PREDICTION OF OPEN FRACTURES IN THE ASMARI FORMATION USING GEOMETRICAL ANALYSIS: AGHAJARI ANTICLINE,DEZFUL EMBAYMENT,SW IRAN

Reservoir quality in the carbonates of the late Oligocene – early Miocene Asmari Formation at oilfields in SW Iran is enhanced by the presence of a well‐developed fracture network. In anticlinal structures, fracture density is partly controlled by geometrical parameters such as the fold curvature. In this study, a geometrical analysis of the Asmari Formation at the NW‐SE oriented Aghajari Anticline in the Dezful Embayment is presented, and is based on inscribed circle and curvature analyses of the fold. Iso‐curvature and fracture potential maps of the Asmari Formation based on the geometrical analysis are compared to the results of fracture density logs determined from image logs at four widely‐spaced wells, and to dynamic mud loss data. The geometrical analysis demonstrates that in areas of high curvature (such as the SE and NW parts of the SW limb of the Aghajari Anticline and the central part of the NE limb), the fracture density is high. Regions of high curvature (in plan or section view) have the greatest potential to develop open fractures. The predicted fracture density distribution based on the geometrical analysis of the Asmari Formation is in good agreement with actual fracture data from the four wells and with mud loss data from the Aghajari oilfield.     

STRONTIUM ISOTOPE STRATIGRAPHY OF THE ASMARI FORMATION (OLIGOCENE - LOWER MIOCENE), SW IRAN

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 ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr 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.     

FLOW UNIT DISTRIBUTION AND RESERVOIR MODELLING IN CRETACEOUS CARBONATES OF THE SARVAK FORMATION,ABTEYMOUR OILFIELD,DEZFUL EMBAYMENT,SW IRAN

Carbonate sediments within the Mid‐Cretaceous Sarvak Formation form an important reservoir at the Abteymour oilfield in the western Dezful Embayment, SW Iran. The poroperm characteristics of this reservoir were controlled by factors including deposition under tropical climatic conditions and early diagenesis, repeated phases of subaerial exposure due to local, regional and global‐scale tectonism, and diagenetic modification during burial. From microfacies analysis, the Sarvak Formation carbonates in the Abteymour field were interpreted in a previous study as having been deposited on a homoclinal ramp‐type platform. Three third‐order sequences were recognized in the middle Cenomanian to middle Turonian part of the formation. The reservoir quality of the carbonates was enhanced both by dissolution (comprising separate phases of eogenetic and telogenetic meteoric dissolution) and dolomitization (especially stylolite‐related dolomitization). In this paper, a rock/pore type approach was used in order to integrate petrophysical data with facies and diagenetic models within a sequence stratigraphic framework. Two different rock‐typing methods for the determination of flow units were considered. Hydraulic flow units (HFUs) were identified firstly using flow zone indicators and secondly using a stratigraphic modified Lorenz plot. The flow units resulting from these two methods are compared, and their close correspondence within the sequence stratigraphic framework is discussed. In addition, the previously‐used large‐scale reservoir zonation scheme for the Abteymour field is correlated with the defined flow units, and four new Integrated Reservoir Zones are introduced. By integrating geological information with petrophysical parameters (including porosity, permeability and saturation) within a sequence stratigraphic framework, field‐scale variations and controls on reservoir quality are described.     

PALAEO‐EXPOSURE SURFACES IN CENOMANIAN – SANTONIAN CARBONATE RESERVOIRS IN THE DEZFUL EMBAYMENT,SW IRAN

Cretaceous carbonates host major hydrocarbon reserves in SW Iran and elsewhere in the Arabian Plate. Tectonic activity combined with eustatic sea‐level changes resulted in periodic exposure of these carbonates which were subsequently modified by meteoric diagenesis under a warm and humid climate. Long‐term exposure led to the formation of several disconformity surfaces within the middle Cretaceous succession which had important effects on the interval's reservoir characteristics. These disconformity surfaces in the Dezful Embayment were investigated using microfacies, diagenetic and geochemical studies at five subsurface sections. Facies differences across these boundaries, together with features such as karstification, palaeosol development and collapse‐dissolution breccias, were used to identify emergent surfaces. Stable oxygen and carbon isotope ratios and trace element profiles indicate intense meteoric diagenesis. Disconformities were dated using biostratigraphic studies. The results indicate the presence of two major erosional disconformities: one is located at the Cenomanian – Turonian boundary separating the middle Sarvak Formation from its upper part; and the other is in the mid‐Turonian at the boundary between the Sarvak and Ilam Formations. The latter disconformity is correlatable throughout the Arabian Plate.     

