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.     

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.     

A THIRD‐ORDER UNCONFORMITY WITHIN LOWER ORDOVICIAN CARBONATES IN THE TARIM BASIN,NW CHINA: IMPLICATIONS FOR RESERVOIR DEVELOPMENT

This paper presents outcrop, petrographic, geochemical, well log and seismic data which together characterise the third‐order T₇⁸ unconformity located between the carbonate‐dominated Lower Ordovician Penglaiba and Yingshan Formations in the Tarim Basin, NW China. Unconformities in Lower Palaeozoic carbonates in this basin are of increasing interest because major reserves of hydrocarbons have recently been discovered at the North Slope field (> 1000 × 10⁶ brls oil and ～ 3050× 10⁸ m³ gas). The reservoir here consists of karstified Lower Ordovician carbonates bounded by a third‐order unconformity. The T₇⁸ unconformity in Tarim Basin represents a short‐term exposure surface (< 1 Ma) controlled both by sea‐level changes and by palaeogeographic location within the basin, and the intensity of karstification varies laterally. The unconformity has had a major influence on porosity development in the underlying Penglaiba Formation carbonates. At two measured outcrop sections at the NW basin margin (Penglaiba and Shuinichang), dissolution porosity was observed in karstified and dolomitised carbonates below the T₇⁸ unconformity surface. A seismic profile shows the presence of reflection anomalies below the unconformity which are interpreted as karst‐related palaeo‐caverns. Geochemical data indicate that the T₇⁸ unconformity is associated with anomalies in stable isotope ratios and in heavy mineral and trace element profiles. Thus there are negative excursions in δ¹³C and δ¹⁸O ratios within the carbonate rocks immediately below the unconformity surface. Similarly, concentrations of major and trace elements such as Li, K, Ti, Rb, Th, Sr, V and Ni are significantly reduced in the underlying carbonates, while there is an anomalously high content of haematite‐limonite.     

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.     

CHARACTERIZATION OF THE LATE APTIAN TOP‐DARIYAN DISCONFORMITY SURFACE OFFSHORE SW IRAN: A MULTI‐PROXY APPROACH

The late Aptian disconformity at the top of the carbonate‐dominated Dariyan Formation is a regional‐scale surface recognised throughout SW Iran and adjacent areas to the south. This study investigates the disconformity using wireline logs, petrographic analyses, SEM observations, and CL and stable isotope data from an oilfield offshore SW Iran. The top‐Dariyan surface in the studied field is characterised by erosional incisions and clastic‐filled fissures which, together with secondary porosity in the underlying carbonates, uranium enrichment and meteoric infill cements, are interpreted as subaerial exposure‐related features. The clastic‐filled fissures were identified from gamma‐ray logs in horizontal wells, and infill material is composed of argillaceous carbonates, clays, quartz and iron‐bearing minerals. The correlation of GR logs in 19 vertical wells resulted in the recognition of erosional incisions (up to about 16 m deep) on the top‐Dariyan surface, together with thickening of siliciclastic sediments in the overlying basal part of the Kazhdumi Formation. Back‐filled sediments in erosional incisions may form potential exploration targets. Secondary porosity in the uppermost 20 m of the Dariyan Formation was created by meteoric processes during subaerial exposure. Cathodoluminescence microscopy of diagenetic cements, carbon and oxygen isotope data and elemental analyses were used to reconstruct the diagenetic history of the carbonates in the Upper Dariyan interval.     

Original Mineralogy and Recognition of Upper Boundary of the Sarvak Formation Based on Geochemistry and Isotope Studies

Carbonate sequence of upper cretaceous (Cenomanian) of Sarvak Formation is a part of Bangestan Group with the thickness of 760 m in Ahvaz oil field (well no. 63). This formation is overlain by the Kazhdumi Formation and is uncomfortably underlain by the Ilam Formation. In this study major and minor elements and carbon and oxygen isotope values and bivariate plot of them indicate that aragonite was the original carbonate mineralogy of the Sarvak Formation. Variations of Sr/Ca and δ¹⁸O values versus Mn also illustrate that they were affected by nonmarine diagenesis in a nearly closed system. Recognition of the exact boundary between the Sarvak and Ilam Formations is difficult, due to similar lithologies. So in this study, elemental and oxygen and carbon isotope analysis were used to determine the exact boundary between these Formations. Geochemical data clearly indicate a sub aerial exposure surface, below which meteoric diagenesis effected the sediments.     

