A REVIEW OF EOCENE NUMMULITE ACCUMULATIONS: STRUCTURE, FORMATION AND RESERVOIR POTENTIAL

Eocene nummulite accumulations, also referred to as nummullte "banks", form Important hydrocarbon reservoirs in Tunisia and Libya and may constitute exploration targets in other parts of North Africa, the Mediterranean and the Middle East. Porosities commonly average 10–20% and permeabilities 10–50md. Foraminifera of the genus Nummulites may comprise up to 98% of the bioclasts in these carbonate reservoirs, although only one or two species may be present. The absence of associated fauna is generally taken to indicate an oligotrophic depositional environment.
In this paper, the palaeoecology of the genus Nummulites is discussed together with depositional models for two nummulitic carbonate reservoirs — the Middle Eocene Seeb Limestone of Oman and the Early Eocene El Garia/Jdeir Formation of Tunisia and Libya. The El Garia and Seeb Limestone Formations were deposited in ramp settings, and comprise a series of amalgamated sheets or low-relief banks. In the Hasdrubal field offshore Tunisia, where the El Garia Formation constitutes the reservoir rock, most of the nummulites have been redeposited from shallow into deeper waters whilst in the Bourri field (offshore Libya) they occur as an in situ "bank".
Nummulite accumulations often show evidence that both physical reworking (scouring, winnowing and imbrication) and biological processes (reproduction strategies and bioturbation) have influenced their formation. A general model is outlined for discriminating between physically and ecologically produced biofabrics, and the implications for reservoir quality are discussed.     

THE PETROLEUM GEOLOGY OF THE EARLY EOCENE EL GARIA FORMATION, HASDRUBAL FIELD, OFFSHORE TUNISIA

The Hasdrubal field (offshore Tunisia) comprises an Early Eocene shallow‐marine nummulitic limestone reservoir (the El Garia Formation) sourced by deep‐marine mudstones and limestones of the generally age‐equivalent Bou Dabbous Formation. The field is located on a NNW‐SSE trending horst between a series of en‐echelon normal to oblique faults, and is dip‐closed except to the north where a stratigraphic pinch‐out into the Bou Dabbous Formation is inferred. Middle Eocene shales and dense limestones of the Cherahil Formation form the main seal. The El Garia Formation reservoirs significant volumes of hydrocarbons in Tunisia and Libya. A detailed micropalaeontological and nannofossil study has been undertaken of the El Garia Formation and the immediately over‐ and underlying formations which together form the Metlaoui Group, using subsurface data from the Hasdrubal field. This has permitted a detailed chronostratigraphic and sequence stratigraphic framework to be developed, including the recognition of three flooding events, which can partly be calibrated with second‐order sequences, thus permitting the correlation of discrete reservoir units across the field. A further six microfaunal events are recognized between the Chouabine Formation and the “Compact Micrite Member” within the Metlaoui Group. Previous depositional models for the El Garia Formation are discussed and a new model is proposed. The model partly explains why a number of wells drilled along the El Garia nummulite “bank” trend have failed to encounter the nummulite reservoir fades, and why, even where this fades was encountered, the limestones were frequently tight and/or contained limited hydrocarbons. It is also suggested that proximity to source is a critical factor, with the development of dissolution porosity by acidic pore waters migrating in advance of hydrocarbons. This is critical for enhancing reservoir quality and thus promoting the capacity to reservoir hydrocarbons, as indicated by the location of existing discoveries.     

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.     

