RESERVOIR PROPERTIES OF BARREMIAN – APTIAN URGONIAN LIMESTONES,SE FRANCE,PART 1: INFLUENCE OF STRUCTURAL HISTORY ON POROSITY-PERMEABILITY VARIATIONS

Upper Barremian – Lower Aptian inner platform “Urgonian” limestones in the Mont de Vaucluse region, SE France, consist of alternating metre-scale microporous and tight intervals. This paper focuses on the influence of structural deformation on the reservoir properties of the Urgonian limestone succession in a study area near the town of Rustrel. Petrographic, petrophysical and structural data were recovered from five fully-cored boreholes, from the walls of a 100 m long underground tunnel, and from a 50 m long transect at a nearby outcrop. The data allowed reservoir property variations in the Urgonian limestones to be studied from core to reservoir scale. Eleven Reservoir Rock Types (RRTs) were identified based on petrographic features (texture, grain size), reservoir properties (porosity, permeability), and the frequency of structural discontinuities such as fractures, faults and stylolites. Tight and microporous reservoir rock types were distinguished. Tight reservoir rock types were characterised by early cementation of intergranular pore spaces and by the presence of frequent structural discontinuities. By contrast microporous reservoir rock types contained preserved intragranular microporosity and matrix permeability, but had very few structural discontinuities. Observed vertical alternations of microporous and tight rock types are interpreted to have been controlled by the early diagenesis of the Urgonian carbonates. Deformation associated with regional-scale tectonic phases, including Albian – Cenomanian “Durancian” uplift (∼105 to 96 Ma) and Pyrenean compression (∼55 to 25 Ma), resulted in the modification of the initial petrophysical properties of the Urgonian limestones. An early diagenetic imprint conditioned both the intensity of structural deformations and the associated circulations of diagenetic and meteoric fluids. Evolution of the Reservoir Rock Types is therefore linked both to the depositional conditions and to subsequent phases of structural deformation.     

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.     

西湖凹陷平湖组砂岩中碳酸盐胶结物形成机制及其对储层质量的影响

碳酸盐胶结物是东海西湖凹陷平湖组储层砂岩中最为重要的自生矿物。薄片鉴定、阴极发光、元素成分及碳氧同位素分析结果表明,西湖凹陷平湖组储层砂岩中碳酸盐胶结物的主要矿物类型是菱铁矿、铁白云石和方解石。其中菱铁矿胶结物主要形成于同生到早成岩早期,具有最高的δ^13和δ^18值,分别为0.75‰和-10.22‰;铁白云石胶结物的δ^13接近同期海水,变化于-2.45‰和-2.98‰之间,δ^18值偏负,最低达-12.39‰,其形成于相对较早的成岩时间。方解石胶结物主要以粒状和连生胶结方式产出,粒状方解石具有复杂的多种成因机制,而连生方解石具有最低的δ^13值（-6.02‰）和δ^18值（-16.94‰）,主要形成于成岩晚期,其成因与有机酸溶解长石等铝硅酸盐矿物及粘土矿物转化作用有关。在对储层质量的影响方面,早期形成的菱铁矿和铁白云石胶结物增加了岩石的机械强度和抗压实能力,对研究区储层发育的影响是正面的,而晚期形成的连生方解石胶结物普遍占据长石溶解空间,对储层的发育是不利的。     

大港滩海地区沙一段下部砂岩储层中方解石胶结物碳、氧同位素研究

对大港滩海地区沙一段下部砂岩储层中方解石胶结物的12个样品进行了碳、氧同位素分析,结果表明碳同位素δ13C值分布范围在-9.58‰～+13.61‰,平均为5.29‰;氧同位素δ18O值分布范围在-12.46‰～+0.09‰,平均为-6.28‰.探讨了碳酸盐岩的成岩流体盐度及碳的来源和成因等问题,认为本区的方解石胶结物成岩流体为高盐度流体,方解石胶结物主要来自层内碎屑碳酸盐颗粒或者邻近碳酸盐岩层的溶解,其碳来源主要为还原有机碳.      

