THE ORIGIN OF THE PERMO-TRIASSIC GAS ACCUMULATIONS IN THE IRANIAN ZAGROS FOLDBELT AND CONTIGUOUS OFFSHORE AREAS: A REVIEW OF THE PALAEOZOIC PETROLEUM SYSTEM

More than 1500 trillion cubic feet (Tcf) of gas reserves have been discovered in Permo-Triassic carbonates sealed by thick Triassic anhydrites in the Zagros Foldbelt (SW Iran), the southern part of the Gulf (Iran, Qatar, and Abu Dhabi) and Saudi Arabia. This paper discusses the origins of this gas in terms of source rock distribution and thermal maturation through time (as indicated by modelling), regional variations in thermal maturation (as indicated by cumulative isopachs), and long-range migration and accumulation of hydrocarbon prior to the Zagros orogeny. The sequence of events leading to the present-day distribution of gas accumulations is reconstructed in detail.
The only important source rocks so far identified in the Late Proterozoic to Late Triassic succession are organic-rich, radioactive shales which are dated as Llandoverian (early Silurian). Oil generation began in the Middle Jurassic in areas of greatest subsidence, while the gas window was reached locally as early as the Middle Cretaceous. Huge volumes of oil, then of gas, accumulated in a few major regional highs and salt-related structures prior to the Zagros orogeny. Part of the gas was lost during Zagros folding as some of the anticlines were breached, and another portion, possibly associated with light oil, remigrated into unbreached Zagros anticlines.
Among critical parameters essential to the appraisal of the numerous Permo-Triassic prospects present in Lurestan, Fars and in the Iranian Offshore, three are discussed in this paper, namely (i) the location of prospects in comparison to pre-Zagros regional highs and to reconstructed pre-Zagros gas accumulations; (ii) the characteristics of potential reservoir intervals in the Dalan/Kangan Formation; and (iii) the extent of the Dashtak evaporitic seal. The distribution of surface oil, bitumen and gas seepages together with indirect hydrocarbon indications provides an additional exploration tool.     

TECTONO‐STRATIGRAPHIC EVOLUTION OF THE NW SEGMENT OF THE ZAGROS FOLD‐THRUST BELT,KURDISTAN, NE IRAQ

The Kurdistan (NW) segment of the Zagros fold‐thrust belt, located in the Kurdistan Region of NE Iraq, forms the external part of the Zagros orogen and is bounded by the Zagros suture to the NE. To the SW is the Arabian Plate into which the deformation front has migrated progressively, beginning in the Late Cretaceous and culminating in the Tertiary. Regional compression resulted in obduction of the Mawat ophiolites and emplacement of the Avroman and Qulqula nappes onto the continental margin, and the formation of the Kurdistan foreland basin. In this paper, structural, stratigraphic and palaeontological data together with new field observations are used to investigate the tectono‐stratigraphic evolution of this basin, and to study the propagation of the deformation front from the Zagros Imbricate Zone in the NE towards the Mesopotamian foredeep in the SW. Six unconformities within the Kurdistan foreland basin succession are recognized: Turonian (base‐AP9; 92 Ma); Danian (base‐AP10; 65 Ma); Paleocene–Eocene (intra‐AP10; 55 Ma); late Eocene (top‐AP10; 34 Ma); middle‐upper Miocene (a local unconformity; intra‐AP11; 12 Ma); and Pleistocene. These unconformities can be divided into two groups; obduction‐related (Turonian, Danian, and Paleocene‐Eocene); and collision‐related (late Eocene, middle‐upper Miocene, and Pleistocene). The geographical position of the unconformities is used to determine the rate of propagation of the deformation front, which is estimated at ca. 3 mm/yr. This is in agreement with previous studies which suggested a NW‐ward decrease in the propagation rate. The rate was most rapid (2.95 mm/yr) in the Low Zagros Fold‐Thrust Zone and slower (2.06 mm/yr) in the High Zagros Fold‐Thrust Zone. The more rapid propagation rate in the former area may be attributed to the presence there of the Miocene Lower Fars Formation which acted as a shallow décollement surface. Within the Zagros fold‐thrust belt, the intensity of deformation decreases towards the foreland (SW). Deformation in the High Zagros Fold‐Thrust Zone is characterized by thrust imbricates and high amplitude fault‐propagation folds at the surface separated by narrow synclines. However, the Low Zagros Fold‐Thrust Zone (Simply Folded Belt) is characterised by detachments and low amplitude fault propagation folds separated by broad synclines. In the foredeep area, folds are confined to the subsurface. Deeply buried Jurassic units, together with Upper Cretaceous – Paleocene siliciclastics, and the evaporite‐dominated Lower Fars Formation may have acted as décollement surfaces in the NW segment of the Zagros fold‐thrust belt, and controlled the structural geometry and evolution of the area.     

