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The carbonate reservoirs in the Late Oligocene-Early Miocene Asmari Formation in the Dezful Embayment of SW Iran are characterized by low matrix permeability, and effective drainage is dependent on the occurrence of open fractures. Limited information on fracture orien- tation and fracture density is available from core and borehole image data, and high-quality/high- resolution three-dimensional seismic is often lacking in this area. Well and core data do not contain information on important fracture parameters like length distribution, crosscutting relationships, fracture density v. lithology and bed thickness. The understanding of fracture distri- bution and formation in the region and their effects on fluid flow has been greatly improved by the use of outcrop analogue data. Exposures of the Asmari Formation in the Khaviz Anticline are in close vicinity to the giant hydrocarbon fields. The Khaviz Anticline has a similar geometry and structural history to the major hydrocarbon fields in the area, and represents an excellent analogue for these. Two types of fracture features were observed: diffuse fracturing and fracture swarms. The diffuse fractures form networks and comprise structures grouped into four fractures sets, which are the typical for this type of anticline. Two orthogonal fracture sets are oriented parallel and perpendicular to the fold axis, and two conjugate fracture sets are oblique to the fold axis with their obtuse angle intersecting the trend of the fold axis. The fractures are typically stratabound, sub-perpendicular to bedding and commonly about the bounding stratigraphic surfaces. To a large extent the density and height of fractures in the Asmari Formation are controlled by the mechan- ical stratigraphy, which is controlled by the depositional environment and cycles. These outcrop data have been essential in the generation of discrete fracture network (DFN) models and the population of the fracture properties in the reservoir models.

The Asmari Formation has been studied in the subsurface at the Bibi Hakimeh, Marun and Ahwaz oilfields and in an outcrop section from the Khaviz anticline. It consists of approximately 400 m of cyclic platform limestones and dolostones with subordinate intervals of sandstone and shale. The method of Sr-isotope stratigraphy is well suited for dating these strata because of the rapid rate of change of marine Sr-87/Sr-86 during Asmari deposition (roughly 32-18 Ma) and the common presence of well-preserved macrofossils.Profiles of age against depth in the four areas show a decrease from higher stratigraphic accumulation rates in the lower Asmari to lower rates in the middle to upper part of the formation. There is also a trend towards less open-marine depositional conditions and increasing early dolomitization and anhydrite abundance above the lower part of the formation. These changes reflect the dynamics of platform progradation across the areas studied, from early deposition along relatively high accommodation margin to slope settings to later conditions of lower accommodation on the shelf top.Ages of sequence boundaries are estimated from the age-depth profiles at each locality, providing a framework for stratigraphic correlation. Asmari deposition began in early Rupelian time (34-33 Ma) in the Bibi Hakimeh area, when the studied areas to the NW were accumulating basinal marl facies. Progradation of the platform across the Marun and Ahwaz areas took place in mid-Chattian time (27-26 Ma) and somewhat later (26-25 Ma) in the more basinward Khaviz area. Depositional sequences have durations of 1-3 Ma, whereas component cycles represent average time intervals of 100-300 Ky.Sr analyses of most dolomite, anhydrite and celestite samples plot close to or below the macrofossil age-depth trend for each locality, indicating formation from waters preserving seawater Sr-87/Sr-86 approximately contemporaneous with or slightly younger than the time of sediment deposition. Local deviations from this trend are interpreted as indicating episodes of seepage-reflux and also a contribution of Sr from non-marine sources during formation of the Gachsaran cap rock anhydrite.

Dolomitisation is an important factor controlling reservoir quality in the Asmari Formation in many producing fields in SW Iran. Dolostones have higher average porosities than limestones. Petrographic and geochemical studies have been used to determine the causes of Asmari dolomitisation at the Bibi Hakimeh and Marun fields and at the Khaviz anticline. The formation is generally characterized by a large-scale trend of upward-decreasing accommodation. Basal strata were deposited under relatively open-marine, high-energy conditions, whereas the Middle to Upper Asmari succession was deposited in relatively protected settings with more frequent evidence of exposure and evaporitic conditions. There is a general upward increase in the abundance of both anhydrite (occurring as nodules and cement) and dolomite. Two main types of dolomite fabric are recognised, reflecting the textures of the precursor limestones: (1) finely crystalline pervasive dolomite (commonly

A new set of descriptions has been prepared for five of the IKU cores, a series of shallow stratigraphic cores penetrating the Upper Carboniferous-Permian Finnmark carbonate platform succession near its southern erosional truncation against the Norwegian mainland. These data are compared with core descriptions previously published from the thicker 'stratigraphic reference section' of the exploration well 7128/6-1 (Ehrenberg et al. 1998a). New fusulinid datings from 7128/6-1 are correlated with existing fusulinid data from the IKU cores to provide a consistent time-stratigraphic framework for landward correlation of depositional sequences previously defined in the 7128/6-1 reference section. These correlations reveal a limited two-dimensional image of depositional sequence geometry for the inner platform. Of the 7 major sequences previously defined in the Kasimovian through Upper Permian section of well 7128/6-1, 2 sequences are suggested to pinch out before reaching the IKU cores, while the remaining 5 se...