RESERVOIR PROPERTIES OF BARREMIAN–APTIAN URGONIAN LIMESTONES,SE FRANCE,PART 2: INFLUENCE OF DIAGENESIS AND FRACTURING

Integrated sedimentological, diagenetic and structural analyses have been carried out on microporous and tight Urgonian (Barremian – Aptian) limestones in a study area in SE France in order to understand the influence of diagenetic changes and structural deformation on the spatial distribution of reservoir properties. A diagenetic history for the carbonates was established and was divided into phases which correspond to episodes of regional geodynamic activity. Petrographic (optical, SEM and cathodoluminescence microscopy), structural and geochemical (δ¹⁸O, δ¹³C) studies were carried out to characterize the cement phases in the carbonates, especially micrite and blocky calcite, and to investigate their relationship with episodes of fracturing. Eleven calcite cement phases and four micritic cement phases were identified in relation to the two main phases of structural deformation which affected the Urgonian limestones. A first phase of micrite cementation occurred early in the diagenetic history and was linked to early marine cementation at the tops and bases of depositional cycles during the Barremian. A major phase of micrite recrystallization, which generated microporosity in carbonates that had previously been preserved from early cementation, was followed by a first phase of blocky calcite which occluded intergranular pore spaces. The blocky cement formed in a shallow burial meteoric environment and contributed to the preservation of microporosity during late Durancian tectonism (Albian – Cenomanian). A second phase of blocky calcite is associated with fracture activation during latest Eocene (Priabonian) – Oligo‐Miocene extension. Reservoir rock‐types (RRTs) proposed in a previous study were consistent with the diagenetic characteristics and the results of δ¹³C / δ¹⁸O analyses. Microporous RRTs formed as a result of early to late shallow burial processes and display low δ¹³C values; whereas cemented RRTs developed both due to early marine cementation (with high δ¹³C values) and/or as a result of cementation related to fluid flow linked to the reactivation of faults and fractures. This suggests that some late diagenetic and microstructural processes were pre‐determined by early diagenetic changes in the carbonates. The resulting stratigraphic architecture consists of a vertical stacking of weakly fractured microporous limestone intervals alternating with highly fractured, cemented limestone units.     