MULTI‐PHASE HORMUZ SALT DIAPIRISM IN THE SOUTHERN ZAGROS,SW IRAN

In the Zagros orogenic belt in SW Iran, a 14 km thick sedimentary succession is decoupled from crystalline basement by the thick Infracambrian Hormuz complex. The complex consists of evaporites (“Hormuz salt”) together with marls, sandstones, carbonates and volcanic rocks and is known from emergent salt diapirs in the Fars and High Zagros areas. In this paper, we document discrete pulses of halokinetic activity using seismic profiles, well data and recent field studies. The earliest phase of mobilization of the Hormuz salt occurred in the pre‐Silurian. Later pulses of salt movement took place during opening of Neo‐Tethys in Permo‐Triassic times and during ophiolite obduction onto the NE Arabian plate margin in the Late Cretaceous. Halokinesis was intense during the Neogene Zagros orogeny when some salt plugs breached the surface.     

THE PRE-ZAGROS INTEGRITY OF THE IRANIAN PLATFORM

The Hormoz Formation of InfraCambrian age is the oldest unit recognized in southern Iran. It consists mainly of salt, other evaporites and débris of basement rocks. Some huge blocks of the uppermost PreCambrian and Lower Cambrian layered rocks were lifted to the surface by uprising Hormoz salt.
A detailed field investigation within three major salt diapirs located in the SE segment of the Zagros Fold Belt was carried out. Some lithologic-stratigraphic correlations between three formations within selected salt domes on the western part of the Zagros crush zone, and the equivalent units on the eastern part, are established.
The Permian transgressive sea deposited basinal clastics unconformably on the Devonian in central Iran, and overstepped the basement rocks in central Arabia. The Pecten species of the Lower Jurassic Neyriz Formation of the Zagros Fold Belt can be correlated with the Nayband Formation of central Iran. The Tertiary evaporites of SSW Iran are correlated with the Tertiary evaporites of central Iran.
Evidence discussed indicates the presence of a single tectonic entity with no geologic break between SW Iran and the plateau of Iran in the NE.     

SEDIMENTARY HISTORY AND PALEOGEOGRAPHY OF LOWER AND MIDDLE JURASSIC ROCKS, CENTRAL SAUDI ARABIA

Eight clastic and carbonate lithofacies of Lower and Middle Jurassic age (the Marrat and Dhruma Formations respectively) have been identified. These lithofacies are mainly shales, shales-and-siltstones, siltstones-and-shales, sandstones, argillaceous limestones, calcarenitic limestones, calcarenites and dolomites. Intraclasts, pellets, oolites, gypsum, algae, and coral reefs were also found to be dominant among these lithofacies. Furthermore, these beds contained either restricted or diversifed biota, with a few sedimentary structures such as lamination, cross-bedding and common bioturbation. Thus, it is presumed that the Lower and Middle Jurassic rocks in Central Saudi Arabia were deposited in very shallow (i.e. tidal flat and lagoon), shallow-neritic, and deep-marine conditions of the Tethys Sea. During Toarcian (Lower Jurassic) time, the Tethys Sea extended towards Arabia, forming an arcuate shoreline around the Pre-Jurassic sediments. The greater part of the Arabian Peninsula was under very shallow marine (Tethys) water as is shown by the presence of tidal-flat and lagoon deposits in northern Oman, Rub al-Khali, parts of the Aden Protectorate and Yemen, central, eastern and NNW Saudi Arabia, the northern edge of Sinai, NW Jordan, Lebanon, central and northern Syria, and west, central and eastern Iraq and Iran (Fig. 5). On the other hand, east and north of this area, very shallow marine deposits overlaid the shallow-neritic Tethys deposits. Further expansion of the Tethys Sea transgression occurred during Bajocian-Bathonian (Middle Jurassic) time, when major parts of the Arabian Peninsula and its neighbouring regions were submerged below shallow-neritic and deepmarine Tethys conditions (Fig. 6). During this time, shallow-nentic Tethys conditions extended to include the Oman Mountains, as far west as western Rub al-Khali, north to include Central Saudi Arabia, running through parts of its northern area and passing under the Saudi Arabia-Iraq Neutral Zone and reaching SW Iraq, turning west to include part of the area of NW Arabia and reaching NW Jordan. Northern Oman, most of central Rub al-Khali, eastern Saudi Arabia, western Iran, central and eastern Iraq, and eastern and northern Jordan, were covered by deepmarine Tethys waters (F&. 6). However, simultaneously, SW Arabia, Yemen and the Aden Protectorate display a marked continuous sedimentation of continental and neritic conditions, as neritic deposits narrow toward the north.     

