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The Upper Permian Dalan Formation is a supergiant gas reservoir in the Persian Gulf Basin. Reservoir quality is controlled by the original facies type, dolomitization, and anhydrite cementation. This study focuses on the texture (petrographic analysis), bulk stable-isotope values, and trace element composition in order to identify the original mineralogy of carbonate grains and cements, and to assess the synsedimentary and diagenetic processes, including the timing and setting of dolomitization. The limestone data suggest that the initial precipitation of isopachous acicular to fibrous aragonite cements in the marine phreatic environment was followed by dissolution of aragonite grains and cements in the meteoric phreatic zone, and neomorphism and low-Mg calcite cementation (blocky and drusy crystals, syntaxial overgrowths) in the active freshwater phreatic zone. The relatively pristine δ18O and δ13C values, low Mn and relatively high Sr and Na contents indicate that this meteoric diagenesis was relatively limited. The remaining porous space was cemented by blocky calcite later during burial. Four types of dolomites are identified within a complex mixture of texturally and isotopically different dolomites: dolomicrite, dolomicrosparite, dolosparite, and saddle dolomite. Dolomicrite formed syndepositionally from the evaporated Permian seawater or by reflux of dense evaporated brines during shallow burial. The second major episode of dolomitization (dolomicrosparite and dolosparite) was driven by reflux of warmer and more saline fluids at greater depth, which affected both previously non-dolomitized and dolomitized sediments. The final diagenetic stabilization during deep burial is associated with extensive pressure solution, fracturing, vein-filling of calcite and dolomite cements, and crystallization of saddle dolomite.

Despite the vast research on various aspects of its stratigraphy, relatively little is known about the source-rock potential of the Upper Jurassic Adriatic platform, southern Croatia. Here a case palynological study has been provided to determine the kerogen type, quality, and thermal maturity, and assess its relative hydrocarbon generative potential. Palynofacies characteristics indicate a low-energy, shallow-marine oxygen-depleted setting. The deposited mud-supported carbonates contain organic matter that is almost entirely composed of a fluorescent amorphous organic matter of planktonic and bacterial origin. A high phycomata/dinocyst ratio suggests relative hydrographic stability in an environment favorable for motile-stage dinoflagellate community with reduced production of dinocysts. The lack of anaerobic degradation and very good preservation of lipoid palynomorphs indicates dysoxic conditions ; the latter also is suggested by a low proportion of algal-phytoplanktic carbohydrates. The lack of oxygen-deficient zone within the water column, coupled with well-preserved lipoid sediment components, suggests that the boundary between the oxygen-rich and oxygen-poor levels was likely located at/near the sediment-water interface. The palyno-organic facies analysis suggests that the organic matter corresponds to the type II (possibly I/II) kerogen (VRo ∼0.3–0.4%) that was protected from degradation before, during and after sedimentation. Well-preserved lipoid components suggest a good hydrocarbon-generative potential, but the low TOC values (

The long (700 m) Hauterivian to Albian Adriatic Platform section mainly from Mljet Island, Croatia underwent little post-Mesozoic burial or little later diagenesis. Consequently its smoothed δ13C and δ18O record from calcite lime mudstone matrix provides one of the most continuous stable-isotope curves from an Early Cretaceous platform. This record captures the carbon isotope excursions (CIEs) and oceanic anoxic events (OAE1a, b, c) evident in published hemipelagic sections, with the added advantage of providing information on the shallow platform response to the OAEs. The platform facies appear to have been little affected by the OAEs, except for OAE1a when deposition of organic-rich laminated limestones occurred in local downwarps.