IMPACT OF DIAGENESIS ON RESERVOIR QUALITY IN RAMP CARBONATES: GIALO FORMATION (MIDDLE EOCENE), SIRT BASIN,LIBYA

The reservoir quality of Middle Eocene carbonates in the intracratonic Sirt Basin is strongly influenced by depositional facies and various diagenetic processes. Based on data from core samples and well logs from five boreholes in the Assumood and Sahl gasfields in the subsurface of the north‐central Sirt Basin, six major carbonate facies (and fourteen microfacies) are distinguished in the Middle Eocene Gialo Formation (thickness ～1100 ft/335 m). Wackestones‐packstones dominate the Gialo Formation with abundant grains of larger benthic foraminifera, especially nummulites, in a matrix composed mostly of finely comminuted bioclastic material. Sediments were deposited as a mosaic of facies on a broad carbonate ramp in moderate‐energy nummulite‐packstone banks, with locally restricted back‐bank lagoons and fore‐bank areas of foraminiferal debris passing to offshore lime mudstones. Marine diagenesis was minor with micritization of bioclasts and rare vadose marine fibrous cements. Sparry calcite cements are ubiquitous and were precipitated during shallow to moderate burial from seawater and/or meteoric water, the latter largely unaffected by surface‐pedogenic processes. This is indicated by the fabrics and pre‐ and post‐compaction precipitation of drusy spar and echinoderm overgrowths, and supported by their bright‐to‐dull luminescence and low negative δ¹⁸O signatures (‐2.68 to ‐4.16%° PDB). Some early neomorphic alteration of calcitic bioclasts is suggested by bright CL and marine to low negative δ¹⁸O values (‐1.06 to –3.93%° PDB). Bioclastic grains have δ¹³C values ranging between 0.76 and 1.19%° PDB, interpreted as marine signatures. Similar low positive δ¹³C values of the cements (0.9 to 1.05 %°) indicate a source of carbonate from dissolution of grains and/or seawater/meteoric water, but without any near‐surface/soil effects. Dissolution of grains and matrix, notably originally aragonitic grains but also the originally high‐Mg calcite nummulites, was a major porosity‐enhancing process, and took place from shallow to moderate burial depths. Fractures may locally have increased porosity too. Shallow‐water packstones/rudstones containing both primary intergranular and secondary biomouldic porosity have the best reservoir quality and these are concentrated in the upper parts (top ～100 ft/30 m) of the wells, with porosity‐permeability decreasing downwards. This trend relates to the broad, large‐scale facies pattern of more mud‐dominated facies giving way upwards to more grainy, nummulite‐dominated facies as a result of an overall shallowing of the depositional environment. In addition, the influx of meteoric waters in the upper part of the Gialo platform, before drowning and deposition of the overlying Augila Shale, increased porosity through dissolution. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Gialo carbonates.     

Geochemical characteristics of the Permian Changxing Formation reef dolomites, northeastern Sichuan Basin, China

The recent discovery of deep and ultra-deep gas reservoirs in the Permian Changxing Formation reefs, northeastern Sichuan Basin is a significant development in marine carbonate oil & gas exploration in China. Reef dolomites and their origins have been major research topics for sedimentologists and oil & gas geologists. The petrography, trace element and isotope geochemistry of the reef dolomites indicated that the dolomites are characterized by low Sr and Mn contents, relatively low Fe contents, very similar δ 13 C and δ 18 O values and very different 87 Sr/ 86 Sr ratios. Although the calculated results of the fluid mixing suggested that a mixture with 85%-95% meteoric water and 5%-15% seawater seemed to be the dolomitizing fluids of the reef dolomites, the low Mn contents, relatively low Fe contents, high δ 13 C values and high homogenization temperatures of the dolomites did not support that there were large proportions of meteoric water in the dolomitization process, and the 87 Sr/ 86 Sr ratios which were close to coeval seawater also did not support the possibility of the mixture of deep-burial circulated fluids from clastic rocks. High temperature deep-burial circulated seawater with low Mn and Fe contents, high Sr content and high δ 13 C values from the dissolution of widely distributed Triassic evaporites during the burial diagenetic processes (including dehydration of water-bearing evaporites) could have been the dolomitizing fluids of the reef dolomites.     

