ORGANIC GEOCHEMISTRY AND SOURCE ROCK CHARACTERISTICS OF THE ZAGROS PETROLEUM PROVINCE, SOUTHWEST IRAN

The Zagros of SW Iran and its continuation into N Iraq forms the tectonised NE margin of the Middle East basin. Sedimentation in the Zagros began in the late Precambrian and continued with comparatively few interruptions until the Pliocene, when strong earth movements affected the area and gave rise to the present day large. elongated NW-SE trending structures. Some of the world's largest structurally-controlled oil fields are located in the Zagros. The most productive pay zone is the Oligo-Miocene Asmari Formation, although significant oil pools are present also in the Cenomanian-Turonian Sarvak limestone and in the Neocomian-Jurassic Khami Group carbonates. Recently, large gas deposits have been discovered in the Permo-Triassic carbonates assigned to the Deh Ram Group. Geochemical studies were carried out five potential source beds of Eocene-Palaeocene (Pabdeh Formation). Coniacian-Neocomian (Garau Formation) and Silurian (Ghakum Formation) age. The results showed that the organic matter in these formations is almost exclusively of marine algal origin, and that the Kazhdumi is the major source of the hydrocarbons in the Asmari and Sarvak reservoirs. The origin of the hydrocarbons in the Khami and Deh Ram reservoirs is at present speculative.     

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.     

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.     

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.     

A SOURCE BED STUDY OF THE OLIGO-MIOCENE ASMARI LIMESTONE IN SW IRAN

Detailed geological and geochemical investigations in the folded belt of the Zagros geosyncline reveal that the source rocks for the very large Asmari oil accumulations may not be the PabdehGurpi and/or Kazhdumi formations as has been suggested; the hydrocarbons are more likely to be indigenous to the Asmari. Most previous investigators have assumed that only organic-rich marls and shales serve as effective source rocks. However, geochemical analysis shows that the organic content of the Asmari carbonate, although not very rich, compares favourably with the source beds proposed by earlier workers.
A rock should not be considered an oil source bed if it does not currently contain at least traces of oil. If the oil had been generated in the Pabdeh-Gurpi and/or Kazhdumi formations, some hydrocarbons should still be present within these rocks.
For all source rocks, there must be a positive relationship between thermal history and oil generation. Consequently, temperature and depth of burial are essential factors in petroleum genesis. The temperature history of the Asmari Formation is related to the thickness of the overlying Teritiary rocks. Maximum burial of the oligo-Miocene Asmari, throughout most of the folded belt, is assumed to have occured during Early Pliocene time before the last phase of Zagros orogeny.
Asmari limestone, therefore, qualifies as a source rock where it is buried deep enough to reach the optimum temperature needed to release its hydrocarbons. Under conditions of lower temprature and shallower burial, the limestones serve only as reservoir rocks.     

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.     

扎格罗斯盆地新生界Asmari-Gachsaran成藏组合地质特征及成藏模式

Asmari-Gachsran成藏组合是扎格罗斯盆地重要的成藏组合。通过对该成藏组合古地理演化、烃源岩、储层、盖层、成藏过程和模式的分析认为,新近纪至第四纪的扎格罗斯褶皱作用使白垩系Kazhdumi组烃源岩埋深加大,大面积进入生烃高峰;同时,构造作用形成了贯穿新生界Asmari组灰岩、Pebdeh组泥岩和白垩系Sarvark组灰岩的开放性裂缝网络,构成油气自烃源岩向储层运移的垂向通道。在大部分地区,Asmari组成藏组合内的油气都是Kazhdumi组烃源岩直接一次充注,而不属于由Sarvark组圈闭中纵向调整而来的次生油气藏。由于裂缝的沟通,Asmari组合Sarvark组储层具有统一的油水界面,在浮力的作用下,浅部的Asmari组储层要比深部的Sarvark组储层更早地充注油气,油气充满度也更高。优质烃源岩、沟通烃源岩与储层的大量开放性垂直裂缝、厚度大的蒸发岩盖层及烃源岩生烃期与构造和裂缝形成期的良好配合是油气富集的主控因素。     

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.     