HYDROCARBON MIGRATION AND CHARGE HISTORY IN THE KARANJ,PARANJ AND PARSI OILFIELDS,SOUTHERN DEZFUL EMBAYMENT,ZAGROS FOLD-AND-THRUST BELT,SW IRAN

This study investigates the charge history of the Oligocene – Lower Miocene Asmari Formation reservoir at three oilfields (Karanj, Paranj and Parsi) in the southern Dezful Embayment, SW Iran, from microthermometric analyses of hydrocarbon-bearing fluid inclusions. The Asmari Formation reservoir was sampled in seven wells at depths of between 1671.5 and 3248.5 m; samples from three of the wells were found to be suitable for fluid inclusion analyses. The samples were analyzed using an integrated workflow including petrography, fluorescence spectroscopy, Raman microspectroscopy and microthermometry. Abundant oil inclusions with a range of fluorescence colours from near-yellow to near-blue were observed. Based on the fluid inclusion petrography, fluorescence and microthermometry data, two episodes of oil charging into the reservoir were identified: 7 to 3.5 Ma, and 3.5 to 2 Ma, respectively. Fluid inclusions in general homogenized at temperatures between 112 and 398°C and with salinities of 14 to 23 wt.% NaCl equivalent. Based on the burial history, the Albian Kazhdumi and Paleogene Pabdeh Formation source rocks in the study area have not reached the gas generation window. The abundant fluid inclusions containing gas-liquid phase observed in the Asmari samples studied are therefore inferred to have been derived from secondary oil-to-gas cracking which resulted from Late Pliocene uplift.     

DOLOMITIZATION AND RELATED FLUID EVOLUTION IN THE OLIGOCENE – MIOCENE ASMARI FORMATION, GACHSARAN AREA, SW IRAN: PETROGRAPHIC AND ISOTOPIC EVIDENCE

Petrographic and stable isotope investigations of Oligocene‐Miocene carbonates in the Asmari Formation from the Gachsaran oilfield and surrounding area in SW Iran indicate that the carbonates have been subjected to extensive diagenesis including calcite cementation and dolomitization. Diagenetic modification occurred in different diagenetic realms ranging from marine, meteoric and finally burial. Asmari carbonates were in general deposited in a ramp setting and are represented by intertidal to subtidal deposits together with lagoonal, shoal and low‐energy deposits formed below normal wave base. Lithofacies include bioclastic grainstones, ooidal and bioclastic, foraminiferal and intraclastic packstones, and mudstones. Multiple episodes of calcite cementation, dolomitization and fracturing have affected these rocks to varying degrees and control porosity. Four types of dolomites have been identified: microcrystalline matrix replacement dolomite (D1); fine to medium crystalline matrix replacement dolomite (D2); coarse crystalline saddle‐like dolomite cement (D3); and coarse crystalline zoned dolomite cement (D4). Microcrystalline dolomites (D1) (6–12 μm) replacing micrite, allochems and calcite cements in the mud‐supported facies prior to early compaction show δ¹⁸O and δ¹³C values of ?4.01 to +1.02‰ VPDB and ?0.30 to +4.08‰ VPDB, respectively. These values are slightly depleted with respect to postulated Oligocene‐Miocene marine carbonate values, suggesting their precipitation from seawater, partly altered by later fluids. The association of this type of dolomite with primary anhydrite in intertidal facies supports dolomitization by evaporative brines. Fine to medium crystalline matrix dolomites (D2) (20–60μm) occur mostly in grainstone facies and have relatively high porosities. These dolomites formed during early burial and could be considered as recrystallized forms of D1 dolomite. Their isotopic values overlap those of D1 dolomites, implying precipitation from similar early fluids, possibly altered by meteoric fluids. Coarse crystalline saddle‐like dolomites (D3) (200–300 μm) partially or completely occlude fractures and vugs. The vugs developed through the dissolution of carbonate components and rarely matrix carbonates, while fractures developed during Zagros folding in late Oligocene to early Miocene times. A final diagenetic episode is represented by the precipitation of coarse crystalline planar e‐s zoned dolomite (D4) (80–250 μm) that occurs in fractures and vugs and also replaces earlier dolomite and post‐dates stylolitization. Fluids responsible for the formation of D3 and D4 dolomites are affected by brine enrichment and increasing temperatures due to increasing burial. Reservoir porosity is dominated by microcrystalline pore spaces in muddy, dolomitized matrix and mouldic and vuggy porosity in grainstone. Porosity was significantly enhanced by the formation of multiple fracture systems.     