Detailed studies of the sedimentology, sequence stratigraphy and diagenesis were conducted on the Permian- Triassic Kangan and Dalan formations of Iran. The objective was to reveal links between sequence stratigraphy, depositional and diagenetic processes. The identified relationships and the methodology used to distribute reservoir properties in a geocellular model is the focus of this presentation. Detailed descriptions of depositional facies and the depositional stacking pattern were followed by characterisation of diagenetic facies. The dataset consisted of more than 800 m of core and more than 800 thin sections from three wells penetrating the Kangan and Dalan formations. The analysis showed that the reservoir quality varies according to depositional facies and is extensively modified by diagenetic processes. However, the diagenetic processes can, to a certain extent, be related to the sequence stratigraphy and depositional facies. The trends are not unique and other controls must be applied when reservoir properties are to be distributed. These controls need to be obtained from standard log suites, because cores are available from only a limited number of wells. In order to model the 3-D distribution of reservoir properties, the modelling work has focused on capturing the spatial distribution of both depositional and diagenetic facies. The depositional facies have been modelled using high-frequency cycle-specific conceptual models. For the distribution of diagenetic facies, a novel approach using trends from calculated volume fractions of calcite, dolomite and anhydrite was used. Alternative geocellular models will be presented and differences between these discussed. Flow simulations have been erformed, and the results were used to identify key sensitivities and controls.

Analysis of previously unpublished data from a major Middle East gas field has resulted in an integrated study of the Khuff Formation. Conceptual geological models for the field will be presented, providing an opportunity for comparison with other published data in the region. Detailed sedimentological work of three extensively cored wells has enabled the identification of a suite of lithofacies, which are grouped into seven facies associations. Depositional settings range from marine grainshoals, through to restricted tidal flat settings with common evidence of exposure. Shallowing-upwards cycles form the basic building blocks in the sequence stratigraphic framework. Cycles are organised into packages, termed high-frequency sequences (HFS), which possibly reflect fourth-order relative sea-level variation. At a larger scale, HFS’s are grouped to form four major depositional sequences (K4 – K1). These are comparable to the thirdorder sequence described by Sharland et al. (2001). Cycles show distinct trends in thickness variation, which can be traced in all cored wells. Thicker cycles typically occur within marine ooid-grainstone facies of the transgressive systems tract (TST) of large scale sequences, whilst thinner cycles are more typical of restricted facies of the highstand systems tract (HST). Diagenesis has significantly modified the primary depositional facies. The key diagenetic processes include: (1) cementation: primarily calcite and anhydrite; (2) dissolution: dissolution of grains, in particular ooids; (3) dolomitisation: both early evaporitive (in late HST) and hydrothermal processes associated with faults. Reservoir potential is related to the interplay of primary depositional facies and subsequent diagenesis. The best reservoir quality is associated with dolostones, although overdolomitisation and anhydrite cementation are commonly detrimental. TST grainstone facies are prone to calcite cementation; however, dissolution of grains significantly improves porosity. Conceptual geological models have been built based on the HFS stratigraphic framework, and these models are the input for the flow-unit and geo-modelling.

Spectral gamma-ray (GR) profiles were examined in well 7128/6-1, the stratigraphic reference section of the entirely subsurface Finnmark carbonate platform. Detailed bulk-chemical profiling of selected GR peaks shows that potassium (K) and thorium (Th) are mutually correlated and are a direct index of siliciclastic (aluminosilicate) content, whereas uranium (U) is uncorrelated with K, Th, and all other chemical components measured. Uranium tends to be enriched in thin shale and argillaceous carbonate layers within otherwise carbonate-dominated intervals. Uranium is thus associated with aluminosilicate minerals and is not particularly concentrated in dolomite. Two types of GR peaks are observed. Potassium-thorium-dominated peaks are suggested to indicate relatively major transgressions during which aluminosilicate detritus was derived from sources interior to the Fennoscandian shield. Uranium-dominated peaks correspond with relatively minor transgressions within intervals of cyclic shallow-water carbonate deposits. Uranium-enriched aluminosilicate detritus is suggested to be the product of extended subaerial exposure of the platform, during which U was concentrated by groundwater movement. These results can be useful as a basis for applying spectral GR signature as a tool for stratigraphic interpretation in uncored or incipiently understood carbonate sections.

This article summarizes the lithostratigraphy of two sections through the Kapp Starostin Formation on Spitsbergen having contrasting distal and proximal depositional settings. These outcrop sections are compared with an approximately age-equivalent spiculitic unit penetrated in two exploration wells 800 km to the southeast on the Finnmark Platform. Eight main facies associations comprise these strata, including limestones (matrix-rich, matrix-poor, and mudstone), siliciclastics (shale and sandstone), spiculite (dark/tight and light-colored/porous), and phosphatized surfaces. A sequence stratigraphic model is proposed wherein siliciclastic-limestone intervals represent lowstands of relative sea level, and the spiculites are mainly highstand deposits. Based on this model, seven depositional cycles (sequences) are recognized in the distal, high-accommodation locality on Spitsbergen (Akseloya), four in the proximal locality (Dickson Land), and two on the Finnmark Platform. A speculative model for correlation between these localities is presented, wherein lateral amalgamation of sequences, due to landward pinch-out of some lowstand intervals, results in recognition of fewer sequences in areas of lower accommodation. On both Dickson Land and the Finnmark Platform, an upward change in spiculite lithology from dark/tight to light/porous is shown, and Dickson Land also shows an upward change in siliciclastic lithology from shale to glauconitic sandstone. These changes record a long-term shoaling trend approaching the Permian-Triassic boundary. This model provides a basis for interpreting the basin-scale (Begin page 2062) distribution of lithofacies and predicting the occurrence of porous zones representing potential hydrocarbon reservoirs.