The Lower Triassic Kangan Formation of the Persian Gulf Superbasin forms one of the largest reservoirs in the South Pars/North Field, the world's largest gas field. The upper Khuff-equivalent, gas-hosting carbonate-evaporite succession was formed on a gently-sloping, homoclinal carbonate ramp in a warm, tropical aragonite sea under a hot-and-arid greenhouse climate. This study explores the impact of vertical variations in depositional facies and diagenetic features on the reservoir quality to improve the prediction of the subsurface facies and hydraulic flow unit distribution. Detailed core logging and petrographic analysis resulted in the recognition of twelve distinct carbonate-evaporite facies that are grouped into four major facies associations, from shallowest to deepest: supratidal to restricted subtidal, lagoon, carbonate shoal, and shallow mid-ramp. The facies associations are stacked into two long-term sequences KS2 (58 m) and KS1 (77 m) bounded by anhydrites and/or unconformities; three higher frequency sequences (KS1a, KS1b, and KS1c) can be identified within the KS1. Based on the petrophysical properties, sedimentary facies characteristics and their diagenetic modifications, five hydraulic flow units (HFUs) are identified, with the best porosity-permeability values and flow- and storage capacities in the late transgressive (TSTs) and the earliest highstand systems tracts (HSTs) of the two long-term sequences. The reservoir quality and the distribution of hydraulic flow units are a product of interactions between primary mineralogy, depositional facies and their stacking, and the early- and late-diagenetic alterations. The reservoir quality decreases in landward direction, being the lowest in the mud-supported textures of the lagoon and supratidal to restricted subtidal facies associations. The seaward improvement in reservoir quality reflects a change to grain-supported textures that formed along the tide-and wave-agitated ooid-bioclastic shoals, whereas the best reservoir quality characterizes the shallow mid-ramp open-marine facies association, where secondary porosity was improved by dissolution and dolomitization. Both the early and late diagenetic dolomitization overall improved the reservoir quality, with the dolomitization pathways controlled by the porosity and permeability heterogeneity in the original depositional facies. The results may be useful in reservoir modeling, recognition of the productive zones, and the further development of the South Pars/North Dome Field.

The upper Katian (Upper Ordovician) Red River Formation is a carbonate-evaporite supersequence that formed in the epicratonic Williston Basin of western North America. This study focuses on the upper part of the formation in the southern, US part of the basin. The studied succession is made up of three transgressive-regressive sequences (RR1, RR2, and RR3), each ∼0.5–∼1 m.y. in duration and composed of relatively few meter-scale high-frequency cycles (parasequences). Sequence boundaries are characterized by intraclastic breccia-laminated dolomite successions that have been truncated and capped by evaporite deposits. Subaqueous anhydrites within each sequence represent evaporite Lowstand Systems Tracts (LSTs) that formed during relative sea-level lows that lead to the precipitation of gypsum (later recrystallized into anhydrite) in a quiet, salina-type environment. Transgressive (TST) and early Highstand Systems Tracts (HST) are composed of subtidal facies of predominantly skeletal mudstone to wackestone-packstone with abundant burrow mottling. Late HSTs are predominantly composed of peritidal, porous laminated dolomite. The study illustrates how the sequence stratigraphic framework of carbonate-evaporite deposits from the Williston Basin relates to the Late Ordovician glacio-eustasy, transitional climates, and low basin subsidence in an arid setting.

The Upper Permian Upper Dalan Formation contains one of the largest gas reservoirs in the world. The formation consists of carbonates with some evaporite intercalations that developed on a gently sloping homoclinal carbonate ramp facing the Late Permian Paleo-Tethys Ocean. This study focuses on the Kish Gas Field (Zagros offshore basin situated between Iran and Qatar), and is based on a 222-m-thick continuous core. Based on the integration of core- and wireline-log data coupled with petrographic analyses of 580 thin sections, three major depositional environments (facies belts) with 11 carbonate microfacies are identified. These include (1) sabkha to tidal flat (laminated to massive anhydrite, dolomudstone with anhydrite nodules, dolomudstone, and intraclastic dolowackestone), (2) lagoon and leeward shoals (bioclastic wackestone/dolowackestone to packstone, and peloid dolopackstone and peloid–bioclastic dolopackstone), and (3) mobile (windward) sand shoal (ooid–peloid dolograinstone, ooid dolograinstone, ooid–intraclast dolograinstone, ooid–bioclast dolograinstone–packstone, and coarse bioclast–intraclast dolograinstone). Diagenetic evolution of the Upper Dalan Formation is associated with evaporative marine, shallow-water normal-marine, meteoric, and burial diagenetic environments. Common diagenetic effects include dolomite and calcite cementation, mechanical and chemical compaction, dissolution, dolomitization, and evaporative (anhydrite) mineralization. Reservoir quality is strongly affected by variations in the original rock fabrics and subsequent diagenetic alterations. The most common pore types include interparticle, moldic, and connected vug (fracture and cavernous). The interparticle porosity–permeability relationship for the studied facies suggests that the reservoir quality is not affected by different crystal sizes and most samples plot in the low porosity and low to high permeability field, or display Lucia class 1 or 2 petrophysical relationships. The study shows that the pervasive pore-filling anhydrite mineralization lead to a significant decrease in porosity and permeability; poikilotopic anhydrite cement reduced matrix porosity, but the pore size was less affected.