AN OVERVIEW OF THE PETROLEUM SYSTEMS IN THE IONIAN ZONE,ONSHORE NW GREECE AND ALBANIA

The Ionian and Gavrovo Zones in the external Hellenide fold‐and‐thrust belt of western Greece are a southern extension of the proven Albanian oil and gas province. Two petroleum systems have been identified here: a Mesozoic mainly oil‐prone system, and a Cenozoic system with gas potential. Potential Mesozoic source rocks include organic‐rich shales within Triassic evaporites and dissolution‐collapse breccias; marls at the base of the Early Jurassic (lower Toarcian) Ammonitico Rosso; the Lower and Upper Posidonia beds (Toarcian–Aalenian and Callovian–Tithonian respectively); and the Late Cretaceous (Cenomanian–Turonian) Vigla Shales, part of the Vigla Limestone Formation. These potential source rocks contain Types I‐II kerogen and are mature for oil generation if sufficiently deeply buried. The Vigla Shales have TOC up to 2.5% and good to excellent hydrocarbon generation potential with kerogen Type II. Potential Cenozoic gas‐prone source rocks with Type III kerogen comprise organic‐rich intervals in Eocene–Oligocene and Aquitanian–Burdigalian submarine fan deposits, which may generate biogenic gas. The complex regional deformation history of the external Hellenide foldbelt, with periods of both crustal extension and shortening, has resulted in the development of structural traps. Mesozoic extensional structures have been overprinted by later Hellenide thrusts, and favourable trap locations may occur along thrust back‐limbs and in the crests of anticlines. Trapping geometries may also be provided by lateral discontinuities in the basal detachment in the thin‐skinned fold‐and‐thrust belt, or associated with strike‐slip fault zones. Regional‐scale seals are provided by Triassic evaporites, and Eocene‐Oligocene and Neogene shales. Onshore oil‐ and gasfields in Albania are located in the Peri‐Adriatic Depression and Ionian Zone. Numerous oil seeps have been recorded in the Kruja Zone but no commercial hydrocarbon accumulations. Source rocks in the Ionian Zone comprise Upper Triassic – Lower Jurassic carbonates and shales of Middle Jurassic, Late Jurassic and Early Cretaceous ages. Reservoir rocks in both oil‐ and gas‐fields in general consist of silicilastics in the Peri‐Adriatic Depression succession and the underlying Cretaceous–Eocene carbonates with minimal primary porosity improved by fracturing in the Albanian Ionian Zone. Oil accumulations in thrust‐related structures are sealed by the overlying Oligocene flysch whereas seals for gas accumulations are provided by Upper Miocene–Pliocene shales. Thin‐kinned thrusting along flysch décollements, resulting in stacked carbonate sequences, has clearly been demonstrated on seismic profiles and in well data, possibly enhanced by evaporitic horizons. Offshore Albania in the South Adriatic basin, exploration targets in the SW include possible compressional structures and topographic highs proximal to the relatively unstructured boundary of the Apulian platform. Further to the north, there is potential for oil accumulations both in the overpressured siliciclastic section and in the underlying deeply buried platform carbonates. Biogenic gas potential is related to structures in the overpressured Neogene (Miocene–Pliocene) succession.     

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.     