鄂尔多斯盆地中央古隆起区中寒武统鲕粒碳酸盐岩

鄂尔多斯盆地中央古隆起区中寒武统鲕粒碳酸盐岩沉积于碳酸盐斜坡中的小规模浅滩,其主要矿物成分为白云石和方解石。根据全岩样品分析结果,鲕粒碳酸盐岩的δ18O_V-PDB值在-11.3‰~-4.9‰之间,平均-6.5‰;δ13C_V-PDB值在-1.7‰~0.3‰之间,平均-0.8‰。该碳酸盐岩沉积于浅水鲕粒滩,因受到淡水淋滤影响,之后又长期处于深埋环境,δ18O值明显偏低。由于成岩作用过程中受有机质影响很小,δ13C值没有表现出异常。白云岩的δ18O值较石灰岩的高。显微激光采样分析结果表明,鲕粒较胶结物有较低的δ18O和δ13C值。前者形成于动荡的浅水环境,形成后常受淡水的淋滤。胶结物大多形成于埋藏期,孔隙水具有海水性质,盐度较高,胶结物沉淀后,受淡水影响较小。由于经受了强烈的埋藏作用,鲕粒碳酸盐岩表现出的特征为Na和Sr含量低、Fe含量高—特高及Mn含量较高。白云石大多数为埋藏作用阶段高Mg含量卤水条件下交代而成,整体上有较低的有序度,MgCO₃含量高。在强烈的埋藏作用和胶结作用下,鲕粒碳酸盐岩现存孔隙度极低。      

DOLOMITIZATION AND ANHYDRITE PRECIPITATION IN PERMO-TRIASSIC CARBONATES AT THE SOUTH PARS GASFIELD, OFFSHORE IRAN: CONTROLS ON RESERVOIR QUALITY

Dolomitization and related anhydrite cementation can complicate the characterization of carbonate reservoirs. Both processes have affected the Permo-Triassic Upper Dalan – Kangan carbonates, the main reservoir at the South Pars gasfield, offshore Iran. The carbonates were deposited in a shallow-marine ramp or epeiric platform and, according to previous studies, underwent intense near-surface diagenesis and minor burial modification. Detailed petrographical and geochemical analyses indicate that dolomitization and anhydrite precipitation can be explained in terms of the sabkha/seepage-reflux models. The early dolomites then re-equilibrated or re-crystallized in a shallow burial setting. Evaluation of poroperm values in different reservoir intervals indicates that replacive dolomitization in the absence of anhydrite precipitation or with only patchy anhydrite has enhanced the reservoir quality. Where anhydrite cement is pervasive and has plugged the rock fabric, poroperm values are significantly decreased. As emphasized in previous studies and confirmed here, dolomitization and anhydrite cementation, together with original facies type, are the major factors controlling reservoir quality in the Dalan – Kangan carbonates at South Pars. When associated with minor anhydrite cementation, replacive dolomitization has enhanced reservoir quality by increasing permeability. However, porosity in fabric-retentive dolomite was apparently inherited from the precursor rock and therefore reflects the original depositional environment. Low-temperature dolomitization is commonly fabric-selective and partially fabric-retentive. Whole rock stable isotope thermometry indicates that fabric-destructive dolomites in the reservoir rocks formed at temperatures above 22°C, whereas fabric-retentive dolomites and associated anhydrites formed in surface and near-surface conditions. Fabric-destructive dolomite or dolomite neomorphism post-date fabric-retentive dolomite and continued to form in deep burial conditions (∼1400m). These observations may explain why fabric-retentive dolomite and anhydrite fabrics are traversed by stylolites.     

川东上二叠统生物礁成岩作用与孔隙演化

川东地区上二叠统长兴组生物礁礁体经历了海底、潮坪、陆上、浅-中埋藏和深埋藏成岩环境,而礁间相和非礁相沉积直接从海底成岩环境进入浅-中和深埋藏成岩环境。礁灰岩中的大量原生孔隙被海底和埋藏成岩环境中多期胶结物充填而丧失殆尽。礁间生物屑灰泥沉积物微孔隙在压实过程中逐渐消失。礁体的白云岩主体是埋藏成岩环境中形成的,一般孔、渗性良好。礁顶发育2—7m厚的潮坪白云岩,则孔、渗性很差。     

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.     

