-curve data only and perhaps lead to improved sequence-stratigraphic models for these reservoirs.Well-log tomography and 3-D imaging provide complementary visualization tools that use existing data to image reservoirs and geologic formations. These imaging tools can be used to improve the design and placement of vertical, deviated, and horizontal wellbores for primary, secondary, tertiary, and gas storage operations in these reefs and other reservoirs.

The Cerro Toro Formation in the Torres del Paine National Park, southern Chile, contains a series of deep-water channel complexes deposited in an elongate Andean foreland basin during the Late Cretaceous. This stratigraphic interval represents an essentially continuous depositional record of migrating, leveed-channel complexes. Collectively, the channel-fill units in the study area form a belt approximately 5 km (3 mi) wide and several hundred meters thick. Within the study area, four sets of channel complexes are identified. This paper focuses on the best exposed of these channel-complex sets (channel-complex set 3). The channels are filled by bedded conglomerate and amalgamated sandstones interpreted to represent the deposits of high-concentration turbidity currents and debris flows. Large-scale cross-beds in some of the conglomerates indicate significant bed-load transport of gravel- and cobble-forming bars in the channels. Channel axis to margin facies changes between clast-supported conglomerate and either (1) thick-bedded sandstone or (2) matrix-supported conglomerate are observed. Channel-fill facies lie on erosional surfaces that cut into adjacent interchannel facies. Beds thin and onlap these surfaces toward the channel margins. Shale or siltstone drapes of the channel cuts are uncommon and laterally discontinuous. Bed continuity between channel and adjacent, interchannel facies is not observed. The interchannel strata are interpreted to represent levee successions that bound the channels. Stratigraphy in the levee units is defined to include (1) basal, sandy lobe deposits comprised of medium- to thick-bedded turbidites and (2) overbank facies consisting primarily of packages of fining- and thinning-upward, fine-grained, thin-bedded turbidites. This vertical succession is transitional. Distal levee facies include mudstones with thin-bedded, laterally continuous sandstones. Proximal levee facies include mudstones with both thin- and thick-bedded sandstones; however, the thick-bedded sandstones have lower lateral continuity. The proximal levee facies have a higher sandstone percentage than the distal levee, but also have greater depositional and postdepositional complexity, with sand-filled crevasses, erosional truncation, and slumped beds. Field observations suggest that these leveed channels formed in stages that are represented by depositional and/or erosional events. In chronological order, these are (1) an initial stage of relatively unconfined, sand-rich deposition; (2) aggradation of a mud-rich, confining levee system resulting from overbank deposition as turbidity flows bypass the area; (3) erosion as the channel becomes entrenched or as the channel migrates; and (4) filling of the channel-margin relief by onlap of channel-fill sediments. These stages appear to have repeated several times during the formation of a series of channel complexes. In these ways, the Cerro Toro Formation appears analogous to leveed-channel systems observed in late Pleistocene submarine fans and subsurface examples.

Conventional cores from Lower Cretaceous outer-shelf and upper-slope prodelta facies in the Alma and Glenelg fields of the Scotian Basin show a wide range of synsedimentary deformation. Storm-dominated prodelta sandstone and mudstone beds have common load casts and structureless sandstone beds overlain by deformed sediment and sandstone dykes that are caused by storm- or earthquake-induced local liquefaction. Blocks of sediment 5–15 m (15–50 ft) thick, principally on the outer shelf, have foliated mudstone at their base, common internal shear zones in mudstone, shear deformation of sandstone, and are capped by intraclast conglomerates that are interpreted as debris-flow deposits. These are all features observed in shallow slides on the modern Mississippi prodelta. At the paleoshelf edge and upper slope, larger slide blocks are recognized with zones of intense internal deformation and high-angle thrusts near their base. The lack of a basal foliated mudstone may be the result of more rapid slide motion than on the shelf, resulting in hydroplaning. In addition, there is early postdepositional interstratal deformation as a result of loading by sandy delta distributaries and by slide blocks on the upper slope. The scale of many deformation structures is below the limits of three-dimensional seismic vertical resolution, yet they are likely to substantially influence reservoir properties.

The early Cenozoic evolution of the Canadian High Arctic was governed by a compressional stress regime related to the opening of Baffin Bay-Labrador Sea. On northeastern Ellesmere Island, the Eureka Sound Group sediments (late Paleocene in age) record the paleogeographic reorganisation of this event. This group is divisible into three formations and are preserved within six outliers in this area. The oldest of these units, the Mokka Fiord Formation is characterised by a brown weathering medium - to coarse grained lithic sandstone occuring as decimeter scale trough cross bedded units. These sediments are to 740 metres thick and are interpreted to be a predominantly fluvial deposit. The overlying and younger Cape Back Formation is interpreted to be fluviolacustrine in origin consisting of interbedded fine grained litho- to calcarenite, siltstone, and mudrock. Current ripples, climbing ripples, ripple and parallel laminations are common throughout this formation which reaches a total thickness of 1100 metres. Polymictic boulder orthoconglomerates of the Cape Lawrence Formation varies in thickness from 700 to 1000 metres. Clast composition consists of a variety of siliciclastics and carbonates. These conglomerate units are interpreted to be proximal gravely rivers which were alluvial fans. These data provide us with a valuable contribution towards advancing our understanding of the dynamics of this syntectonic siliciclastic depositional system.

The Cretaceous-preCretaceous unconformity, i. e., base Zuni supersequence boundary, crops out across much of central and west Texas. Topographic features on the unconformity surface are exhumed along the erosional out-crop edge of the nearly flat-lying Cretaceous strata that overlie the surface within Lampasas and Mills counties, Texas. Digital elevation data displayed as a topographic profile atop Pennsylvanian outcrops adjacent to the Cretaceous outcrop edge indicate that paleotopography was influenced by northwest-dipping Pennsylvanian strata that underlie the unconformity surface. Differential erosion of Pennsylvanian limestone, sandstone, and shale prior to Cretaceous deposition created a trellis network of northeast-trending valleys and divides. Local tectonic features, climate, and rising sea level influenced the nature of the Cretaceous fluvial fill that directly overlies the unconformity surface. The Sycamore Conglomerate is the earliest Cretaceous deposit, and accumulated in a sub-humid or drier climate as a downfilling braided alluvial wedge shed off the Llano Uplift. The Travis Peak Formation, a conglomeratic sandstone, onlaps and disconformably overlies the Sycamore Conglomerate, and was deposited as an aggradational meandering river complex during sea level rise.

A primary objective in exploration for and development of fluvial reservoirs is determining the thickness and width of sandstone-conglomerate bodies (mainly channel-belt deposits). Most of the existing techniques for estimating the dimensions of fluvial reservoirs have major drawbacks. A fresh approach to the problem is made using recent theoretical, experimental, and field studies. This new approach involves (1) new models for the lateral and vertical variation of lithofacies and petrophysical-log response of river-channel deposits with explicit recognition of the different superimposed scales of strata, (2) distinction among single and superimposed channel bars, channels, and channel belts, (3) interpretation of maximum paleochannel depth from the thickness of channel bars and the thickness of sets of cross-strata formed by dunes, and (4) evaluation of various methods for estimation of widths of sandstone-conglomerate bodies that represent either single or connected channel belts (outcrop analogs; correlation of sandstone-conglomerate bodies between wells; use of empirical equations relating channel depth, channel width, and channel-belt width; theoretical models; and three-dimensional seismic data).