DATING AND CORRELATION OF THE BALUTI FORMATION,KURDISTAN, IRAQ: Implications for the regional recognition of a Carnian “marker dolomite”, and a review of the Triassic to Early Jurassic sequence stratigraphy of the Arabian Plate

The Baluti Formation is a lithostratigraphic unit defined at outcrop in the Zagros foldbelt in Kurdistan, North Iraq, where it consists of a thin, shale‐dominated interval between the thick carbonates of the overlying Sarki and the underlying Kurra Chine Formations. New biostratigraphic results presented in this study confirm that firstly, the Baluti shale marks the top of the Carnian at outcrop; and secondly, that the correlative subsurface shale occurs well above the base of the subsurface‐defined Butmah Formation. As such, the Butmah is not directly correlative to the Sarki as has previously been assumed, and its definition overlaps that of the Baluti and Kurra Chine Formations. These results have regional implications. A distinctive low Gamma Ray dolostone unit in the Kurra Chine A can be correlated across Kurdistan. Similar dolostones are recognised widely across the Arabian Plate and have been considered to be correlative by some workers, but not by others. This study establishes the age of this “Marker Dolomite” as late Carnian on the basis of its position below the Baluti Formation and confirms its regional development. Correlation is demonstrated to the Sefidar Dolomite (Iran), the “Muss”/Mulussa D (Syria), the upper Abu Ruweis Formation (Jordan), the Jilh Formation (Kuwait, Saudi Arabia, Oman), and the upper Gulailah Formation (UAE). Regional sequence stratigraphic studies have assigned the Marker Dolomite to different sequences in different areas. The recognition that it represents a single, regionally‐correlatable event provides the starting point for a sequence stratigraphic review based on regional log correlation and published dating. An updated sequence stratigraphic framework is presented for the Triassic to Early Jurassic, defining additional sequences (e.g. Tr55, Tr90, Tr100, J01, J05) and recognising that significant depositional breaks are associated with SBs Tr60, Tr70, Tr80, Tr100 and J01. The hiatus below Tr80 reflects a Carnian–Ladinian regional high in the Southern Gulf. This represents the barrier between the open Neo‐Tethys and the saltern in which the Carnian evaporites accumulated. The implications for petroleum exploration are discussed in the light of these revisions. There are two established Triassic plays in Iraq and Syria: firstly, in fractured carbonate reservoirs sealed by evaporites and shales, which is productive in both countries; and secondly, in fluvial sandstones sealed by interbedded shales, which is productive in Syria. A key observation regarding the carbonate play is that the main Triassic exploration target in Iraq has been the Kurra Chine B fractured carbonates, located above the main Carnian evaporites; whereas in the Syrian Palmyrides, the main discoveries are generally located below the Carnian evaporites. The sub‐evaporite fractured carbonate play is underexplored in Iraq. With regard to the clastic play, the correlations presented in this study indicate that the Mulussa F and Minjur represent a single clastic pulse, and equivalent sandstones are likely to be developed in the Sarki Formation in Central‐South Iraq which remains largely untested. Sequence correlations indicate regionally developed source rock potential at the levels of the Kurra Chine B Member and Geli Khana Formation.     

BURIAL HISTORY RECONSTRUCTION AND THERMAL MODELLING AT KUH-E MOND, SW IRAN

At the Kuh-e Mond anticline (Fars Province, SW Iran) and in nearby offshore structures, large volumes of natural gas are reservoired in the Permian — Early Triassic Dehram Group while heavy oil has been discovered in the Cretaceous Sarvak and Eocene Jahrum Formations. In this paper, we use data from six exploration wells and from nearby surface exposures to reconstruct the burial history at Kuh-e Mond. Regional observations show that the thick sedimentary fill in this part of the Zagros Basin was subjected to intense tectonism during the Zagros Orogeny, with a paroxysmal phase during the late Miocene and Pliocene. Thermal modelling and geochemical data from Kuh-e Mond and adjacent fields allows possible hydrocarbon generation and migration mechanisms to be identified. Maturities predicted using Lopatin's TTI model are in accordance with maturities obtained from vitrinite reflectance measurements.
We show that formations which have source potential in the nearby Dezful Embayment (including the Pabdeh, Gurpi, Gadvan and Kazhdumi Formations) have not reached the oil window in the Mond wells. Moreover, their organic carbon content is very low as they were deposited in oxic, shallow-water settings. Underlying units (including the Ordovician and Cambrian) could have reached the gas window but contain little organic matter. Silurian shales (Sarchahan Formation), which generate gas at Kuh-e Gahkum and Kuh-e Faraghan (north of Bandar Abbas) and in Saudi Arabia and elsewhere in the Middle East, are absent from the Mond structure.
The absence of source rocks suggests that the gas and heavy oil accumulations at Kuh-e Mond and at nearby fields have most probably undergone long-distance lateral migration from distant source kitchens.     