DEPOSITIONAL ENVIRONMENT AND DIAGENESIS OF THE EOCENE JDEIR FORMATION, GABES-TRIPOLI BASIN, WESTERN OFFSHORE, LIBYA

The late Ypresian (early Eocene) Jdeir Formation was deposited in the Mesozoic‐Cenozoic Gabes‐Tripoli Basin, offshore Libya. The basin developed on the northern passive margin of the African Plate and was relatively unstable being affected by syn‐sedimentary tectonic movements. Deposition was coeval with a relative rise of sea‐level and the subsequent highstand. A lower, thinly‐developed nummulitic bank facies with restricted distribution records the transgressive event and is succeeded by more micritic sediments that record the time of maximum flooding. The succeeding sea‐level highstand is represented by a thick, and widely developed, progradational‐aggradational nummulitic sequence that displays lateral changes across WE‐ESE trending facies belts. Three major lithofacies are recognized in the Jdeir Formation: Nummulites packstone‐grainstone, Alveolina‐Orbitoliteswackestone‐packtone, andFragmental‐Discocyclina‐Assilina wackestone‐packstone, depositedin bank, back‐bank, and fore‐bank environments, respectively. The formation passes to the NNE into the pelagic lithofacies of the Hallab Formation; landward, to the south, it passes into shoreline evaporitic facies of the Taljah Formation. The lithofacies were structurally controlled by contemporaneous and/or syndepositional tectonic movements, with nummulitic facies tending to develop on uplifted areas. Petrographic and petrophysical studies indicate that porosity in the Jdeir Formation is controlled by depositional environment, tectonic setting and diagenesis. The combined effects of salt tectonics, a major unconformity at the top of the formation and meteoric diagenesis have produced excellent‐quality reservoir facies at the Bouri oilfield and in other areas. Porosity is highest in the nummulitic bank facies and lowest in the Alveolina‐Orbitolites micrite facies. Good to excellent reservoir quality occurs in the upper part of the nummulitic packstone‐grainstone facies, especially where these sediments overlie structurally high areas. High rates of dissolution found at the crests of domes and anticlines suggest that early diagenetic processes and features are, in part, structurally controlled. Future exploration success will depend on investigation of similar structures within the Gabes‐Tripoli Basin. Both porosity initiation and preservation are related to early depositional and diagenetic processes. The wide time‐gap between hydrocarbon generation and reservoir formation points to the role of the seal in porosity preservation and rules out the assumption that early emplacement of oil had preserved the porosity.     

准噶尔盆地南缘安集海河组砂岩碳酸盐胶结物特征

准噶尔盆地南缘古近系安集海河组砂岩普遍存在碳酸盐胶结作用,是导致储层物性变差的重要因素.利用镜下观察、碳氧同位素分析等方法,在分析碳酸盐胶结物岩石学特征的基础上,探讨了碳酸盐胶结物形成环境、时期及其对储层物性的影响.安集海河组砂岩碳酸盐胶结物主要为含铁方解石,形成于成岩晚期.受有机质演化及其流体运移的影响,δ13 CPDB值大多为-10‰～-3‰,δ18OPDB值多为-15‰～-10‰.古盐度计算结果表明碳酸盐胶结物形成于淡水环境;大部分方解石为晚期较高温度下形成的.有机酸溶蚀形成的次生孔隙发育带对应于δ13 CPDB值负偏移的井段.碳酸盐胶结物充填粒间孔隙,使储层的有效孔隙度大大降低.     