Mississippian chert of variable character and diagenetic history defines a unique and prolific series of reservoirs in south-central Kansas and northern Oklahoma. Known collectively as the "chat," these reservoirs occur beneath, and are partly associated with, a major regional unconformity separating Mississippian and Pennsylvanian deposits. Chat fields occur in an arcuate fairway extending approximately 160 km along the flanks of the southward-plunging Pratt anticline, a southern extension of the Central Kansas uplift. Initial discoveries were made in the 1920s and 1930s, with major expansion of the trend in the 1940s and 1950s. Chat wells have been among the most productive in the southern midcontinent, yielding oil and gas at rates up to 1500 bbl, or 40 mmcf, per day from depths of 909-1515 m. The better reservoirs have exceptional porosities (30-50%), but moderate permeabilities (20-100 md); fracturing is assumed to play an important role in reservoir performance. Few detailed characterizations of chat reservoirs exist to date. Recent study has identified three reservoir types: (1) in-situ spiculitic chert, (2) in-situ brecciated and partly weathered spiculitic chert, and (3) highly weathered, transported chert conglomerate. These types can be broadly categorized according to paleostructural position. Current depositional models interpret the chert as mainly primary in origin, related to subtidal sponge spicule bioherms along a subtle shelf break. Entrapment is predominantly stratigraphic in nature; bioherms grade laterally into impermeable carbonate and are truncated updip beneath the sub-Pennsylvanian unconformity. Total production from the chat trend is on the order of 381 million bbl oil and 2.3 tcf gas. New infill drilling, field expansion, and enhanced recovery projects in the 1990s have helped revitalize interest in the "chat." Future exploratory and development efforts should add significant new data to the characterization of these complex and highly profitable reservoirs.

The Neogene sediments of eastern Azerbaijan and western Turkmenistan contain significant hydrocarbon reserves in deltaic reservoirs. Sedimentary logging of these reservoirs at outcrop in Azerbaijan has outlined four facies associations that encompass a range of paleoenvironments, from alluvial braided river sandstones and conglomerates to delta-front siltstones and mudstones. The facies associations suggest a river-dominated braid delta and are arranged into an architecture controlled by base-level changes: parasequences, parasequence sets, and sequence boundaries are key elements. The facies associations and stratigraphic architecture, together with outcrop observations of cementation and faulting, are summarized in terms of four idealized reservoir models: fluvial, delta plain, proximal delta front, and distal delta front. Each reservoir model has distinctive grain-size and shale distributions. Comparison of nearby oil fields with the studied outcrops suggests that the outcrops form good analogs from which to model reservoir architecture, barriers and baffles to fluid flow, reservoir heterogeneity, and reservoir quality variations.

Excellent three-dimensional exposures of the Upper Jurassic Salt Wash Sandstone Member of the Morrison Formation in the Henry Mountains area of southern Utah allow measurement of the thickness and width of fluvial sandstone and shale bodies from extensive photomosaics. The Salt Wash Sandstone Member is composed of fluvial channel fill, abandoned channel fill, and overbank/flood-plain strata that were deposited on a broad alluvial plain of low-sinuosity, sandy, braided streams flowing northeast. A hierarchy of sandstone and shale bodies in the Salt Wash Sandstone Member includes, in ascending order, trough cross-bedding, fining-upward units/mudstone intraclast conglomerates, single-story sandstone bodies/basal conglomerate, abandoned channel fill, multistory sandstone bodies, and overbank/flood-plain heterolithic strata. Trough cross-beds have an average width:thickness ratio (W:T) of 8.5:1 in the lower interval of the Salt Wash Sandstone Member and 10.4:1 in the upper interval. Fining-upward units are 0.5-3.0 m thick and 3-11 m wide. Single-story sandstone bodies in the upper interval are wider and thicker than their counterparts in the lower interval, based on average W:T, linear regression analysis, and cumulative relative frequency graphs. Multistory sandstone bodies are composed of two to eight stories, range up to 30 m thick and over 1500 m wide (W:T > 50:1), and are also larger in the upper interval. Heterolithic units between sandstone bodies include abandoned channel fill (W:T = 33:1) and overbank/flood-plain deposits (W:T = 70:1). Understanding W:T ratios from the component parts of an ancient, sandy, braided stream deposit can be applied in several ways to similar strata in other basins; for example, to (1) determine the width of a unit when only the thickness is known, (2) create correlation guidelines and maximum correlation lengths, (3) aid in interpreting the controls on fluvial architecture, and (4) place additional constraints on input variables to stratigraphic and fluid-flow modeling. The usefulness of these types of data demonstrates the need to develop more data sets from other depositional environments.

Following a 36-fold seismic survey that covered 460 km, two exploratory wells were drilled between July 1991 and August 1993 in the Ahuas area, on the Patuca tectonic belt, in the Mosquitia savannah in northeastern Honduras. The Embarcadero 1 well encountered only dense, barren, gray and red siliciclastics and some phyllite at total depth. The Raiti-Tara 1 well also drilled mostly barren, but less dense, red beds that included some Upper Cretaceous limestone conglomerate in the lower section. We did not find source or reservoir rocks in either well, nor did we find hydrocarbon shows. The absence of Lower Cretaceous limestone in both wells is significant because more than 1500 m of limestone are exposed 35-50 km southwest in the Colon Mountains. The lithology of the clastics in the Embarcadero well is similar to Middle and Upper Jurassic formations in central Honduras. The lithology of the softer red beds in the Raiti-Tara well suggests they are Tertiary fill in a pull-apart basin. The Mosquitia basin, including the Ahuas area, probably was on the seaward side of the Chortis block (once part of Mexico) and received only Jurassic sediments until it was elevated by arc magmatism in the Early Cretaceous. However, thick Lower Cretaceous platform carbonates were deposited some distance inland. Lateral forces in the early Late Cretaceous caused the outer edge of Chortis to break up, carrying the Colon carbonate block up to 50 km northwest by sinistral fault movement. Later, antithetic dextral displacement offset the various blocks and created pull-apart basins that filled with Tertiary sediments. In the early Paleocene, compression from a spreading center to the southeast ruptured the Jurassic rocks, creating a decollement and later thrusting. No complete petroleum system seems to exist along the axis of the uplifted Patuca tectonic belt largely because of the lack of organic-rich source rocks and the presence of complicated young structures.