THE INFLUENCE OF TECTONICS ON THE ENTRAPMENT OF OIL IN THE DEZFUL EMBAYMENT, ZAGROS FOLDBELT, IRAN

The timing of the orogenic events associated with the closure of South Tethys significantly influenced the generation, migration and entrapment of petroleum in the Zagros Foldbelt of Iran. This influence was particularly important in the Dezful Embayment, which is one of the world's richest oil provinces, containing some 8% of global oil reserves in an area of only 60,000 sq. km. In the Dezful Embayment, oil and associated gas occur in two carbonate reservoirs ‐ the Sarvak Formation of Cenomanian to Turonian age, and the Oligocene ‐ Early Miocene Asmari Formation, sealed by the evaporites of the Gachsaran Formation. The oil and associated gas are trapped in large “whaleback” anticlines which formed during the Neogene Zagros orogeny. Two excellent source rocks, the Albian Kazhdumi Formation and the upper part of the Pabdeh Formation (Middle Eocene to Early Oligocene), supplied the Asmari and Sarvak reservoirs and with them form the Middle Cretaceous to Early Miocene Petroleum System. This system was found to be independent of older petroleum systems. Two particular problems are addressed in this paper. The first is the relative timing of trap formation versus oil expulsion from the source rocks. If oil expulsion occurred prior to Zagros folding, the oil would have migrated along gently dipping ramps towards the Persian (Arabian) Gulf and Southern Iraq, and would have been trapped a long way from the source kitchen. By contrast, if oil expulsion took place when the whaleback anticlines already existed or had at least begun to develop, the oil generated would have moved almost vertically towards the nearest anticline. Secondly, we assess the type of heatflow to be used for modelling. This could be either variable or constant, depending on the stability or instability of the Arabian Platform and on subsidence variations during source rock maturation. Our conclusions can be summarized as follows. First, the paroxysmal phase of Zagros folding commenced in the Dezful Embayment towards the end of the Middle Miocene around 10 Ma ago and continued throughout the Late Miocene and Pliocene. Second, bearing in mind the remarkable stability of the Arabian Platform for some 260 Ma, during which there was almost continuous gentle subsidence between the Permian transgression and the Early Miocene, a constant heatflow was used for modelling. Burial profiles and maturity indices, such as vitrinite reflectance and Rock‐Eval parameters, demonstrate that the Kazhdumi and Pabdeh source rocks reached the onset of oil expulsion during deposition of the Agha Jari Formation between 8 and 3Ma, depending upon the location. This chronology means that oil migrated from source rocks into preexisting Zagros structures. Therefore, oil migrated over short distances to nearby traps within well‐defined drainage areas, the geometry of which can be deduced from seismic data. Moreover, the Zagros folding induced prominent fracturing which can be observed both at outcrop and in wells. This fracturing, which affects lime‐stones as well as marls, enhanced subvertical migration of hydrocarbons towards the reservoirs. As a result of this short distance migration, oils can directly be linked to the source rocks which generated them by oil‐oil and oil‐source rock correlations based on stable isotope (σ¹³C, σ³⁴S) and biomarker data. Modelling of each drainage area provides estimates of the amount of oil expelled by each source rock. Calculated estimates can then be compared to the actual oil‐in‐place of the corresponding field. An example of this modeling procedure is given in this paper.     