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.     

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.     

COMPARATIVE PETROLEUM GEOLOGY OF SOUTHEAST TURKEY AND NORTHEAST SYRIA

In terms of their regional tectonic setting, SE Turkey and NE Syria belong to the Persian Gulf basin, and the essential structural features of SW Iran and N Iraq are readily discernible here. Large, elongated folds of Alpine (Mio-Pliocene) age dominate the structure of the region, and the intensity of deformation increases towards the north, in the direction of the Taurus orogenic zone. Five major depositional cycles have been recognized in the sedimentary record of SE Turkey and NE Syria, and the stratigraphy of the area has been outlined. The depositional cycles are late Precambrian, Cambro-Devonian, Permo-Carboniferous to Upper Jurassic, Lower-Cretaceous to Lower Eocene and Middle Eocene to Recent in age. Important regional unconformities are present at the base of the Permo-Carboniferous to Upper Jurassic and Lower Cretaceous to Lower Eocene cycles. The effects of the latter unconformity are particularly marked north of the Diyarbakir-Mardin swell in southeast Turkey, and it has adversely affected the petroleum potential of this region. Several giant oilfields have been discovered in NE Syria, while in SE Turkey the accumulations are small by Middle East standards and are often only marginally commercial. The difference in size of accumulations is due to more favourable source, reservoir and caprock relationships in the former area. Many of the structures in NE Syria can be regarded as multi-objective with Tertiary, Upper Cretaceous, Lower Jurassic and Triassic prospects, whereas in SE Turkey the effects of the late Jurassic–early Cretaceous uplift and erosion seem to have restricted the potential pay zones largely to the Upper Cretaceous–Palaeocene section. Geochemical evidence suggests separate sources for the Tertiary, Cretaceous and Lower Mesozoic crudes of NE Syria, and in SE Turkey much of the currently produced oil is believed to have been generated by late Maestrichtian–Palaeocene basinal shales. However, in both areas there is some evidence that at least part of the oils now trapped in Upper Cretaceous-Tertiary reservoirs in certain fields may be of deeper origin.     

INTEGRATED GRAVITY AND SEISMIC INVESTIGATION OVER THE JABAL HAFIT STRUCTURE: IMPLICATIONS FOR BASEMENT CONFIGURATION OF THE FRONTAL FOLD-AND-THRUST BELT OF THE NORTHERN OMAN MOUNTAINS

A gravity survey was conducted over and around Jabal Hafit, located on the border between the United Arab Emirates (UAE) and Oman on the western edge of the northern Oman Mountains, as part of a study to investigate the subsurface structures and sedimentary sequences of the area. This new data, together with outcrop geology, well data and measurements of physical properties of rock samples, was integrated with a new interpretation of reprocessed commercial seismic reflection profiles recorded across the Jabal Hafit anticline. We recognize five major tectonostratigraphic sequences in the seismic profiles: Mesozoic shelf carbonates; Upper Cretaceous foreland (primarily Fiqa Formation); the Hawasina allochthon; and Upper Cretaceous to Lower Tertiary, and Upper Tertiary sequences. The seismic sections suggest that Jabal Hafit represents a backfolded anticline associated with a steep east‐vergent thrust that probably lies above a blind thrust fault along the base of the foreland basin sequence at depth. The lack of onlapping and thinning in the Lower Tertiary sequence suggests that the initiation of folding may have started after the Oligocene‐Miocene and corresponds to the beginning of the collision of Arabia and central Iran along the Zagros suture in Iran. In addition, the seismic sections suggest a pop‐up structure beneath the Jabal Hafit structure at the base of the foreland basin sequence. The structure is bounded by high‐angle listric reverse faults that cut downward through the Mesozoic shelf carbonates and are interpreted to be related to the inversion of deep structures. NNW to SSE trending positive residual gravity anomalies of up to +8 mGal occur on the Jabal Hafit and Al‐Ain anticlines. These positive gravity highs are attributed to uplifted basement structures which probably resulted from the reactivation of deep‐seated fault blocks. The residual Bouguer anomaly values decline gradually to the east and west of Jabal Hafit probably due to the deepening of the basement. A large negative gravity anomaly of 相似文献   