缅甸睡宝盆地A井区始新统与渐新统碎屑岩储层成岩作用与孔隙演化的差异

缅甸睡宝盆地始新世和渐新世均为辫状河三角洲入海的沉积环境,但是储层物性差异很大。利用钻井的大量岩石薄片、地球化学等测试资料,结合区域地质特征,对该盆地A井区始新统和渐新统储层成岩作用及孔隙演化的差异性进行了分析。研究认为,成岩演化序次特别是碳酸盐胶结的时间早晚、晚期构造压实效应对孔隙度的影响、构造抬升与大气淡水溶解作用的匹配,这三点是造成始新统、渐新统储层物性差异的主要原因。从成岩演化序次和构造压实效应上看,渐新统在岩石没有充分压实之前经历大量的碳酸盐胶结,甚至现今仍保留大量的未遭受溶蚀的碳酸盐胶结物,使得部分颗粒呈基底式胶结。虽然早期的胶结作用充填了部分孔隙,但却在很大程度上阻碍了压实作用的进程,而且为后期次生孔隙的形成奠定了物质基础。而始新统碳酸盐胶结作用发生在岩石充分压实之后,分布在剩余的粒间孔隙中,胶结物含量低。始新统除了接受上覆沉积物的重力压实外,还受到晚期的区域构造挤压应力对始新统形成压实效应,使得原生孔隙度遭到强烈破坏。从构造抬升与大气淡水溶解作用上看,渐新世末期构造抬升而暴露地表接受大气淡水的淋滤,碳酸盐胶结物溶解形成次生孔隙。并且现今A井区古近纪地层处于中成岩A1—A2期,有利于保护次生孔隙。     

NEURAL NETWORK PREDICTION OF PERMEABILITY IN THE EL GARIA FORMATION, ASHTART OILFIELD, OFFSHORE TUNISIA

The Lower Eocene El Garia Formation forms the reservoir rock at the Ashtart oilfield, offshore Tunisia. It comprises a thick package of mainly nummulitic packstones and grainstones with variable reservoir quality. Although porosity is moderate to high, permeability is often poor to fair with some high permeability streaks. The aim of this study was to establish relationships between log‐derived data and core data, and to apply these relationships in a predictive sense to uncored intervals. An initial objective was to predict from measured logs and core data the limestone depositional texture (as indicated by the Dunham classification), as well as porosity and permeability. A total of nine wells with complete logging suites, multiple cored intervals with core plug measurements together with detailed core interpretations were available. We used a fully‐connected Multi‐Layer‐Perceptron network (a type of neural network) to establish possible non‐linear relationships. Detailed analyses revealed that no relationship exists between log response and limestone texture (Dunham class). The initial idea to predict Dunham class, and subsequently to use the classification results to predict permeability, could not therefore be pursued. However, further analyses revealed that it was feasible to predict permeability without using the depositional fabric, but using a combination of wireline logs and measured core porosity. Careful preparation of the training set for the neural network proved to be very important. Early experiments showed that low to fair permeability (1–35 mD) could be predicted with confidence, but that the network failed to predict the high permeability streaks. “Balancing ” the data set solved this problem. Balancing is a technique in which the training set is increased by adding more examples to the under‐sampled part of the data space. Examples are created by random selection from the training set and white noise is added. After balancing, the neural network's performance improved significantly. Testing the neural network on two wells indicated that this method is capable of predicting the entire range of permeability with confidence.     