THE MESOZOIC SEQUENCE IN SOUTH-WEST IRAN AND ADJACENT AREAS

New outcrop and subsurface data from SW Iran have permitted a review of the stratigraphy of the area to the SW of the Zagros Crush Zone and a comparison with neighbouing areas. The Triassic sequence consists mainly of an evaporite and dolomite sequence in the coastal areas of the Persian Gulf which is the extension of the evaporite basin of Saudi Arabia and Iraq. Towards the high Zagros in the northeast, the evaporites are replaced by dolomites. Two unconformities are found at the base and top of the Triassic. The Jurassic in Fars and eastern Khuzestan consists of an argillaceous interval representing early Liassic time, overlain by a thick development of neritic carbonates of early to late Jurassic age. An evaporite unit developed in the upper Jurassic is present in coastal/subcoastal areas of Fars and eastern Khuzestan and is the north-eastwards extension of Hith Anhydrite of Saudi Arabia. The end of the Jurassic was marked by uplift and erosion, giving rise to an unconformity over a large area. In western Khuzestan and Lurestan, the Lower Jurassic is a sequence of alternating evaporites and dolomites. The Middle Jurassic is represented by deeper water bituminous shales and argillaceous limestones of the Sargelu Formation, which is cut by a regional unconformity in this area. The Upper Jurassic is represented by the evaporites of the Gotnia Formation which is terminated by the possible Upper Jurassic unconformity. The Jurassic sequence of this area can be correlated well with that of eastern Iraq. In the high Zagros area to the south of the Crush Zone, the Jurassic consists of a thick development of shelf carbonates with no evaporites. The Cretaceous System in SW Iran is divided into Lower (Neocomian-Aptian), Middle (Albian-Turonian) and Upper (Coniacian-Maastrichtian). The Lower Cretaceous is mainly made up of two shelf carbonate unit separated by shales in Fars and eastern Khuzestan. Towards Lurestan, the carbonates pass into deeper water black shales and limestones with radiolaria. The top of the Lower Cretaceous is marked by a regional unconformity in Fars and the Persian Gulf area. The Middle Cretaceous began with a transgression forming the shales and limestones of the Kazhdumi Formation which was followed by a shallowing of the sea and the deposition of Cenomanian and Turonian shelf carbonates over the entire area of Fars and Khuzestan. The Lurestan basin retreated northwards and northwestwards and covered only central Lurestan during Albian- Turonian time, with the deposition of dark grey to black shales and pelagic limestones of the Garau and the Oligostegina bearing limestones of the Sarvak Formation. At least two pronounced regional unconformities have been recognized, between the Cenomanian and Turonian and between the Turonian and Coniacian. The Upper Cretaceous is represented by limestones at the base and a transgressive shale unit at the top, which is terminated by a regional unconformity at the Cretaceous/Tertiary boundary. Isopach and lithofacies maps of various units and correlations of outcrop and subsurface sections indicate several important unconformities and facies changes in SW Iran during the course of the Mesozoic. The general stratigraphy of the region shows similarities to the Mesozoic sequence of Iraq and Saudi Arabia, with a gradual facies change from carbonates to sandstone towards Saudi Arabia. This change is most evident in the Upper Triassic and in the Barremian-Cenomanian. The Upper Cretaceous sequence of SW Iran changes from mainly argillaceous sediments of deeper marine environment into carbonates of shallow water origin towards Saudi Arabia. The correlation of the Mesozoic sequence of SW Iran with those to the northeast of the Zagros Crush Zone indicates a rather abrupt change from the Upper Triassic onwards.     

THE ORIGIN OF DOLOMITE IN THE ASMARI FORMATION (OLIGOCENE-LOWER MIOCENE), DEZFUL EMBAYMENT, SW IRAN

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 <20μ) replacing mud‐rich facies; and (2) combinations of finely crystalline replacive dolomite and surrounding areas of coarser dolomite cement (crystals up to 100μ) in grain‐supported facies. Fluid inclusion data indicate that finely crystalline dolomites formed at low temperatures (ca. <50°C), while the coarser dolomite formed at higher temperatures (50–;140°C). Whole rock‐carbonate oxygen and carbon isotope analyses of pure dolostone samples show no apparent correlation with either depositional or diagenetic textures: δ¹⁸O is generally 0 to 2.7‰ PDB, and δ¹³C is ?1 to 4‰ PDB. The importance of evaporated seawater to Asmari dolomitisation is indicated by the ubiquitous occurrence of felty‐textured anhydrite nodules in dolostone beds and the presence of high‐salinity fluid inclusions in dolomite. The derivation of dolomitising fluids from contemporaneous seawater is supported by the general correspondence between age estimates derived from the strontium isotope composition of anhydrites and dolomites and those derived from stratigraphic considerations. This suggested synsedimentary dolomitisation. Dolomitisation of the upper half of the Asmari Formation may have occurred as a result of two syn‐sedimentary mechanisms: (1) by the reflux of evaporative brines concentrated in shallow lagoons or sabkhas, through immediately underlying strata (mainly during highstands); and (2) by the flushing of platform‐top carbonates by basinal evaporated waters during lowstand/early transgression. Continued dolomitisation during deeper burial is supported by the presence of high‐temperature fluid inclusions and iron‐rich crystal rims. Dolomite within the lower part of the Asmari Formation probably mostly formed during burial as a result of compaction of, and fluid exclusion from, the underlying Pabdeh marls and shales.     