To understand the stratigraphic nature of the Potter Formation reservoirs, a multi-well study along a west-east cross-section was done using dipmeter, Formation MicroScanner, and FMI[trademark] (Fullbore Formation Microlmager) data. Thermal EOR wells are drilled only into the heavy oil-bearing portion of the reservoir, thus each well does not penetrate the complete Potter sequence, but penetrates successively younger sandstone and conglomerate members from west to east. Wireline logs and continuous core data were used for correlation and calibration of the dip and image data. The image data revealed detailed stratification, bedding, and sedimentary structures, identifying conglomeratic, bioturbated and stratified sequences. Correlation with continuous core clarified which features were more interpretive. The image data, available on three of the eleven wells, was then correlated to dipmeter data, both in the form of computed dip data and interpolated electrical images. Each of the eleven wells penetrated only one or two members of the Potter Formation. Using the dip data, it was possible to reconstruct a true stratigraphic thickness type log of the Potter Formation. The interpreted stratigraphic data, revealing bedded, bioturbated, and conglomeratic depositional systems was combined into a single type log of the Potter Formation. The resultant type log can be usedmore » for various sequence stratigraphic, engineering, and thermal EOR reservoir characterization applications in the Midway-Sunset Field.« less

Ephemeral-fluvial sandstones form the main reservoirs in many oil and gas fields. Production histories and development difficulties suggest these deposits are more complex than is commonly supposed in the petroleum industry. There is inadequate published information to account for this observation. Relevant sedimentological literature on such deposits is qualitative and unsuitable for detailed reservoir characterization studies; therefore, integrated sedimentological, petrophysical, and fluid-flow simulation studies have been conducted on outcrop analogs in the southwestern United States. Our key objectives were to define flow units and to find areas of sensitivity inherent both in the sediments and in the analytical and statistical procedures used to describe and model s ch systems. We report results of integrated studies on the Lower Jurassic Kayenta Formation in southeastern Utah. The Kayenta Formation is a relatively proximal, sand-rich succession dominated by channel-fill sandstones. Sedimentological analysis identified eight main genetic units. In contrast, reservoir simulations, based on probe-permeameter data collected at outcrop, show that six of the main genetic units have uniform flow behavior. Intraformational mudclast conglomerates are the dominant permeability barriers, both because of the clasts themselves and because they act as sites of preferential cementation. Stochastic permeability models conditioned to the outcrop data produce results little different from models based on averaged values for each facies. The most sensitive feature of the mode s concerns the assumptions made about the mudclast conglomerates. The model results show that only by integrating analytical approaches can we understand the full character of an outcrop analog in such a way that we can use it predictively.

New surface mapping at a 1:12000 scale adjacent to the 2 billion barrel Midway Sunset Field has revealed complex intraformational stratigraphy within the upper Miocene Santa Margarita Formation (Tms). Locally known as the Potter and Spellacy Formations in the subsurface, these sandstone and conglomerate heavy oil reservoirs produce the majority of Midway Sunset daily production of 164,000 barrels of oil via thermal EOR processes. The Tms consists mostly of conglomerate inserted into the Belridge Diatomite (Tmb) interval. The stratigraphically lower intervals of the Tms clearly fill deeply incised valleys or submarine canyons cut into Tmb and locally into the underlying Antelope Shale (Tma). The basal intervals of Tms; are very coarse grained, containing boulders of granitic and metamorphic rock as large as 4 meters that were derived from the Salinian block west of the San Andreas Fault. The upper intervals of Tms are more sheet-like and interbedded containing clasts less than 50 cm in length. The incised valleys have a spacing of about one mile in outcrop, with a gap located in the area of the older Republic Sandstone (Tmr). Paleocurrents from Tms regionally suggest sediment transport to the northeast. The sedimentary structures of Tms suggest deposition in deep-water conditions,more » probably a slope (bathyal) setting. Shelf environments should have been present to the southwest (now stripped away by erosion) and submarine-fan and basin-floor environments to the northeast.« less

Lowstand fan deposits on Barbados were studied in outcrop to construct a conceptual reservior model for prediction of facies assemblages. Lowstand fan and wedge deposits are found in terrigenous clastic strata of the Basal Complex of Barbados, and are of Eocene age. The Barbados lowstand fan model consists of five parts, discussed below. (1) Residual lag facies with contrasting texture (conglomerate and coarse-grained sandstone beds interbedded with pebbly mudstone and mudstone) and depositional processes (debris flows, high- and low-density turbidity currents, suspended-load deposits) are interpreted to characterize an area largely of sediment bypass (bypass corridor 1) occurring updip of the lowstand fan. Channel-fill complexes, isolated channel-fills and interchannel deposits are recognized by different types of upward-fining vertical successions and associations of lithologies and compose the lowstand fan proper. (2) Conglomerate, pebbly sandstone and high- and low-density turbiditic sandstone forming hierarchical upward-fining successions, characterize channel-fill complexes in the proximal fan, and are typically amalgamated with other channel-fill complexes. (3) High- and low-density sediment gravity-flow deposits in well-developed hierarchial upward-fining packages characterize amalgamated channel-fill deposits, and are interstratified with mud-rich facies strata representing abandoned channel deposits and interchannel deposits. Together, the amalgamated channel-fill deposits interchannel deposits characterize deposition in the middle fan.more » (4) A residual lag facies, similar to that observed in bypass corridor 1, characterized by cross-bedded lenticular tractional deposits, interbedded with low-density sediment gravity flow deposits, defines bypass corridor 2: this occurs at the transition from the middle fan to the distal fan. (5) Low-density sediment gravity-flow deposits in tabular beds are observed in the distal fan.« less

Lower Pennsylvanian paleovalley-confined sandstones are important petroleum reservoirs in the Midwest. In Kansas, such reservoirs have produced approximately 220 million bbl of oil and 1.7 tcf of gas. Valley-fill successions tend to become muddy upward, but there can be considerable local heterogeneity in which reservoir sandstones pass laterally into muddy sandstones or nonreservoir shales. The lack of understanding of this reservoir heterogeneity can lead to low drilling success rates. The Tonganoxie paleovalley (Upper Pennsylvanian, northeastern Kansas) contains facies very similar to Lower Pennsylvanian (Morrowan) valley fills, and can provide an outcrop- and subsurface-based model of sandstone deposition. The Tonganoxie paleovalley was incised during lowered sea level and filled during the subsequent transgression. The main paleovalley is approximately 41 m deep, 11 km wide, and 240 km long, and was fed by 1-km-wide tributary valleys oriented roughly normal to the trunk valley. Sandstones occur in four distinct architectural elements that were deposited during different phases of transgression. Type I sandstone consists of a belt of sandstone and conglomerate 3-18 m thick and confined to the trunk valley and wider portions of tributary valleys. Type I sandstone consists of amalgamated channel fills, has little or no mud, and has the highest porosity and permeability. The type I sandstone is overlain by estuarine deposits of sandstone (type II sandstones), rippled argillaceous sandstone to sandy mudstone, and coal. Most of the paleovalley was filled during this stage. The type II sandstones are narrow (1.5 km wide) arcuate bodies up to 8 km long and were likely deposited in tidal point bars near the fluvial to tidal transition, are either isolated sandstone bodies or are incised into type I sandstone. The higher mud content is expected to reduce porosity and permeability compared to fluvial facies. Type III sandstone bodies occur at the upstream limits of narrow tributaries and are probably bay-head deltas. Well logs indicate a range of mud content. Type IV sandstone is a thin (3 m) discontinuous sheet of marine sandstone deposited after most of the paleovalley had been filled.