BURIAL HISTORY RECONSTRUCTION USING LATE DIAGENETIC PRODUCTS IN THE EARLY PERMIAN SILICICLASTICS OF THE FARAGHAN FORMATION, SOUTHERN ZAGROS, IRAN

In spite of the increasing importance to hydrocarbon exploration and production of the Palaeozoic succession in the Zagros area of SW Iran, few burial history and palaeothermal modelling studies of the interval have been carried out. This paper attempts to assess the burial and palaeotemperature history of the Lower Permian Faraghan Formation which is composed of stromatolitic dolomites overlain by mainly cross‐bedded sandstones. The formation grades up into the thick bedded carbonates of the Upper Permian Dalan Formation. The Faraghan and Dalan Formations are major hydrocarbon reservoir units in SW Iran and are time‐equivalents of the Unayzah and Khuff Formations in Saudi Arabia, respectively. The Faraghan Formation consists of shallow‐marine siliciclastics and foreshore deposits, including tidal‐flat and tidal‐channel, estuarine, sabkha, shoreface and offshore facies. In this study, diagenetic constituents are used to evaluate the formation's burial history in the Southern Zagros, an area for which only limited subsurface data is available. A burial history diagram for the formation was constructed for well Finu # 1 using WinBury^TM software. The diagram shows that the formation underwent progressive burial at variable rates between its deposition and the mid‐Tertiary, since when it has undergone rapid uplift. Burial diagenetic products in the Faraghan Formation comprise saddle, ferroan and zoned dolomites, together with dickite, illite/sericite and chlorite minerals. Additional burial‐related features include stylolites and dissolution seams. Isotopic signatures (δ¹⁸O versus δ¹³C) of the ferroan dolomites suggest a burial trend for the formation. Reconstruction of the paragenetic sequence together with the burial history diagram suggests a maximum burial depth of about 5000 m and a wide palaeotemperature range of 80‐160°C. However considering the saddle dolomites as a palaeothermometer, the temperature range narrows to 78 to 138 °C. The burial depth and temperature ranges closely correlate with the main stage of oil generation to the dry gas zone.     

AN AEOLIANITE IN THE UPPER DALAN MEMBER (KHUFF FORMATION), SOUTH PARS FIELD,IRAN

A laterally continuous, 3m thick oolitic grainstone has been studied in cores from two wells from the South Pars field (offshore Iran). This high porosity but low permeability interval occurs at the top of the gas-bearing succession in the Permian Upper Dalan Member, and is equivalent to the informally-defined K4 unit of the Khuff Formation. This interval can easily be traced between the wells and overlies high-energy marine deposits. It is composed of oomouldic, fine-grained azooic grainstones with cm-thick coarser-grained layers. Horizontal to oblique lamination or steep foresets were observed together with pinstripe lamination. Petrographic observations indicate a clean oomouldic grainstone with very thin chitonic rims associated with pedogenetic imprints as first-generation cements. Later cements include early vadose meniscus and pendant cements in coarser-grained layers and pseudophreatic cements in the finer-grained material with a tighter pore network, prior to ooid dissolution. Rhizoliths were observed in cores and thin-sections. The pedogenic imprints and the early vadose cementation, both related to emergence, as well as the presence of pinstripe lamination, suggest an aeolian depositional setting. This interval is the first aeolianite recorded within the Khuff Formation or equivalent units, and the first hydrocarbon-bearing carbonate aeolianite described in a hydrocarbon-producing unit. The discovery of aeolianites has important implications for regional sequence-stratigraphic interpretations and reservoir volume calculations. These deposits do not conform to classic subaqueous sequence stratigraphy and do not record eustatic variations in the associated marine basin. Their recognition is crucial for well-to-well correlations.     