Dark-gray oolitic units characterized by oversized ooids with primary radial calcite fabrics occur in the interior of the Late Jurassic Late Tithonian, Adriatic Platform, a large Mesozoic, Tethyan isolated platform in Croatia. They differ from open-marine, platform-margin ooid grainstones in their dark color, cerebroid outlines, broken and recoated grains, abundant inclusions, highly restricted biota, and lack of cross-stratifiThey have been interpreted as being of vadose origin (“vadoids”) at tops of upward-shallowing parasequences. However, detailed sections show that most oolitic units occur at bases of precessional parasequences, overlying erosional surfaces on fenestral carbonates. The oolitic units are similar to quiet-water ooids that form today in low-energy settings. They developed in an arid climate during initial transgression of supratidal fl ats, along lowenergy shores seaward of tidal fl ats, and along the margins of restricted lagoons and intertidal ponds. Superimposed fenestral fabrics, meniscus micrite cements, and grain breakage occurred as they aggraded to high-tide level and were subjected to wetting and drying, thermal expansion and contraction, and wind transport. They migrated landward with transgression, forming extensive sheets, and were overlain by subtidal lagoonal facies that shallow up into fenestral carbonates. These distinctive facies may have been overlooked in the geological record, or their geological distribution requires juxtaposition of calcite seas, high-calcite supersaturation states, arid climate, and presence of fl at-topped carbonate platforms in a greenhouse world.

Carbonate platforms are sensitive recorders of environmental change through geologic time. Climatically induced changes in sea level or changes in subsidence are expressed through migration of sediment belts that are recorded in the accumulating sedimentary succession. Changes in nutrient levels are recorded in shifts in the biotic community and, if eutrophic levels are reached, platforms may drown due to breakdown in carbonate production (Follmi et al., 1994). Our group has been focusing on the ∼60 Ma Late Jurassic–Early Cretaceous period, which was originally considered to have had uniform greenhouse conditions (Fischer, 1982). Available climate proxy data, however, such as oxygen isotope data (δ18O) from deep sea cores, the paleontologic data (e.g., Frakes et al., 1992), and the 87Sr/86Sr and carbon isotope data of Jurassic and Cretaceous carbonates (Jenkyns and Wilson, 1999), indicate that there were major cooling and warming events that likely affected global ice volume, sea level, and the accumulating sedimentary and microfossil record of carbonate platforms. Numerous carbonate platforms were developed within the tropical–subtropical belt of the circumequatorial Late Jurassic–Early Cretaceous Tethys Ocean. Many of these platforms have comparable shapes, sizes, facies, subsidence rates, and geologic structure to the present-day Bahamas Banks, a commonly used modern-day analogue (e.g., Bosellini, 2002, and references therein). In addition, the outcropping Tethyan platforms provide important clues to the poorly known subsurface Mesozoic carbonates that underpin the modern Bahamas platform. The Tethyan platforms were characterized by high rates of sedimentation (from 100 m/myr; D'Argenio et al., 1999) and exhibit meter-scale shallowing-upward cycles or parasequences generated during high frequency, small sea-level fluctuations within the Milankovitch band (Strasser, 1991; Schulz and Schafer-Neth, 1997; Lehmann et al., 1999; Strasser et al., 1999; Immenhauser et al., 2004; Husinec and …

Fischer plots are plots of accommodation (derived by calculating cumulative departure from mean cycle thickness) versus cycle number or stratigraphic distance (proxies for time), for cyclic carbonate platforms. Although many workers have derived programs to do this, there are currently no published, easily accessible programs that utilize Excel. In this paper, we present an Excel-based spreadsheet program for Fischer plots, illustrate how the data are input, and how the resulting plots may be interpreted. The plots can be used to derive periods of increased accommodation, shown on the plots as a rising limb (which commonly matches times of more open marine, subtidal parasequence development). Times of decreased accommodation, shown on the plots as a falling limb, generally are coincident with thin, shallow, peritidal parasequences.

The shallow marine carbonate deposits on the island of Cres, overlyingdeeper-water Cenomanian–Turonian limestones, are characterizedby an assemblage of rudists, benthic foraminifera, and associatedmicrofossils. The paucispecific character of the fossil association suggestsdeposition in shallow areas of a carbonate platform, with lowcurrent-energies and restricted circulation. Similar assemblages indicatingsimilar palaeoenvironments, are common in the Upper Cretaceousdeposits of the Adriatic Carbonate Platform and adjacent areas.The assemblage of rudists (hippuritids) and microfossils indicatethe Turonian to (?)Coniacian age of the investigated carbonate succession.The biostratigraphic importance of the so-called “primitive”hippuritids within the micropalaeontologically poorly defined biostratigraphyof deposits of this age, is accentuated.