Early Pennsylvanian wrenching along the Red River-Matador Arch (Tectonic Zone) created a braided series of en echelon faults and folds with associated pop-up structures and pull-apart basins. Local extension, or overstepping, in Southeast Cottle County, Texas, has produced the deepest pull-apart basin along the arch with over 10,000` of structural relief. The emerging Wichita-Amarillo Uplift, to the north, provided an abundant sediment source, which prograded rapidly southward as an alluvial fan-braided river complex. Exposure of basement rocks and lower Paleozoic sediments along the Red River-Matador Arch, also contributed to the basin fill. Syntectonic sedimentation led to the accumulation of over 6000` of Bend (Atoka-lower Des Moines) sediments within the basin. Deposition was dominated initially by alluvial fan to fluvial siliciclastics. As basin subsidence was further amplified by sediment loading, accommodation exceeded sedimentation capturing a large segment of the southward prograding Wichita-Amarillo derived clastic wedge. Encroachment of the late Atoka to lower Des Moines epeiric sea promoted further evolution of depositional environments to fan deltas, marine dominated clastics and, later, localized carbonate development. Type III kerogen rich organic shales produced abundant gas prone source rocks. The extreme depth of the basin combined with the local geothermal gradient provided for significant hydrocarbonmore » generation. By early 1988 new well control helped revise previous stratigraphic correlation demonstrating a rapidly expanding lower Des Moines to Atokan section. The drilling of the Gunn Oil Company-Brothers No. 1 to a total depth of 10,301` in the Mississippian Chappel Limestone, encountered 2025` of Bend sediments, with 279` of gross Bend Conglomerate (162` of net pay). The Brothers No. 1 was potentialled on 11/19/89 with a CAOF of 6.0 MMCFD and filed as the field discovery for the Broken Bone (Bend Conglomerate) field.« less

The analysis of lithostratigraphic and geophysics data suggest that during Permian-Lower Triassic and Upper Triassic at least in the northern part of the Moesian Platform has been developed a failed continental rift. This structogenetical evolution of the Moesian Platform has performed a continental environment during of the Permian-Lower Triassic and Upper Triassic as well as predominantly marine one during of the Middle Triassic. The depositional model for the Permian-Lower Triassic and Upper Triassic consists of facies belts of fluviatile, aeolian, evaporite/lacustrine deposits roughly paralleling the main uplifts of the Moesian Platform. The facies belts pass from alluvial fan facies, best developed in proximity to the main uplifts, through fluviatile sands, dune and inter-dune aeolian sands and finally into evaporite/lacustrine facies at the center of the main depressions. The facies associations consist predominantly of the fining-up type sequences. The marine environments for the Middle Triassic consist predominantly of carbonate facies. The best reservoir types consists of conglomerate-, sand- and carbonate-dominated facies associations located at the bottom of Permian-Lower Triassic, Upper Triassic and Middle Triassic respectively. In the north-western part of the Moesian Platform there are twelve oil and gas fields stored into these reservoir types.

A regional sedimentological study of Cretaceous sequences in the Mid-Norway region (Norwegian Sea) and in the Agat region (Agat field area, northern North Sea) reveals that these sequences were predominantly deposited in an upper continental slope environment by slumps and debris flows. Examination of nearly 500 m of core from 14 wells shows eight distinct lithofacies: facies 1 (contorted conglomerate and pebbly sandstone) represents deposits of sandy slumps and debris flows, possibly in a channel setting; facies 2 (contorted sandstone) is the most widespread and is the product of sandy slumps and debris flows; facies 3 (contorted mudstone) indicates deposition from muddy slumps and debris flows; facies 4 (rippled sandstone) suggests bottom-current reworking; facies 5 (gr ded sandstone) represents turbidity-current deposits and is very rare; facies 6 (laminated mudstone) is a product of pelagic or hemipelagic deposition; facies 7 (cross-bedded sandstone) is indicative of tidal processes; and facies 8 (laminated sandstone) represents delta-front and shelf deposits. These facies and their association suggest a shelf-edge delta to upper slope environment of deposition. Existing core data document deltaic facies only in the Mid-Norway region. The proposed shelf-edge delta and upper slope model has important implications for sand distribution. (1) This model provides an alternative to the conventional submarine-fan model previously applied to these sequences. (2) Although slump and debris-flow emplaced sands are usually discontinuous and unpredictable, highly amalgamated slump and debris-flow sands may develop thick reservoirs. (3) By using the Eocene Frigg Formation as an analog, it is predicted that externally mounded seismic facies in the study area may be composed of sandy slumps and debris flows.

The base of the Ellesmerian sequence in the northeastern Brooks Range is a widespread terrigenous clastic succession assigned to the Kekiktuk Conglomerate (late Tournaisian-Visean). The Kekiktuk is situated above a regional angular unconformity and is the base of a transgressive succession that is thought to record coastal onlap associated with the formation of a passive continental margin. Six depositional units have been recognized in the Kekiktuk Conglomerate. Unit A consists of clast-supported and rare matrix-supported pebble and cobble conglomerate with minor interbeds of pebbly sandstone. It records deposition at the base of incised paleovalleys from high-sediment-concentration flows and debris flows. Units B, C, and D consist of multistory fining-upward cycles that record deposition within incised paleovalleys from low-sinuosity bed-load-dominated streams, low to moderate-sinuosity bed-load-dominated streams, and high-sinuosity mixed-load streams, respectively. Unit E consists of multistory fining-upward cycles and laminated sandstone with thin conglomerate stringers. It records deposition in the distal regions of incised paleovalleys in a variety of marginal-mari e settings, including fluvial channel, channel-mouth bar, beach ridge, and coastal swamp environments. Unit F consists of horizontally bedded sandstone, interbedded sandstone, and organic-rich mudstone. It records deposition over paleotopographic highs on the sub-Mississippian unconformity from sheetfloods and in associated coastal swamp and mud-flat settings. Relative sea level rise is thought to have initiated fluvial deposition in the northeastern Brooks Range by the latest Tournaisian-earliest Visean. Coal-bearing fluvial channel-fill and flood-basin successions and palynologic data suggest that this part of Arctic Alaska was situated in a humid climatic zone during the Early Carboniferous. The stratigraphic and sedimentologic organization of the Kekiktuk Conglomerate indicates that once base-level rise began to affect the northeastern Brooks Range, local and regional variations in paleotopography along the sub-Mississippian unconformity exerted primary control on accommodation and consequent distribution of Kekiktuk depositional environments. The stratigraphic position of the Kekiktuk Conglomerate above a regional angular unconformity nd below marginal-marine and shallow-marine shales, combined with its limited thickness, widespread distribution, and suite of paleoenvironments, suggests deposition in an upland region situated landward of the tectonic hinge zone along a steadily subsiding passive continental margin. Thermal subsidence combined with eustatic sea level rise were probably responsible for relative sea level rise in Arctic Alaska and the resulting regional transgression. End_Page 679

The first Miocene terrestrial vertebrate fauna from Louisiana has been recovered from exposures within Fort Polk in central western Louisiana. Remains of eight orders of mammals, including insectivores, rodents, perissodactyls, artiodactyls, and proboscideans, as well as crocodile and fish remains, have been recovered. Small vertebrate teeth are recovered from several conglomeratic layers in the Castor Creek Member of the Fleming Formation, and isolated bones and teeth of sheep-sized to horse-sized mammals are found in both the conglomerates and the surrounding nodule-rich mudstones. Nodules winnowed from the mudstone formed the layers of conglomerate. Small, resistant vertebrate remains from the normal accumulation on the land surface were concentrated and incorporated in them. After dissolution in dilute acetic acid and screening, the rocks yield a diverse fauna of terrestrial microvertebrates, particularly rodent teeth. Fossils found so far indicate an early late Barstovian Land Mammal Age.