BURIAL,TEMPERATURE AND MATURATION HISTORY OF CRETACEOUS SOURCE ROCKS IN THE NW PERSIAN GULF,OFFSHORE SW IRAN: 3D BASIN MODELLING

This study presents a 3D numerical model of a study area in the NW part of the Persian Gulf, offshore SW Iran. The purpose is to investigate the burial and thermal history of the region from the Cretaceous to the present day, and to investigate the location of hydrocarbon generating kitchens and the relative timing of hydrocarbon generation/migration versus trap formation. The study area covers about 20,000 km² and incorporates part of the intra‐shelf Garau‐Gotnia Basin and the adjacent Surmeh‐Hith carbonate platform. A conceptual model was developed based on the interpretation of 2700 km of 2D seismic lines, and depth and thickness maps were created tied to data from 20 wells. The thermal model was calibrated using bottom‐hole temperature and vitrinite reflectance data from ten wells, taking into account the main phases of erosion/non‐deposition and the variable temporal and spatial heat flow histories. Estimates of eroded thicknesses and the determination of heat‐flow values were performed by burial and thermal history reconstruction at various well and pseudo‐well locations. Burial, temperature and maturation histories are presented for four of these locations. Detailed modelling results for Neocomian and Albian source rock successions are provided for six locations in the intra‐shelf basin and the adjacent carbonate platform. Changes in sediment supply and depocentre migration through time were analyzed based on isopach maps representing four stratigraphic intervals between the Tithonian and the Recent. Backstripping at various locations indicates variable tectonic subsidence and emergence at different time periods. The modelling results suggest that the convergence between the Eurasian and Arabian Plates which resulted in the Zagros orogeny has significantly influenced the burial and thermal evolution of the region. Burial depths are greatest in the study area in the Binak Trough and Northern Depression. These depocentres host the main kitchen areas for hydrocarbon generation, and the organic‐rich Neocomian and Albian source rock successions have been buried sufficiently deeply to be thermally mature. Early oil window maturities for these successions were reached between the Late Cretaceous (90 Ma) and the early Miocene (18 Ma) at different locations, and hydrocarbon generation may continue at the present‐day.     

PALAEO‐EXPOSURE SURFACES IN THE APTIAN DARIYAN FORMATION,OFFSHORE SW IRAN: GEOCHEMISTRY AND RESERVOIR IMPLICATIONS

Palaeo‐exposure surfaces within and at the top of the carbonate‐dominated Aptian Dariyan Formation have been poorly studied in the Iranian sector of the Persian Gulf. This paper presents an integrated sedimentological and geochemical study of the Dariyan Formation at four oil and gas fields located in the western, central and eastern parts of the Gulf. Facies stacking patterns in general indicate shallowing‐upwards trends toward the exposure surfaces, which are interpreted to correspond to unconformities. The Dariyan Formation in the study area is divided into upper and lower carbonate units by a deep‐water, high‐gamma shale‐marl interval. At fields in the western and central Gulf, significant diagenetic changes were recorded in the top of the upper carbonate unit, including meteoric dissolution and cementation, brecciation and paleosol formation. An exposure surface is also present at the top of the lower carbonate unit in all the fields in the study area, and is associated with meteoric dissolution and cementation of grain‐dominated facies. Age calibration of studied intervals was carried out using microfossil assemblages including benthic and planktonic foraminifera. Negative excursions of both δ¹⁸O (?10‰ VPDB) and δ¹³C (?0.66‰ VPDB) were recorded in weathered intervals located below the unconformity surfaces. A sequence stratigraphic framework for the Dariyan Formation was established by integrating sedimentological, palaeontological and geochemical data. The δ¹³C curve for the formation in the Iranian sector of the Persian Gulf can be correlated with the reference curve for the northern Neotethys and used as a basis for regional stratigraphic correlation. Where the top‐Aptian unconformity is present, it has resulted in an enhancement of the reservoir characteristics of the underlying carbonate succession. Accordingly, the best reservoir zones in the Dariyan Formation occur in the upper parts of the lower and upper carbonate units which are bounded above by significant palaeo‐exposure surfaces.     