THE OCCURRENCE AND δ34S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS

Minor amounts of authigenic pyrite are common throughout the Middle Jurassic Brent Group, often being well developed within the delta-plain fades of the Ness Formation. Petrographic analyses of pyrite samples from the Lyell and Murchison fields reveal two stages of pyrite authigenesis, both of which occurred during an early stage of burial. The first phase of pyrite authigenesis is represented by the development of nodules and finely-disseminated cement within shale, mudstone, and siltstone fades, and can include the development of bladed marcasite nodules. These cements are interpreted as having formed within organic-rich sediment soon after deposition, and pre-date all other authigenic precipitates (typically calcite, quartz and kaolinite). “Early” pyrite cements display a wide range of δ³⁴S(-14.9 to +42.5% CDT), and some nodules analysed display isotopically-heavy cores and considerably lighter margins. These isotopically-zoned nodules are problematical. They could be interpreted as forming via progressive “closed system” bacteriogenic reduction and isotopic fractionation of sea-water sulphate, with incorporation of heavy H₂S into the earliest-formed pyrite occurring as a result of its upward diffusion through the sediment column, following isotopic fractionation at depth. However, there are a number of problems with this type of interpretation. These include the fact that diffusion will favour the ³²S isotope, so that the first-formed cements should still be isotopically “light” not “heavy”; also, there is a mass-balance problem, in that no low δ³⁴S sulphides were detected. Coarsely-crystalline pyrite euhedraform nodular cements mainly within sandstones (δ³⁴S -2.6 to +12.8%CDT), and are interpreted as a “later” diagenetic phase. This second phase of sulphide cementation also pre-dates the main phases of quartz and kaolinite authigenesis within sandstones, but post-dates an early phase of kaolinite, and may have been partly coeval with some early calcite authigenesis. High organic contents and the early establishment of reducing conditions led to early formation of pyrite within fine-grained brackish and marine sediments. The lowest measured end-member δ³⁴S of sandstone-hosted pyrite cements is less isotopically depleted than that within fine-grained argillaceous fades, perhaps indicating that sandstone-hosted pyrite cements began to form subsequent to the onset of burial and bacteriogenic reduction within mudstones. Compaction of marine/brackish-water shales and mudstones (or sulphate diffusion from these fades) may have supplied an already isotopically-fractionated source of sulphate to porous sandstone lithologies, where “later” pyrite cements precipitated.     

乐山—龙女寺古隆起洗象池群有利储集层发育机制

四川盆地乐山-龙女寺古隆起受基底隆起的控制,在其形成与演化过程中经历了多期或多旋回的同沉积隆起与剥蚀隆起.该区中上寒武统洗象池群发育一套碳酸盐岩储集层,岩性较为致密,基质孔隙度和渗透率极低,但局部存在高孔隙层和溶洞层,高孔隙层段一般多出现在针孔状白云岩和粗结构白云岩中;裂缝的发育可以极大地改善储集层的渗流条件,高渗透率值往往与裂缝的发育有关,即古岩溶层与构造裂缝共生的区域是储集层发育的有利区带.对储集层建设性的成岩作用主要是沉积期与风化期的岩溶作用.古岩溶斜坡带岩溶孔洞发育,流体渗透量大,岩溶作用强烈.因而,研究区古岩溶风化壳及岩溶斜坡带与构造裂缝发育带共存区是有利储集层的主要发育带.图5表1参11     

STRATIGRAPHY AND RESERVOIR POTENTIAL OF THE MESOZOIC KHORAT GROUP, NE THAILAND: Part 2: Diagenesis and Reservoir Quality

The Mesozoic Khorat Group in NE Thailand consists of a series of continental red-beds, divided here into five formations, which unconformably overlie the lithologically-similar Nam Phong Formation. The reservoir quality of the Khorat Group decreases progressively with increasing age, owing to burial compaction and diagenesis. Compactional fabrics suggests a maximum burial depth of more than seven km, although the Sao Khua Formation (in the middle of the Khorat Group) shows early calcite cements which stabilised the sediment fabric at burial depths of about one km. Porosities vary from 11% in the uppermost part of the Khorat Group (Khok Kruat Formation) to 4.9% in the Nam Phong Formation. The ratio of secondary grain-dissolution porosity to primary porosity increases with age (and depth). Authigenic minerals consist of quartz (1.3 - 9.3%), calcite (0 - 26.5%) and kaolinite (0 - 4.0%). The occurrence of detrital clays (0.8–9.6%) in particular causes downgrading of the porosity of many of the sandstone samples examined.     