In southwestern Choctaw County, Alabama, the Eagle Mills Formation consists of alluvial-fan red beds deposited in a Mesozoic graben. The Exxon No. 1 Smith Lumber Co. well penetrated the upper 2,210 ft of the Eagle Mills, and it is divided into eight lithostratigraphic units designated A through H from bottom to top. Unit A consists of numerous upward-fining and -coarsening sequences of sandstones, conglomeratic sandstones, and conglomerates of variable thickness separated by red mudstones. They were deposited on the distal fan by ephemeral streams and sheetfloods and are composed of low-grade metamorphic rock fragments probably derived from the Talladega slate belt. Unit B consists of red mudstone with interbedded upward-fining and -coarsening sandstones and conglomerates with abundant calcite concretions and paleocaliche. Thin-section analyses of three sandstones of Unit B show that two are phyllarenites and one is a subphyllarenite. They vary from very fine to coarse-grained and are moderately sorted and submature, with angular to subangular framework grains. Quartz (subequal monocrystalline and polycrystalline) is the dominant framework grain with lesser amounts of schist and phyllite. In the two shallower sandstones, quartz content decreases as chert and dolomite marble rock fragments increase. Scanning electron microscope examination of one sandstone shows the presence of large well-developed pores lined with illite and authigenic quartz. Examination of drill cuttings from other Unit B sandstones, however, shows them to be poorly sorted, highly indurated, and apparently of low porosity. Unit B was deposited on the distal fan by sheetfloods and ephemeral streams. Faulting and igneous activity related to rifting occurred during the latter stage of deposition of Unit B. Units C through G are dominantly conglomerates; Units C, D, and F contain abundant igneous rock fragments. Unit C contains plutonic (syenite) and volcanic (rhyolite and dacite) rock fragments, and the igneous clasts of Units D and F are volcanic. Rare poorly defined graded beds occur in Unit D cores. Units C and D exhibit a general upward coarsening of sediments, indicating progradation of the fan. Unit E coarsens upward to a sedimentary (chert and limestone) and metamorphic (dolomite marble and lesser amounts of phyllite and schist) rock-fragment-bearing cobbly pebble conglomerate. Unit F is a mixed volcanic and sedimentary-rock-fragment-bearing conglomerate. Unit G is a massive sedimentary and metamorphic rock-fragment-bearing cobbly pebble conglomerate. Cores taken from the upper half of this unit show this portion to be a poorly sorted clast- to matrix-supported conglomerate. Although Unit G is in general massive, rare poorly defined graded beds and rare thin-bedded sandstones are also present. Units D through G were deposited in the mid-fan subenvironment. In Unit H, high log porosities and well-rounded sand grains in drill cuttings indicate the presence of eolian deposits. Log responses and the presence of anhydrite in cuttings show that the Eagle Mills is overlain by the Werner Formation. The provenance for Eagle Mills sediments was the southern Appalachian system, with sedimentary rock fragments being derived from folded Paleozoic rocks, whereas metamorphic rock fragments came primarily from metasedimentary rocks of the Talladega slate belt. Igneous rock fragments were derived from rocks emplaced in the upper-fan area as a result of igneous activity associated with rifting. End_Page 117------------------------ FIGURE 1. [Grey Scale] Unit G cobbly pebble conglomerate. Clasts are chert and dolomite marble. FIGURE 2. [Grey Scale] Upward-fining conglomerate channel deposit. Light-colored clasts are quartzite; dark elongate clasts are hematite-stained phyllite and schist. FIGURE 3. [Grey Scale] Unit D porphyritic rhyolite clast. White phenocryst is alkali feldspar altered to illite. Matrix is hematite-stained mud. FIGURE 4. [Grey Scale] Thin-section photomicrograph of oolitic sandstone clast. Most ooid nuclei are quartz coated with hematite. Clast is from Silurian Clinton Group. End_of_Record - Last_Page 118

The Castner Marble occurs within roof pendants in the Red Bluff Granite complex, and is the oldest Proterozoic unit exposed in the Franklin Mountains. This contact-metamorphosed metasedimentary unit consists of six lithologies: (1) stromatolitic limestone, (2) cryptalgalaminite, (3) massive limestone, (4) hornfels, (5) rhythmite, and (6) flat-pebble conglomerate. Based on the distribution of these lithologies, the Castner Marble can be divided into three distinct intervals: (1) lower section, consisting of stromatolitic, cryptalgalaminite and massive limestone, (2) middle section, characterized by thick hornfels and massive limestone beds, and (3) upper section, consisting of interbedded rhythmite and flat-pebble conglomerates. The lower section represents a shallow-water, intertidal to subtidal deposit, whereas the upper section was deposited in deeper water (below wave base) by turbidite or storm-related processes. The middle section may be a transition between the two, in that it contains lithologies found in the upper and lower sections. These three sections appear to record a transgression on a low-energy, carbonate ramp/platform. The contact between the Castner Marble and the overlying basaltic Mundy Breccia marks a catastrophic event or series of events in which megabreccias of deep-water, subtidal carbonate facies were deposited onto underlying rhythmite intervals.

Chert conglomerates, and associated sediments, of the Bend Conglomerate in Taylor Draw field, Upton Co., Texas, were deposited in a series of fan deltas that prograded into an open-marine system. The distribution of reservoir quality in these deposits is facies controlled. Permeabilities range from near zero to 30,000 md. Low-permeability conglomerates contain either a mud matrix or abundant calcite fossil fragments. The calcite grains serve as nucleation sites for calcite cementation and, through dissolution, as a source of calcite cement. High-permeability, sandy (chert and quartz) conglomerates have no mud matrix or calcite grains. Some of them are cemented with a thin rim of chalcedony, while others have no cement and show strong pressure solution at grain contacts. They are commonly friable and are recovered as loose gravels in the core barrel. The mineralogy of matrix gains is related to the distribution of the depositional facies within the fan delta. The low-permeability conglomerates were deposited in the subaerial to subaqueous distal fan-delta complex where calcareous marine organisms or mud were mixed with the conglomerate. The high-permeability conglomerates were deposited as thick channel-gravels in the subaerial proximal and mid-fan sections of the fan deltas. Depositional facies within the fan-delta complex controlled themore » distribution of calcite grains in the matrix and hence reservoir quality. The fossiliferous conglomerates became tightly cemented with calcite during burial, whereas the nonfossiliferous conglomerates remained permeable. Facies maps of the fan-delta complex therefore can be used to delineate high-permeability conglomeratic reservoirs.« less