鄂尔多斯盆地西南缘奥陶系马家沟组白云岩成因研究

基于岩石薄片的系统鉴定,将钻井岩心和地表露头样品相结合,运用多种分析、测试手段对鄂尔多斯盆地西南缘奥陶系马家沟组白云岩的成因进行深入研究,认为该组白云岩主要由准同生白云岩和埋藏白云岩组成。准同生白云岩主要发育于马家沟组一、三、五段,受古地理环境限制明显,由蒸发泵白云石化形成;埋藏白云岩厚度大、分布广、岩石类型多,构成马家沟组白云岩的主体,成因模式包括浅埋藏期的回流白云石化以及中—深埋藏期的压实水流白云石化和地形驱动水流白云石化。热液白云石在盆地西缘的定边地区、南缘的富县、黄陵、麟游均有发现,通常呈脉状充填裂缝,其成因与深部热流体沿断裂系统上涌有关,而且对储层的发育具有指示意义。在上述成因模式研究的基础上,对各类白云石的结构演化进行了讨论。     

CARBONATE RESERVOIR ROCKS AT GIANT OIL AND GAS FIELDS IN SW IRAN AND THE ADJACENT OFFSHORE: A REVIEW OF STRATIGRAPHIC OCCURRENCE AND PORO‐PERM CHARACTERISTICS

SW Iran and the adjacent offshore are prolific petroleum‐producing areas with very large proven oil and gas reserves and the potential for significant new discoveries. Most of the oil and gas so far discovered is present in carbonate reservoir rocks in the Dehram, Khami and Bangestan Groups and the Asmari Formation, with smaller volumes in the Dashtak, Neyriz, Najmeh, Gurpi, Pabdeh, Jahrum, Shahbazan, Razak and Mishan (Guri Member) Formations. The Permo‐Triassic Dehram Group carbonates produce non‐associated gas and condensate in Fars Province and the nearby offshore. The Jurassic – Lower Cretaceous Khami Group carbonates are an important producing reservoir at a number of offshore fields and in the southern Dezful Embayment, and are prospective for future exploration. Much of Iran's crude oil is produced from the Oligo‐Miocene Asmari Formation and the mid‐Cretaceous Sarvak Formation of the Bangestan Group in the Dezful Embayment. This review paper is based on data from 115 reservoir units at 60 oil‐ and gasfields in SW Iran and the adjacent offshore. It demonstrates that the main carbonate reservoir units vary from one‐another significantly, depending on the particular sedimentary and diagenetic history. Ooidal‐grainstones and rudist‐ and Lithocodium‐bearing carbonate facies form the most important reservoir facies, and producing units are commonly dolomitised, karstified and fractured. In general, reservoir rocks in the study area can be classified into six major types: grainstones; reefal carbonates; karstified, dolomitised and fractured carbonates; and sandstones. The stratigraphic distribution of these reservoir rocks was principally controlled by the palaeoclimatic conditions existing at the time of deposition. A comparative reservoir analysis based on core data shows that dolomitised and/or fractured, grain‐dominated carbonates in the Dehram Group, Lower Khami Group and Asmari Formation typically have better reservoir qualities than the Cretaceous limestones in the Upper Khami and Bangestan Groups.     

A REVIEW OF PERMO‐TRIASSIC RESERVOIR ROCKS IN THE ZAGROS AREA,SW IRAN: INFLUENCE OF THE QATAR‐FARS ARCH

Four “supergiant” and numerous giant gasfields have been discovered in the Zagros area of SW Iran. The gasfields are concentrated in the eastern part of the foldbelt, in Fars Province and the adjacent offshore, and produce from Permo‐Triassic carbonates equivalent to the Khuff Formation. The carbonates belong to the upper member of the Dalan Formation and the overlying Kangan Formation. Reservoir rock quality is strongly influenced by tectonic setting and depositional environment, and also by diagenesis. The highest quality reservoirs occur in oolitic shoal facies; fracturing (especially in onshore fields) and dolomitisation (in offshore fields) have also influenced reservoir quality. Anhydrite plugging is common in reservoirs in offshore fields, while calcite cementation is dominant in onshore reservoirs. Facies variations in the Dalan‐Kangan Formations appear to correspond to syndepositional palaeohighs and depocentres. In the Eastern Zagros (Fars area), thickening of the Dalan Formation corresponds to a Mid‐Late Permian depocentre referred to here as the Permian Fars Basin. As a result of sea level fall, this depocentre evolved into a hypersaline lagoon with evaporite deposition (Nar Member). In the Triassic, the depocentre evolved into a palaeohigh as indicated by thinning and facies changes in the Kangan Formation. The results of this study draw attention to variations in the reservoir quality of the Dalan‐Kangan Formations. Much of this variation was due to the influence of the Qatar‐Fars Arch.     