PETROLEUM GEOLOGY OF THE UNITED ARAB EMIRATES

The Permain to Holocene sediments of the United Arab Emirates consist mainly of epeiric shelf carbonates, associated with minor clastics and evaporites, reflecting major cycles of transgression and regression. These were deposited on the eastern margin of the Arabian Sheild, which lay along the southern margin of the Tethys Ocean during the Mesozoic-Cenozoic eras. Sedimentation patterns were controlled by prominent regional structural features, epeirogenic movements and/or sea-level fluctuations.
The tectonic history of the UAE in the Mesozoic-Cenozoic is connected with the opening (Triassic) and closure (Upper Cretaceous-Paleogene) of the southern Neo-Tethys Ocean.
The distinctive structural style, together with the tripartite development of source-reservoir-seal, has produced in the UAE one of the world's richest Jurassic - Cretaceous oil habitats. Significant oil discoveries have also been made in the Permian; Middle and Upper Jurassic; Lower-Middle-Upper Cretaceous and Oligo-Miocene carbonates.
Two main source rocks have been identified. One is the Upper Jurassic Diyab/Dukhan Formation, which supplies the most prolific reservoirs in the Upper Jurassic (Arab Formation) and Lower Cretaceous (Thamama Group). The other is the Middle Cretaceous Shilaif/Khatiyah Formation, which feeds both Mishrif and Simsima reservoirs. Other minor potential source rocks have also been identified in the study area.
There are two principal sealing formations - the Hith Anhydrite and the Nahr Umr shale; these are the main seals for the oil and gas accumulations in the underlying Arab Formation and Thamama Group, respectively. Secondary seals and barriers also exist in the stratigraphic sequence.     

HYDROCARBON SOURCE ROCKS AND GENERATION HISTORY IN THE LUNNAN OILFIELD AREA, NORTHERN TARIM BASIN (NW CHINA)

In this paper we report on source rocks and maturation history at the Lunnan oilfield, northern Tarim Basin (NW China), using a combination of organic petrographic and geochemical techniques. Three separate source rock intervals are present here: Cambrian mudstones and argillaceous limestones; Middle and Upper Ordovician argillaceous limestones; and Triassic mudstones. Reservoir rocks comprise Lower Ordovician carbonates, Carboniferous sandstones, and Triassic and Jurassic sandstones. Structural traps were formed principally during the Silurian and Jurassic.
The Lunnan field is located on a small-scale palaeo uplift which developed during the Early Palaeozoic. Hydrocarbons migrated updip from source areas in surrounding palaeo-lows along faults and unconformities. Major phases of hydrocarbon generation and migration occurred in the Early Silurian — Late Devonian, Cretaceous — Early Tertiary and Late Tertiary. Uplift and intense erosion at the end of the Devonian destroyed Early Palaeozoic oil and gas accumulations sourced from the Cambrian source rocks, but hydrocarbons generated by Middle and Upper Ordovician source rocks during the Mesozoic and Tertiary have been preserved. At the present day, accumulations are characterized by a range of crude oil compositions because source rocks from different source areas with different maturation histories are involved.     

HYDROCARBON POTENTIAL OF THE INTRACRATONIC OGADEN BASIN, SE ETHIOPIA

The intracratonic Ogaden Basin, which covers one-third of the Democratic Republic of Ethiopia, developed in response to a tri-radial rift system which was active during Late Palaeozoic to Mesozoic times. Thick Permian to Cretaceous sequences, which principally occur in the SW and central parts of the basin, have proved petroleum potential. Reservoir rocks are mainly Permian to Lower Jurassic sandstones (the Calub and Adigrat Formations), and Callovian limestones (the Upper Hamanlei Formation). Source rocks are organic-rich Permian, Lower Jurassic and Callovian-Oxfordian lacustrine and marine shales.
This paper reviews the petroleum geology of the Ogaden Basin and assesses potential exploration targets. Successful exploration can be expected in view of the recent discovery of the Calub gas and gas/condensate field, and the occurrence of significant shows in the centre of the basin together with seeps along the margin.     