The Lower Cretaceous Liard basin in western Canada covers an area of 9500 sq km (3668 sq mi) but is relatively unexplored despite its size. The present-day expression of the basin, which formed during the latest Cretaceous to early Tertiary, trends north-south and is delineated by the outcrop of the coarse-clastic Upper Cretaceous Dunvegan Formation. The lowermost Cretaceous unit, herein named the Chinkeh Formation, is up to 32 m (105 ft) thick and unconformably overlies older Paleozoic strata. The Chinkeh Formation contains four major lithotypes: (1) conglomeratic breccia interpreted as debris-flow or talus deposits, (2) interbedded coal, carbonaceous shale, rooted sandstone, and conglomerate interpreted as nonmarine valley fill or channel deposits, (3) conglomeratic lag related to marine transgression, and (4) upward-coarsening sandstone interpreted as abandoned shoreline deposits. The lower part of the Chinkeh Formation consists locally of an angular-chert breccia overlain by medium- to coarse-grained sandstone and conglomerate with coal and rooted beds. The uppermost Chinkeh Formation consists of a basal conglomeratic layer overlain by an upward-coarsening, fine- to medium-grained, well-sorted marine sandstone, with bioturbated shale. Sediment was recycled repeatedly with the dominant sediment source being from the east with perhaps some western sources. Cretaceous strata in the Liard basin have good petroleum source-rock and reservoir potential, and hydrocarbons may be present in sandstone of the Chinkeh Formation. Potential play types include stratigraphic traps formed by incised-valley deposits and shallow-marine sandstone pinching out laterally into marine shales of the Garbutt Formation. A potential structural play may occur along the Bovie fault zone where reservoirs may abut against a shale seal on the eastern side of the fault. Most of Lower Cretaceous strata falls within the hydrocarbon generation window, with increasing maturity to the southwest. Pyrobitumen is locally abundant and constitutes 30% of the bulk rock volume in one sandstone. Potential source rocks include the lowermost Garbutt Formation and underlying Triassic oad and Grayling formations. The Chinkeh Formation sandstone has porosity values of 8-18%.

A confined trunk fan delta and its structurally controlled tributary fan deltas are extremely well exposed along the southern margin of the Rosario embayment. This fan-delta complex consists of nonmarine to deep marine deposits almost continuously exposed for over 20 km in the down-paleoslope direction. Facies and stratal patterns in the fan-delta complex were controlled by local faulting, climatic variation, and eustasy, resulting in relative sea level fluctuations. Basin bathymetry and drainage patterns were controlled by a series of half-grabens that formed along north-south-trending faults that lay along the northern margin of an east-west-trending depression. Breccias were initially shed into the north-south-trending half-grabens; axial drainage systems were later established within the grabens, making up the tributary fan deltas. These tributary fan deltas fed a voluminous trunk fan delta confined to the east-west-trending depression. The trunk and tributary fan delta deposits show two major progradational to retrogradational cycles that record relative sea level fluctuations. Progradation in the shallow-marine environment is represented by conglomerate channels cut into ripple-laminated or bioturbated siltstone and HCS sandstones, overlain by conglomerate mouth bar deposits interstratified with nearshore sandstone deposits. Retrogradation in the shallow marine environment is recorded by either a vertical clastic facies transition or amore » clastic-carbonate facies transition. The clastic facies transition consists of nearshore sandstone deposits overlain by offshore bioturbated siltstones. The clastic-carbonate facies transition consists of the development of red algal patch reefs and rhodoliths on top of fan-delta conglomeratic lobes, mudstone and sandstone bank channel margins, or paleobasement highs. The clastic-carbonate facies transition reflects low sediment supply controlled by climatic conditions.« less

Vertical sequences characterized by upward decreases in the grain size and bed thickness of turbidites are commonly attributed to laterally migrating channel-levee-overbank systems. A probable Paleocene example of such a sequence is superbly exposed at Pebbly Beach, Point Lobos, California. Contact relations indicate that the Paleocene deposits fill a steep-walled sinuous valley carved into underlying granodiorite of late Cretaceous age. The few fossils found in the Paleocene rocks indicate deposition in water depths of 100 m (328 ft) or more and suggest that the sediment accumulated in a submarine canyon. Although most of these Paleocene deposits are conglomeratic, the upper part of the section exposed at Pebbly Beach consists of a 30-m (98-ft) thick fining-upward sequen e from conglomerate through sandstone to mudstone. About 10 m (33 ft) of predominantly thick-bedded sandstone grades upward through a transitional sequence of about 2 m (6 ft) of thin-bedded sandstone into End_Page 438------------------------------ more than 15 m (49 ft) of mudstone with thin sandstone interbeds. The thick-bedded sandstone beds in the lower part of the sequence are typically several tens of centimeters thick, graded, and display Bouma Ta-b or Ta-b-c intervals. Although many of these sandstone beds are visibly lenticular, the associated thin mudstone interbeds extend laterally across the exposure, unchanged in thickness or lithology. This thin-bedded transitional sequence consists of graded sandstone beds, 5 to 15 cm (2 to 6 in.) thick, that displays Bouma Tb-c intervals. The mudstone unit that caps the succession consists of bioturbated mudstone with thin (mostly less than 5 cm, 2 in.) sandstone interbeds. Paleocurrents in the upper and lower parts of the succession (predominantl as recorded in ripples and ripple bedding) trend dominantly southwest. The transitional sequence, however, displays a consistent 30° deflection of currents to the south; in addition, these strata are inclined slightly more steeply to the south than the beds above or below. I interpret this overall fining-upward sequence as resulting from lateral migration of a channel within the submarine canyon. The thick-bedded sandstone beds represent deposition on the flank of the channel; the lenticularity of these beds suggests that they were not deposited by a pure turbidity current but were carried by additional (or other) mechanisms, such as fluidized flow. The mudstone interbeds reflect predominantly pelagic deposition in the intervals between sediment gravity flows. The thin-bedded sandstone sequence represents a levee facies deposited primarily from tractive currents associated with the gravity flows that spilled over the channel and were deflected slightly to the south by the slope on the outer side of the levee. The upper, predominantly mudstone part of th section was deposited as pelagic sediment interspersed with overbank flows that traveled down the general slope of the submarine-canyon floor. An erosional surface with 3 to 4 m (10 to 13 ft) of relief near the top of the thick-bedded sandstone, covered by a mudstone breccia talus, records an episode wherein the channel reversed its migration direction and locally eroded the upper channel-flank deposits. End_of_Article - Last_Page 439

Studies of the depositional relations of the Umpqua and Tyee Formations (Eocene) in southwestern Oregon indicate a need for reassessing the correlations and currently used terminology. The Umpqua Formation (as herein restricted) consists of as much as 10,000 ft (3,000 m) of mudstone, sandstone, and conglomerate of nonmarine to deep marine origin. A basaltic basement that underlies the sedimentary rocks in most of the area and was formerly included in the Umpqua is herein considered a separate unit and assigned to the Siletz River Volcanics. A proposal to subdivide the Umpqua into three unconformity-bounded formations (in ascending order: Roseburg, Lookingglass, and Flournoy Formations) in the area west of Roseburg, Oregon, is not recognized in this report because of quest onable correlations and limited extent of some units. Foremost of these questionable correlations is that of the Flournoy Formation with rocks formerly assigned to the Tyee Formation in areas as far north as the latitude of Salem, Oregon. My investigations of depositional facies and lithologic criteria do not substantiate this correlation. The Tyee Formation, which conformably overlies the Umpqua, is a predominantly sandstone unit about 6,000 ft (1,800 m) thick, deposited in environments ranging from shallow marine and nonmarine deltaic on the south, to slope and deep marine basinal to the north. Deposition across the Umpqua-Tyee boundary contact represents a change in tectonic setting from active plate margin of the Umpqua to more stable marginal basin deposition of the Tyee. This change is also reflected in a change in provenance as indicated by the characteristic arkosic and micaceous aspects of Tyee sandstones.