HYDROCARBON-INDUCED DIAGENETIC DOLOMITE AND PYRITE FORMATION ASSOCIATED WITH THE HORMOZ ISLAND SALT DOME,OFFSHORE IRAN

Hormoz Island, a salt diapir in the eastern Persian Gulf, is dominated by the Infracambrian Hormoz Complex comprising both evaporites (Hormoz Salt) and carbonates, siliciclastics and volcanic rocks. Minerals include black, white and grey dolomites, pyrite, gypsum, anhydrite, apatite and iron oxides. Formation of some of the dolomite crystals is interpreted to be linked to the oxidation of hydrocarbons. The δ¹³C values of black dolomite crystals range from −0.8 to −2.07‰ VPDB, indicating that little if any of their carbon is derived from hydrocarbon oxidation but that sea water has provided carbon and Mg for dolomite precipitation. The δ¹⁸O values for these dolomites range from −9.2 to −15.3‰ VPDB, reflecting a temperature effect on isotopic fractionation. By contrast, δ¹³C values for white to grey dolomites range from −17.81 to −35.68‰ VPDB, indicating that the carbon may be derived from the oxidation of hydrocarbons. Based on the δ¹⁸O_dolomite and temperatures obtained from fluid inclusion studies (215°C), the calculated δ¹⁸O_water in equilibrium with these dolomites (+2 < δ¹⁸O_fluid < +12‰) indicates the involvement of brines evolved via the interaction of seawater with the Hormoz Salt and associated sedimentary rocks. Some of the dolomite may have precipitated from post-Cambrian seawaters at lower temperatures (ca.100 °C). Thus, the dolomites may have begun to form during deep burial but have also formed during salt diapirism at more shallow depths. Pyrite and native sulphur are interpreted to have formed in reducing conditions where the source of sulphur was H₂S produced by the thermochemical reduction of sulphate in the Hormoz Salt evaporites. Heavy δ³⁴S values for the anhydrites (ranging from 28.7 to 30.8‰) and for sulphides (ranging from 17.2 to 23.4‰) preclude a major contribution of sulphur from volcanic sources or from Early Cambrian shales. Pyrites, apatites and dolomites formed at depth within the diapir. It is envisaged that hydrocarbons leaked along the flanks of the Hormoz Island salt dome, resulting in reducing conditions which promoted the formation of diagenetic minerals.     

FLOW UNIT CLASSIFICATION FOR GEOLOGICAL MODELLING OF A HETEROGENEOUS CARBONATE RESERVOIR: CRETACEOUS SARVAK FORMATION, DEHLURAN FIELD, SW IRAN

Flow unit classifications can be used in reservoir characterization and modelling of heterogeneous carbonate reservoirs where there is uncertainty and variability in the distribution of porosity and permeability. A flow unit classification requires the integration of geological and petrophysical data, together with reservoir engineering and production data. In this study, cores and thin sections from the upper part of the Cretaceous Sarvak Formation at the Dehluran field, SW Iran, were studied to identify flow units which were then used in reservoir modelling. Eight flow units were defined based on a classification of depositional environments and diagenetic processes and an evaluation of porosity and permeability. In lagoonal deposits, two flow units were distinguished in terms of dissolution effects (i.e. low or high values of vuggy porosity). In shoal/reef deposits, three flow units were distinguished in terms of cementation volumes and grain frequency. In open‐marine deposits, two flow units were identified with different degrees of dissolution; while intrashelf basinal deposits were characterized by a single flow unit with no observable reservoir potential. Each flow unit was characterized by unique values of porosity, permeability, water saturation and pore throat distribution. Grain‐supported deposits from high energy depositional environments (shoals) had the highest porosities and permeabilities. However, these rocks were frequently cemented with a consequent reduction in porosity and permeability. By contrast, low permeability mud‐supported deposits had undergone dissolution, forming highly permeable flow units. Capillary pressure curves from mercury injection were used to determine the distribution of pore throat sizes and the pore characteristics of the flow units, and were used to give an indication of the productivity of each flow unit. Flow units were modelled using a pixel‐based modelling tool. Modelled reservoir characteristics were mainly controlled by facies changes in the vertical direction, and by diagenetic variations in the horizontal direction. Input values for the geometry of the flow units were based on information from geological and diagenetic models of the reservoir, and from thickness maps of the flow units derived from well data.     