Genetic mechanism and development of the unsteady Sarvak play of the Azadegan oil field,southwest of Iran

The upper Cretaceous Sarvak reservoir in the Azadegan oil field of southwest Iran has its oil–water contact nearly horizontal from the north to the center and dips steeply from the center to the south.The purpose of this paper is to interpret this abnormal reservoir feature by examining the accumulation elements,characteristics,and evolution based on the 3D seismic,coring,and well logging data.Generally,in the field,the Sarvak reservoir is massive and vertically heterogeneous,and impermeable interlayers are rare.The distribution of petrophysical properties is mainly dominated by the depositional paleogeomorphology and degrades from north to south laterally.The source is the lower Cretaceous Kazhdumi Formation of the eastern Dezful sag,and the seal is the muddy dense limestone of the Cenozoic Gurpi and Pebdeh Formations.Combined with the trap evolution,the accumulation evolution can be summarized as follows: the Sarvak play became a paleo-anticlinal trap in the Alpine tectonic activity after the late Cretaceous(96 Ma) and then was relatively peaceful in the later long geologic period.The Kazhdumi Formation entered in the oil window at the early Miocene(12–10 Ma) and charged the Sarvak bed,thus forming the paleo-reservoir.Impacted by the ZagrosOrogeny,the paleo-reservoir trap experienced a strong secondary deformation in the late Pliocene(4 Ma),which shows as the paleo-trap shrank dramatically and the prelow southern area uplifted and formed a new secondary anticline trap,hence evolving to the current two structural highs with the south point(secondary trap) higher than the north(paleo-trap).The trap deformation broke the paleoreservoir kinetic equilibrium and caused the secondary reservoir adjustment.The upper seal prevented vertical oil dissipation,and thus,the migration is mainly in interior Sarvak bed from northern paleo-reservoir to the southern secondary trap.The strong reservoir heterogeneity and the degradation trend of reservoir properties along migration path(north to south) made the reservoir readjustment extremely slow,plus the short and insufficient re-balance time,making the Sarvak form an ‘‘unsteady reservoir'which is still in the readjustment process and has not reached a new balance state.The current abnormal oil–water contact versus the trap evolutionary trend indicates the secondary readjustment is still in its early stage and has only impacted part of paleo-reservoir.Consequently,not all of the reservoir is dominated by the current structure,and some parts still stay at the paleo-reservoir form.From the overview above,we suggest the following for the future development: In the northern structural high,the field development should be focused on the original paleoreservoir zone.In the southern structural high,compared with the secondary reservoir of the Sarvak with the tilted oil–water contact and huge geologic uncertainty,the lower sandstone reservoirs are more reliable and could be developed first,and then the deployment optimized of the upper Sarvak after obtaining sufficient geological data.By the hints of the similar reservoir characteristics and tectonic inheritance with Sarvak,the lower Cretaceous Fahliyancarbonate reservoir is also proved to be an unsteady reservoir with a tilted oil–water contact.     

Estimating Total Organic Carbon Content and Source Rock Evaluation,Applying ΔlogR and Neural Network Methods: Ahwaz and Marun Oilfields,SW of Iran

Abstract

Well logging is a useful method for sedimentary basin and source rock evaluation. Source rocks have special responses in porosity and resistivity logs that can make them distinguishable from surrounding rocks. Therefore, well logging data and diagrams can be used as indicators of determination of source rock potential. Characterizations of Kazhdomi, Pabdeh, and Gurpi source rocks have been determined by geochemical analysis in some Iranian oilfields, but no total organic carbon (TOC) zonation and interpretation have been carried out in these formations yet. Several studies have confirmed the petroleum potential of the Kazhdomi formation in Dezful Embayment, but Pabdeh formation (more significant) and Gurpi (less significant) have been always the topics of Iranian petroleum geologist discussions in order to deermine whether these formations have the potential of generating oil and what the organic matter properties of these formations are. The purpose of this article is to calculate TOC values of the Pabdeh formation in Ahwaz and Marun oilfields using a combination of sonic and resistivity logs (ΔlogR method) and a neural network method. Then these TOC values were compared with TOC from geochemical analysis. Finally, the zonations of source rock in terms of TOC richness were carried out and TOC changes in the oilfields were shown by plotting Iso-TOC maps. It was found that due to the high local temperature gradient Pabdeh reaches an oil window level in some parts of Ahwaz and Marun oilfields. Hence, Pabdeh acts as a source rock for these two oilfields in some sections.