The Milligen Formation at the type area in the Wood River Valley of south-central Idaho has been dated as Devonian rather than Mississippian. For this reason the McGowan Creek Formation was defined for use in east-central Idaho for Lower Mississippian rocks that formerly were called Milligen. In the area between the Wood River Valley and the region where McGowan Creek rocks are present, the lowest formation in the Mississippian Copper Basin Group also has been assigned to the Milligen. Because these rocks differ in age and lithology from the Devonian Milligen and Mississippian McGowan Creek rocks, a revision in nomenclature is required. The Little Copper Formation is proposed as the new name for these rocks. The type section, the basal part of the type section for the Cop er Basin Group, is above Little Copper Creek about 11 km (7 mi) north of the old Muldoon townsite in Blaine County, Idaho. This section formerly was proposed as a reference section for the Milligen Formation. The Little Copper consists of 1,120 + m (3,700 ft) of thin to medium-bedded, dark-gray argillite with some interbeds of dark quartzite, granule-size conglomerate, and a few limestone lenses. The stratigraphic base of the formation is not exposed and the top is gradational with the overlying Drummond Mine Limestone of late Kinderhookian age. The Little Copper is of Early Mississippian (Kinderhookian) age, and it grades eastward into the lower part of the McGowan Creek Formation. The stratigraphic relation of the Little Copper to the Devonian Milligen is not known, although these older rocks are in thrust contact with various formations of the Copper Basin Group at several localities within the Pioneer Mountains.

The southern margin of the present Gabes-Sabratha basin roughly parallels the coastline of northwest Libya and southeast Tunisia and is contiguous to the northern margin of a Lower Cretaceous subaerial delta platform. The Lower Cretaceous Chicla and Cabao formations, exposed along the Jebel Nefusa escarpment and in the subsurface of the onshore Ghadames basin in Libya, represent a thin (0 to 200 m), laterally extensive, fluvio-deltaic sequence. A detailed study of 15 measured outcrop sections indicates each formation is a transgressive fluvio-deltaic package. A complete package can be subdivided into three major sedimentary facies from base to top: (1) braided fluvial delta plain (facies 1) or braided fan delta plain (facies 1a), (2) delta front-prodelta slope (facies 2), and (3) prodelta shelf (facies 3). Figure The Chicla and Cabao fluvio-deltaic packages were derived from two sediment source areas: (1) a large, undetermined area far to the south, and (2) a smaller, proximal area, 50 to 100 km inland of the Libyan coast. End_Page 555------------------------------ A system of large to small northwesterly flowing braided streams, fed from the quartz-rich southern source, deposited clean, laterally persistent, reservoir sands (braided fluvial and delta front facies) and impervious sealing shales (prodelta slope and prodelta shelf facies) across the slowly subsiding Saharan Platform. These deposits can be traced northward in the subsurface, across the entirety of western Libya, into the outcrop in the Jebel Nefusa, the northern limit of onshore control. Deposition ultimately extended northward into the more rapidly subsiding, organic rich, Gabes-Sabratha basin. Simultaneously, the locally sourced reservoir facies were deposited, north-northwesterly, from the region of the Garian basement high. Coarse conglomerate quartz sands (braided fan delta plain facies) were transported a short distance to the Jebel Nefusa and northward, forming a wedge of clean, reservoir quality sands (braided fluvial and delta front facies) and sealing shales (prodelta slope and prodelta shelf facies) lying below well control in the Gabes-Sabratha basin area immediately adjacent to the Libyan coast due west of Tripoli. End_of_Article - Last_Page 556

The Lower Cretaceous conglomerate sandstone, sandstone, and mudstone interval stratigraphically between the Upper Cretaceous Dakota Sandstone and the Upper Jurassic Brushy Basin Member of the Morrison Formation in the Chama and northern San Juan basins, New Mexico and Colorado, is tentatively called the Burro Canyon(?) Formation. In this area the Burro Canyon(?) consists of a basal fluvial sequence deposited in braided-stream environments and an upper fluvial sequence deposited in braided to meandering-stream environments. The Burro Canyon(?) is unconformably overlain by the Dakota Sandstone and may disconformably overlie the Brushy Basin Member of the Morrison Formation. The Burro Canyon(?) thins southward from the Chama and northern San Juan basins because of truncation of the formation under the End_Page 567------------------------------ pre-Dakota erosion surface. The relation of the Burro Canyon(?) to the underlying Morrison Formation is less clear and has been the subject of controversy; the Burro Canyon(?) has at times been equated with the Jackpile-type sandstone at the top of the Brushy Basin Member in the San Ysidro area and along the east side of the San Juan basin. However, several lines of evidence, including facies relations, differing styles of sedimentation and sedimentary structures, and differences in composition of sandstone and mudstone of the upper part of the Brushy Basin Member and of the Burro Canyon(?) Formation in the San Juan and Chama basins, indicate that the Burro Canyon(?) Formation occurs stratigraphically above similar-appearing sandstones at the top of the Brushy Basin along the east side of the San Juan basin. End_of_Article - Last_Page 568

The Eocene El Rito Formation of north-central New Mexico is the basal Tertiary unit of the Chama basin and the north-western Espanola basin. Deposition of the El Rito occurred in the final stages of the Laramide orogeny. The formation is an alluvial-fan deposit, showing two principal facies, quartzite conglomerate and quartzose sandstone. Paleocurrent directions determined from imbricated clasts, cross-bedding, and parting lineations indicate that the primary sources for the sediment were in the north and northeast. The clasts of the early, conglomeratic facies were derived from the reworking of sediments eroded from the Precambrian crystalline highlands of northern New Mexico early in the Laramide orogeny. The conglomerates represent debris-flow, sieve and high-energy, braided-stream gravel deposits. The immature, sand-sized particles were derived from proximal sources in the Brazos uplift during the final stages of the Laramide orogeny. The sandstone units represent sheet-flow sediments deposited from lower energy, braided streams with more easterly sources than the sources for the conglomerates. Point counts of thin sections of four sandstone samples show high QFL% quartz and QpLvmLsm% metamorphic lithics. These results are consistent with a provenance in the Precambrian metamorphic core of the Brazos uplift. Extensive diagenetic effects typical of Cenozoic desert alluvium include calcite, zeolite, and hematite pore-filling and replacement of grains, alteration of feldspars, and removal of heavy minerals. End_of_Article - Last_Page 563