CHEMOSTRATIGRAPHY OF CENOMANIAN–TURONIAN CARBONATES OF THE SARVAK FORMATION,SOUTHERN IRAN

Stable‐isotope and trace‐element analyses from five surface and subsurface sections of the mid‐Cretaceous Sarvak Formation in southern and offshore Iran confirm the presence of the regional‐scale Turonian unconformity and of a more local Cenomanian–Turonian unconformity. The geochemical results indicate the presence of previously unrecognized and/or undifferentiated subaerial exposure surfaces. Sarvak Formation carbonates at or near palaeo‐exposure surfaces show varying degrees of diagenetic modification, and more extensive alteration is associated with longer periods of exposure. The subaerial exposure and associated diagenetic processes greatly influenced reservoir quality and amplified karstification and evolution of porosity in the Upper Sarvak Formation. The palaeo‐exposure surfaces are identified by their negative δ¹³C values (as low as – 6.4%) and negative δ¹⁸O values (as low as –9.4%), together with low Sr concentrations and relatively high ⁸⁷Sr/⁸⁶Sr ratios. These geochemical characteristics are interpreted to be the result of the interaction of the Sarvak Formation carbonates with meteoric waters charged with atmospheric CO₂. The meteoric waters also caused karstification and soil formation which in some places extends a few metres below the exposure surfaces. Depleted carbon values were not recorded in areas where palaeosols are not well developed or where the uppermost layers of the Sarvak Formation have been removed by erosion.     

伊朗卡山地区库姆组储层特征及控制因素分析

渐新统-中新统库姆组海相碳酸盐岩是伊朗卡山地区目前最有利的一套储层,且已获得了高产工业油气流。通过钻井、测井、地质、地球物理等方法,对这套储层进行了系统的研究和评价,结果发现,库姆组储层以台地相生物灰岩为主,有效储集空间为溶孔和裂缝,孔隙类型是裂缝＋孔隙型,具有双重介质特征。利用地震属性分析、波阻抗反演等方法,对库姆组有利储层的分布进行了平面上的预测,发现有利储层主要分布在断裂、构造发育部位和褶皱的转折端,但平面上和垂向上都具有强非均质性,认为沉积环境、成岩作用和构造活动是控制该区储层发育的3个主要因素。     

DEPOSITIONAL HISTORY AND SEQUENCE STRATIGRAPHY OF OUTCROPPING TERTIARY CARBONATES IN THE JAHRUM AND ASMARI FORMATIONS, SHIRAZ AREA (SW IRAN)

The Oligo-Miocene Asmari Formation is one of the most important petroleum reservoir units in the Zagros Basin of south and SW Iran. It mainly consists of limestones and dolomitic limestones with interbedded shales, together with a few intervals of sandstone and gypsum assigned to the Ahwaz and Kalhur Members, respectively. The Asmari Formation rests on the thin-bedded limestones of the Jahrum Formation (Paleocene-Eocene). In this paper, we report on the lithofacies characteristics of these two formations using data from three measured outcrop sections near Shiraz in SW Iran. From field and petrographic data, we have identified four major lithofacies and twelve subfacies which are interpreted to have been deposited in open-marine, shoal, lagoon and tidal flat settings.
We show that the Asmari and Jahrum Formations constitute two separate depositional sequences which are separated by a thin palaeosol, representing a type-one sequence boundary which can be correlated with global curves of relative sea-level. Each depositional sequence is composed of many metre-scale shallowing-upward parasequences. This is the first time that the Asmari and Jahrum Formations have been differentiated in the study area. We hope that this study will lead to a better understanding of the Asmari Formation in the subsurface in other parts of the Zagros Basin.