柴北缘九龙山地区侏罗系砂岩中碳酸盐胶结物特征及意义

综合运用岩石学、矿物学和地球化学等方法,对柴达木盆地北缘中段九龙山地区侏罗系砂岩中碳酸盐胶结物的成岩温度、形成环境和物质来源等进行了系统研究。结果表明,九龙山地区侏罗系砂岩中存在4种自生的碳酸盐胶结物类型:方解石、含铁方解石、白云石和铁白云石。测定其碳同位素(δ¹³C)值分布范围在-15.1‰～-1.3‰之间,平均值为-6.01‰;氧同位素(δ¹⁸O)值分布范围在-17.8‰～-11.2‰之间,平均值为-15.06‰,推算古盐度(Z)值分布范围为87.71～117.77,平均值为107.5。说明九龙山地区侏罗系砂岩在成岩阶段早期有少量有机碳的加入,且成岩流体主要是来自碎屑析水和大气中的淋滤淡水。碳酸盐胶结物的形成温度在78.5～126.55℃之间,平均值为105.86℃。根据成岩特征及形成温度说明侏罗系砂岩的埋藏深度应该大于2 500m,结合目前侏罗系的埋藏深度在1 600m左右,说明九龙山地区侏罗系砂岩在成岩过程中早期被深埋藏后,又经历了后期构造运动的强烈改造,被抬升1 000m左右。      

DOLOMITIZATION OF THE EARLY EOCENE JIRANI DOLOMITE FORMATION, GABES-TRIPOLI BASIN, WESTERN OFFSHORE, LIBYA

Dolomitization in the early Eocene Jirani Formation in the Gabes‐Tripoli Basin (offshore western Libya) occurred in two stages. Stage I dolomites are composed of two types, one associated with anhydrite (Type I) the other anhydrite free (Type II,). The stratigraphic and sedimentological settings together with petrographic and geochemical criteria suggest that dolomitization was effected by refluxed evaporative seawater. Stable isotope and trace element analyses suggest dolomitization of both Types from a fluid of near‐surface seawater composition under oxidising conditions modified by evaporation. Non‐luminescence and lack ofzonation of all the dolomite indicate that the dolomitizing fluids maintained a relatively constant composition. The geologic setting during the early Eocene, interpreted as hypersaline lagoon, supports an evaporative reflux origin for the anhydritic dolomite Type I. Type II developed under less saline conditions in the transition zone between lagoon and open marine shelf. Stage II dolomitization is recorded by negative isotope values in both Types I and II indicating their dissolution and recrystallization (neomorphism) by dilute solutions. A period of exposure of the overlying Jdeir Formation following a relative sea‐level fall allowed ingress of meteoric waters into both the Jdeir and the underlying Jirani Formations. Flushing by meteoric waters also resulted in development of excellent secondaly porosity and caused major dissolution of anhydrite to form the anhydritic‐free dolomite facies typical of Type II. Following, and possibly during, both Stages I and II, low temperature dolomites (Type IIIa) precipitated in pore spaces from residual jluids at shallow burial depths, partially occluding porosity. In the late stage of basin evolution, medium clystalline, pore‐filling saddle dolomite precipitated, causing some filling of mouldic and vuggy porosity (Type IIIb). Very light oxygen isotopic signatures confirm that it developed from high temperature fluids during deep burial diagenesis. Calculation of temperatures and timings of the dolomitization and cement phases show that the main dolomitization phases and Type IIIa cements occurred in the early Eocene, and that the saddle dolomite precipitated in the Miocene; these results are consistent with age relationships established from stratigraphic, petrographic and geochemical signatures. The most common porosity includes intercrystal, vuggy and mouldic types. Porosity is both pre‐dolomitization and syn‐dolomitization in origin, but the latter is the most dominant. Hence, reservoir quality is largely controlled by fluid dynamics.     

鄂尔多斯盆地上古生界砂岩储层方解石胶结物特征

鄂尔多斯盆地南部上古生界山西组—下石盒子组砂岩储层岩性致密,方解石的后期胶结作用是导致储层致密化的重要因素之一.镜下观察、包裹体测温、氧碳同位素分析和电子探针微量元素分析证实,方解石主要形成于成岩晚期,在遭受强压实作用和早期胶结作用之后,储集岩的孔隙度和渗透性降低,成岩环境开放性变差,伴随着有机质的降解而形成大量的二氧化碳,造成方解石沉淀.这种后期的方解石胶结物主要充填剩余粒间孔隙,使储层的有效孔隙度大大降低.并且由于方解石胶结物形成时代晚,其后储层未受到其他建设性成岩作用的明显改造,所以方解石胶结物的发育程度对储集物性具有明显的控制作用.     