The extensive matrix facies of the Hensel Formation consist of calcretized (calichified) red mudstone, and thin, sheet sandstone, interpreted as paleosols and overbank deposits. Framework facies include cross-stratified lenses of conglomerate and sandstone that represent low-sinuosity to meandering streams of small fluvial systems. These systems were active during the transgression that resulted in deposition of Glen Rose and Fredericksburg Group carbonates. The pervasive development of calcrete, desiccation cracks, and pseudo-anticlines within the Hensel paleosols, as well as the overall depositional style of the unit, suggest an arid to semiarid, seasonal climate. Paleopedogenesis resulted in the formation of authigenic kaolin and illite, intense red-orange coloration, and various types of calcrete. Paleocalcrete within the mudstone facies commonly assumed a nodular habit. It is frequently observed to be coalesced into honeycomb structures. Locally calcretized, thin-bedded sandstone and mudstone were buckled into pseudo-anticlines. These structures were partly produced by fluctuating ground water and other inferred soil-forming processes. Paleocalcrete accumulated in the sandstone facies as undulating hardpans and vertically oriented rhizoliths. Microscopic textures characteristic of all these calcretes include low-magnesian calcite, crystallaria, and circumgranular cracks. Carbon and oxygen isotopic values from each type of calcrete confirm a pedogenic origin for the authigenic calcite. A "heavier" oxygen isotopic value in the dolomite, peculiar to the pseudo-anticline calcrete, denotes enrichment of meteoric ground water by extreme evaporation. These conditions would have existed near a continental playa, an environment compatible with the seasonal, arid to semiarid climate. End_of_Article - Last_Page 295

The Brigham Group in southeastern Idaho comprises over 3000 m (9843 ft) of dominantly quartzose sandy strata above the late Proterozoic Blackrock Canyon Limestone of the Pocatello Formation and below fossiliferous Cambrian carbonate. The Brigham was formerly thought of as a conformable sequence of quartzites deposited in the shallow marine Cordilleran miogeocline. Detailed study shows, however, that it contains braided fluvial, lacustrine, beach, intertidal and subtidal deposits; local and regional disconformities; and major influxes of coarse, angular feldspar. Braided-stream facies characterized by channels, discontinuous beds, gravel-size lag deposits, and poor sorting occur in the upper Caddy Canyon Quartzite, Mutual Formation, and lower Camelback Mountain Quartzite. Lacustrine facies of the Mutual Formation contain maroon and green laminated argillite with flaser-bedded sand. Both the Camelback Mountain and Caddy Canyon Quartzite contain low-angle wedge cross-beds of shoreface facies, tabular deposits of planar foreset beds belonging to the offshore sand-wave facies, and storm-produced hummocky cross-stratification. These two formations and the fine grained marine Inkom Formation contain submarine channels filled with coarse conglomerate. Tidal facies of the Papoose Creek Formation contain flaser beds, bimodal planar cross-beds, and synaeresis cracks. Feldspar is locally abundant (up to 40%). Its percentage varies both vertically within stratigraphic units andmore » laterally in the same unit over tens of kilometers. It is absent only in the uppermost Camelback Mountain Quartzite at the top of the Brigham Group. Synsedimentary uplift of nearby basement source areas is suggested.« less

Ramba field in the South Sumatra basin produces oil from lower Miocene coral and foraminifera-rich wackestones with secondary moldic and vuggy porosity. These wackestones form two low-relief carbonate banks separated by a deeper water, intrabank channel at least 10 km long and 1 km wide. Sediments infilling the channel include (1) carbonate mudstone, (2) carbonate conglomerate (calclithite) composed of dolomite and micrite intraclasts, (3) foraminiferal packstone to grainstone, and (4) terrigenous shale rich in planktonic foraminifera. The channel formed during fault-controlled downwarping during (or followed by) growth of the adjacent carbonate banks. Slow rates of carbonate deposition in the clay-rich environment of the channel resulted in deposition of relatively tight carbonates that form a nonproductive barrier between the A and B oil pools in Ramba field. The channel-fill deposits are economically important for two reasons: (1) where tight, they act as a lateral seal to the reservoir, and (2) the carbonate conglomerate has sufficient porosity and permeability to have provided the spillpoint for oil in the A pool. Thus, although not sufficiently permeable to provide commercial production, it is the distribution of this conglomerate in the channel that limits the height of the oil column in the Ramba field's A pool to only 48 m.

A section of Upper Cretaceous (Maestrichtian) nonmarine sandstone, conglomerate, and siltstone with associated coal is exposed near Saddle Mountain on the northwest flank of Cook Inlet basin, the only known surface exposure of nonmarine Upper Cretaceous rocks in the Cook Inlet area. The section, at least 83.3 m thick, unconformably overlies the Upper Jurassic Naknek Formation and is unconformably overlain by the lower Tertiary West Foreland Formation. These Upper Cretaceous rocks correlate lithologically with the second or deeper interval of nonmarine Upper Cretaceous rocks penetrated in the lower Cook Inlet COST 1 well.

The lower to middle Eocene Ione Formation in southern Madera County was deposited in an alluvial-fan, braided-stream complex bordering the ancestral Sierra Nevada. Sandstone and conglomerate occur as a thin veneer unconformably overlying a deeply weathered Mesozoic igneous and metamorphic basement. Two facies are recognized: a braided-stream sandstone and a proximal to midfan conglomerate with sandstone interbeds. Braided-stream facies display both sandstone and thin matrix-supported conglomerate (debris flows). Sandstone units contain tabular cross-bedded and parallel-laminated bedding units, which may be capped by small-scale trough cross-beds. This longitudinal-bar sequence may locally occur adjacent to large-scale festoon cross-beds of channel origin. Locally, lebensspuren (trace fossils) are abundant in the sandstone units and these traces represent biologic activity on emergent longitudinal bars or inactive channels. The proximal to midfan facies consists of interbedded conglomerate and sandstone. Matrix-supported conglomerate units were deposited by debris flows and cross-bedded sandstone units were deposited in alluvial channels. Framework-supported conglomerate may have resulted by reworking of the sediment, removing the clay (forming sieve deposits), or as alluvial channel gravels. The Ione Formation in Madera County is part of a long, narrow alluvial plain between the ancestral Sierra Nevada and the ocean. This part of the Ione Formation differs from the mixed marine and nonmarine environments of the Ione at its type section. End_of_Article - Last_Page 1697

Commercial quantities of gas have been produced from several Miocene conglomerates in the southwestern part of the Veracruz basin. These conglomerates, with their associated sandstones and shales, make up a zone 1,000 m thick, all of which was deposited in a bathyal environment. The conglomerates consist of recycled Upper Cretaceous limestone clastic rocks containing minor amounts of igneous and metamorphic rock fragments. The clastic material was derived from the west, where a very thick section of carbonate and terrigenous rocks, Jurassic to Paleocene in age, was uplifted, folded, and thrust faulted during the Laramide orogeny. Erosion of the deformed source materials is indicated by two significant unconformities. One unconformity truncates sedimentary rocks from the Paleocene to the Upper Cretaceous, whereas the other is at the base of the Miocene. Most of the conglomerate beds are in the lower Miocene. A reconstruction of the pre-lower Miocene erosional surface serves to locate paleodrainage patterns and the areas of debouchment of deep-water fanglomerat s. Sediments derived from the source area west of Veracruz basin were transported in a general northeasterly direction by fluvial currents which fed into deep submarine canyons. Through these canyons most of the coarse clastic sediments were carried down to the basin floor and deposited as a series of prograding submarine fans.