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.     

THE PETROLEUM RESOURCES OF THE MIDDLE EAST: A REVIEW

The area under review is the Arabian Peninsula, Fertile Crescent and parts of SE Turkey and SW Iran (Arabian Plate plus Levant). Petroleum production is essentially from the eastern part or the Iraq-Iran-Arabia basin. Published proved oil reserves at the start of 1985 were estimated at 398.7 B? brl (barrels) and those for natural gas at 869.95 T? cu. ft (53 B tonnes and 24.8 T cum respectively) amounting to 57% of estimated world oil reserves and 25.6% of world gas reserves. The region produced 21.2% of world production in 1984 (but had produced 38.8% in 1974, the drop being due to non-technical reasons). The oil reserves are attributable to some 290 producing fields and undeveloped discoveries, 25 of which are “supergiant” and 69 of which are “giant” in size. Only about 122 of these fields and discoveries are actually producing (excluding Turkey's 30 which account for little of the total). The reasons for the prolific oil abundance lie principally in the repeated extensive deposition of organically-rich source rocks under anoxic conditions in the right juxtaposition with very permeable extensive reservoirs and sealed by regionally extensive efficient seals over several intervals of geological time, charging extremely large anticlinal traps formed by a variety of structure-forming mechanisms available at optimum hydrocarbon maturation and migration times. Accumulations occur in Paleozoic sands in southern Oman charged from Infra-Cambrian source rocks, and in Middle and Upper Jurassic carbonates and Lower and Middle Cretaceous carbonates and sanhtones in the central part of the basin (the Gulf), and the Zagros fold belt, as well as in the Oligo-Miocene carbonates in the latter charged by vertical migration from the Cretaceous. The NW-most part has heavier Middle-Upper Cretaceous and some Miocene oil of differing origin and Triassic light oil, gas and condensate extending into central Syria. Subordinate Paleogene heavy oil occurs in the Kuwait-Basra area. Other new discoveries of Mesozoic and Paleogene age have occurred in Jordan and North and South Yemen. All Middle East countries with the exception of Lebanon now either have established production or discoveries under assessment. Even in the main basin, exploration has not been intensive and future efforts including enhanced recovery methods will lead to considerable success and are expected to add new reserves equal to those now established for oil and probably more than those now estimated for gas, with a 50% chance of success. Saudi Arabia, Iraq and UAE are ranked highest, but small basins outside the main area will yield important finds by utilisation of the latest in geophysical and geochemical methods.     

STRATIGRAPHY AND PETROLEUM GEOLOGY OF THE CROATIAN PART OF THE ADRIATIC BASIN

Coastal parts of Croatia are dominated by the SW‐verging Dinaric foldbelt, to the west and SW of which is the Adriatic Basin (the stable foreland). In both areas, the stratigraphic column is dominated by a thick carbonate succession ranging from Carboniferous to Miocene. Four megasequences have been identified: (i) a pre‐platform succession ranging in age from Late Carboniferous (Middle Pennsylvanian: Moscovian) to Early Jurassic (Early Toarcian; Bru?ane and Ba?ke Ostarije Formations); (ii) an Early Jurassic to Late Cretaceous platform megasequence (Mali Alan Formation); (iii) a Paleogene to Neogene post‐platform megasequence (Ra?a Formation); and (iv) a Neogene to Quaternary (Pliocene to Holocene) megasequence (Istra and Ivana Formations). A number of organic‐rich intervals with source rock potential have been identified on‐ and offshore Croatia: Middle and Upper Carboniferous, Upper Permian, Lower and Middle Triassic, Lower and Upper Jurassic, Lower and Upper Cretaceous, Eocene, and Pliocene – Pleistocene. Traps and potential plays have been identified from seismic data in the Dinaric belt and adjacent foreland. Evaporites of Permian, Triassic and Neogene (Messinian) ages form potential regional seals, and carbonates with secondary porosity form potential reservoirs. Oil and gas shows in wells in the Croatian part of the Adriatic Basin have been recorded but no oil accumulations of commercial value have yet been discovered. In the northern Adriatic offshore Croatia, Pliocene hemi‐pelagic marlstones and shales include source rocks which produce commercial volumes of biogenic gas. The gas is reservoired in unconsolidated sands of the Pleistocene Ivana Formation.     

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.