塔里木盆地塔河南地区良里塔格组成岩环境及油气成藏期次

塔河南地区上奥陶统良里塔克组主要是一套碳酸盐岩,通过对该组岩心、薄片观察发现,其依次经历了大气渗流、混合带、浅埋藏、表生岩溶、构造后浅埋藏、深埋藏成岩环境的改造.分别研究各环境下的流体包裹体,发现在浅埋藏、深埋藏及成岩晚期有3期油气活动.结合沉积相、泥浆漏失统计、岩溶现象、储层物性、产出油气数据等资料,认为塔河南地区良里塔格组储层以岩溶孔洞为主,具有一定的勘探开发潜能.      

THREE‐DIMENSIONAL SEISMIC GEOMORPHOLOGY OF PALEOKARST IN THE CRETACEOUS MACAÉ GROUP CARBONATES,CAMPOS BASIN,BRAZIL

Three‐dimensional seismic data from an oilfield in the Campos Basin, SE Brazil, was used to characterize a karstified unconformity surface at the top of the Albian – Cenomanian Macaé Group. Macaé Group carbonates underwent intense karstification associated with subaerial exposure during a period of some 10 to 15 Ma resulting in the development of canyons, valleys, sinkholes and cave systems. The carbonates host commercial oil accumulations at a number of important oil fields. Understanding how the karstification process has affected the carbonates' reservoir properties is essential for future exploration, not only to improve recovery rates but also to avoid drilling‐related issues such as thief zones. In this context, this study aims to characterize the top‐Macaé Group paleokarst system by investigating the morphology of the associated reflectors recorded on seismic data, together with endokarst features in the underlying carbonate succession such as cave systems and collapse structures. The top‐Macaé Group seismic horizon can be divided into two principal geomorphological domains: highlands, characterized by abrupt relief with well‐developed erosional features; and lowlands, marked by a smoother topography. Collapse sinkholes occur in both domains and take the form of closed circular depressions. The study of endokarst features from an analysis of amplitude anomalies (bright spots) indicates the presence of heterogeneous cave systems. The interpreted data contributes to a better understanding of the spatial distribution of the paleokarst system in the Macaé Group carbonates and may assist with future hydrocarbon exploration in the Campos Basin.     

Classical observations and stable isotopes for identification the diagenesis of Jeribe formation at Jambour oil Fields-Kurdistan Region-Iraq

Jeribe Formation in two subsurface sections were studied using samples from two wells of Jambour oil field, Kirkuk area-northern Iraq to determine its petrographical characteristics, microfacies, depositional environment, and diagenetic signatures based on δ¹⁸O and δ¹³C isotope analysis. Three main types of associated microfacies were detected: mudstone, wackestone, and packestone/grainstone. The inspected formation experienced various diagenetic processes. The well-preserved dolomitization and pressure-solution and stylolitic microstructures bring a new insight in form of the paragenetic sequence. The isotopic signature data revealed that the positive covariance of ¹⁸O and ¹³C isotopes is closely associated with infiltration of meteoric water during the mesogenesis.     

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.     

鄂尔多斯盆地三叠系油砂中芴酮系列化合物成因

从鄂尔多斯盆地三叠系延长组6个油砂样品的抽提物中,通过色谱-质谱(GC-MS)分析,首次检测出一系列C_0-5芴-9-酮化合物和苯并芴-9-酮化合物,它们是芴和苯并芴在储集层中发生氧化-还原反应的产物。这个过程是由于在晚成岩A期,陆表暴露时期,伴随着含氧和细菌的大气淡水的渗入,在延长组储集层中的铝硅酸盐矿物参与下,油砂中的烷基芴和苯并芴在碱性成岩环境下氧化的结果。      