不同埋藏深度交代白云石晶体结构及其对白云岩储层研究的意义

交代白云石化作用是除了原生沉淀外白云石的主要成因,对于不同埋藏深度的交代白云石晶体结构的研究有助于分析白云石被交代的过程及主要影响因素。选取加勒比海Cayman岛中新统Cayman组、南海中新统宣德组和四川盆地上震旦统灯影组3类保留了原岩颗粒结构的交代白云石进行研究,3类白云石均未受到陆源物质影响。研究手段为:X衍射分析成分、有序度、晶胞参数;透射电镜观察晶体微形貌特征。随着埋藏深度从几米至几百米再至几千米依次加深,晶体自形程度逐渐变差,反映了埋藏成岩作用对交代白云石自由生长具有抑制作用;白云石有序度逐渐升高,晶胞参数逐渐向理想晶胞参数值接近,反映了后期稳定的埋藏成岩作用有利于白云石向有序转化。白云石的晶形与有序度之间不具有必然联系。通过X衍射发现,Cayman组白云岩样品中含有高镁方解石(HMC)和高钙白云石(HCD)、低钙白云石(LCD),其岩石学特征和晶体结构特征反映了交代白云石的形成是从最初的高镁方解石转化为高钙的白云石,再转化为低钙的白云石,交代白云石化可以被认为是镁离子逐渐进入方解石晶格的过程。经历足够长时间的稳定演化后,逐渐向理想白云石接近。3类交代白云石的镁离子来源为海水。表生岩溶作用不仅对于交代白云石化作用具有一定的催化作用,且经历白云石化后的岩石有利于保证原有的孔隙不易被破坏,形成了良好的储集空间,有利于后期油气的保存和运移。     

渤中坳陷古近系中、深部碎屑岩储层碳酸盐胶结物分布特征及成因机制

碳酸盐胶结物是渤中坳陷古近系砂岩储层中最重要的胶结物.薄片分析表明,中、深部储层中碳酸盐胶结物主要为菱铁矿、白云石、方解石、铁白云石、铁方解石.统计显示,随着层位和深度加深,碳酸盐含量逐渐增加,含量突增段主要发生在东营组;碳酸盐含量超过10%,对储层物性影响较大.由薄片分析及碳、氧同位素分析可知,菱铁矿多以孔隙充填和颗粒环边方式产出,主要形成于同生到早成岩早期;方解石主要以粒状或呈连生胶结方式产出,方解石相对含量与δ¹⁸O值呈明显的负相关关系,说明大部分方解石形成较晚;铁方解石主要以粒状产出,以铁方解石为主的样品对应着较低的δ¹⁸O值,且沉淀温度高;白云石主要以斑块状充填粒间孔隙,其相对含量与δ¹⁸O值呈明显正相关性,且沉淀温度较低、形成较早;铁白云石主要以分散的粒状产出,其相对含量与δ¹⁸O值呈明显的正相关关系,且沉淀温度相对较低.     

准噶尔盆地腹部西侧侏罗系三工河组储层成岩作用及控制因素

准噶尔盆地现今的地温梯度低(约为2.3℃/hm),其腹部西侧侏罗系三工河组辫状河三角洲砂体的埋深差异极大(约为2100 m)、成岩作用复杂,影响油气勘探开发部署.通过岩石薄片、阴极发光、扫描电镜和包裹体荧光显微鉴定,结合黏土矿物的演化特征,深入分析了准噶尔盆地腹部西侧沙窝地、莫西庄及征沙村3个小区三工河组储层的成岩作用...