DIAGENETIC HISTORY OF LATE PALAEOCENE POTENTIAL CARBONATE RESERVOIR ROCKS, KOPET-DAGH BASIN, NE IRAN

The Upper Palaeocene (Thanetian) Chehel-Kaman Formation in the Kopet-Dagh Basin of NE Iran is principally composed of carbonates with minor siliciclastics and evaporites. Six stratigraphic sections were measured and more than 1,000 samples were collected for petrographic analysis, together with analyses of carbon and oxygen isotopes and trace element content. Four major carbonate lithofacies (and 13 subfacies) have been identified in previous studies and are interpretd in terms of deposition in a shallow-marine environment.
The petrographic analyses indicate that the Chehel-Kaman Formation carbonates have undergone a complex diagenetic history which includes compaction, cementation, micritization, dissolution, silicification, dolomitization, neomorphism and fracturing. δ¹⁸O and δ¹³C values in Chehel-Kaman Formation limestones range between +0.8 and -15.1%₀ PDB, and -2.82 and +3.5%₀ PDB, respectively. These variations are interpreted to reflect meteoric and burial diagenetic processes. Variations in trace-element concentrations (Fe and Mn increased while Na decreased) also indicated the effects of meteoric flushing. The limestones appear to have been formed at about 28°C.
Chehel-Kaman Formation dolomites were divided into d1 (finely-crystalline) and d2 (coarsely-crystalline) types. Petrographic and geochemical results indicated that the d1 dolomites formed under sabkha conditions from a parental solution at around 26°C, while d2 dolomites formed during burial diagenesis with much hotter pore fluids (around 72°C). The paragenetic sequence indicates that primary porosity decreased during early stages of diagenesis, although secondary porosity was subsequently created improving the reservoir quality of the carbonates.     

A NEW MODEL FOR THE FORMATION OF DOLOMITE IN THE TRIASSIC DOLOMITES,NORTHERN ITALY

The Pale di San Martino and Pale di San Lucano (referred to together as the “Pale”) are remnants of an originally more extensive carbonate platform in the Dolomite Mountains of northern Italy. The platforms are composed of Middle Triassic dolomites and limestones up to 1.6km thick. Limestones comprise 2–3% of the platform carbonates and are restricted to narrow corridors (tens to a few hundred metres wide, hundreds of metres long and high) within the dolomite. The mainly sucrosic dolomites of the Pale are interpreted as the result of recrystallization of a depositional, nearly stoichiometric Mg calcite under burial temperatures of ca. 40–70°C. The principal arguments are:

• The quantitative composition indicates that all platform carbonates are composed mainly of micritic crusts (45%; boundstone fabric prevails) and early cement (35%; microcrystalline, fibrous). The platform carbonates were probably mainly bacterial precipitates and tight at the sediment‐water interface (porosities <5%, permeabilities in the micro‐Darcy range).
• The limestone‐dolomite transitions (centimetres to decimetres wide) lack dolomite gradients. The lack of evidence for flowing fluids causing dolomitization suggests stagnant pore waters.
• The δ¹³C of average dolomite is 1.3‰ heavier than that of coeval limestone (666 analyses). The difference corresponds to a primary difference of 50mol% MgCO₃ and is interpreted as the result of fractionation. It suggests a dolomite precursor of very high Mg calcite, whereas present‐day limestone of the Pale was probably deposited as a basically Mg‐free polymorph (aragonite and/or calcite).
• The dolomite δ¹⁸O (+1 to ?11‰ VPDB) values show a scatter over the platform thickness and preserve randomly distributed values around 0‰. The scatter is probably due to selective re‐setting of δ¹⁸O near pore spaces and is mainly a sampling effect.
• The observation that ⁸⁷Sr/⁸⁶Sr ratios (77 analyses) of limestone and dolomite are either slightly higher or lower than Middle Triassic seawater, but almost never “normal marine”, suggests that the platform carbonates of the Pale were deposited from seawater contaminated with artesian freshwater. The limestone corridors are probably caused by artesian springs of somewhat higher than ambient depositional temperature, with low Mg calcite and/or aragonite deposited in or near fracture zones. The volumetrically subordinate cycle‐cap dolomite is possibly a primary precipitate.

     

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.     

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.