黔西北毕节地区中二叠统碳酸盐岩成岩作用

黔西北毕节地区中二叠统栖霞组和茅口组碳酸盐岩发育,地层出露良好。通过野外剖面的观察和实测,室内铸体薄片的鉴定、阴极发光分析以及碳氧同位素分析,结合区域地质背景和前人研究成果,认为黔西北毕节地区中二叠统碳酸盐岩的成岩作用具有如下特征：①中二叠统主要发育颗粒结构的灰岩和白云质灰岩（豹斑灰岩）,岩石样品整体显示不发光—弱发光的阴极发光特征,碳同位素比值为1.2‰~4.6‰,表明岩石未受大气淡水的显著影响;②中二叠统碳酸盐岩受成岩改造作用强烈,发育海水潜流环境的泥晶化作用、海水渗流环境的等厚环边和马牙状方解石胶结作用、淡水渗流和潜流环境的等轴粒状—块状胶结和方解石的共轴生长、以及硅化作用、白云化作用和白云石的溶解作用、破裂作用、压实压溶作用等成岩作用,据此建立了相应的成岩演化序列;③硅化作用和白云化作用可能受到局部特殊热事件的影响,但具体的影响程度需要进一步的研究。     

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.     

DIAGENESIS OF SILURIAN REEFAL CARBONATES, KUDIRKA OILFIELD, LITHUANIA

Upper Silurian carbonate reservoir rocks at the Kudirka oilfield, southern Lithuania, have a complex diagenetic history. This consists of the following stages: compaction and dewatering; early lithification; minor fracturing? first generation leaching; precipitation of first‐ and second‐generation inter‐ and intragranular calcite cements; recrystallization of lime mud, sparry calcite cement and bioclastic grains; chemical compaction and the formation of stylolite‐associated fractures; precipitation of dolomite, pyrite and silica; a second phase of leaching, with stylolite surfaces acting as conduits for aggressive fluids; and finally oil emplacement. The Upper Silurian carbonates are interpreted to correspond to a variety of reefal deposits. In all the types of reef rock analysed, late diagenetic leaching has enhanced reservoir quality. However, no clear correlation could be found between rock texture and reservoir quality. This is mainly because the very common crinoid fragments acted as substrates for the growth of pore‐occluding syntaxial calcite cement. The lack of correlation between rock texture and reservoir quality can also be attributed to subordinate localized stylolite‐related dolomitization.     

IMPACT OF DIAGENESIS ON RESERVOIR-QUALITY EVOLUTION IN FLUVIAL AND LACUSTRINE-DELTAIC SANDSTONES: EVIDENCE FROM JURASSIC AND TRIASSIC SANDSTONES FROM THE ORDOS BASIN, CHINA

The reservoir quality of Jurassic and Triassic fluvial and lacustrine‐deltaic sandstones in the intracratonic Ordos Basin is strongly influenced by depositional facies and various types of diagenetic modifications. The fluvial sandstones have higher average He‐porosity and permeability (14.8% and 12.7 mD, respectively) than the deltaic sandstones (9.8% and 5.8 mD, respectively). In addition to extensive mechanical compaction, eodiagenesis (220–97 Ma; depth < 2000 m; T < 70°C) has resulted in dissolution and kaolinitization of detrital silicates in the Jurassic fluvial sandstones, and in smectite infiltration and minor cementation by calcite and siderite in the Triassic fluvial and deltaic sandstones. Pervasive eogenetic carbonate cementation (> 20 vol.%) occurred in Triassic deltaic siltstones and very fine‐grained sandstones which are closely associated with organic‐rich mudstones. Mesodiagenesis (97–65 Ma), which occurred during rapid subsidence to depths of 3700–4400 m, resulted in the albitization of plagioclase, checmical compaction, the conversion of kaolinite into dickite, and cementation by quartz overgrowths, chlorite, illite, ankerite (δ¹³C_VPDB=?2.4‰ to +2.6‰; δ¹⁸O_VPDB=?21.5‰ to ?10‰) and calcite (δ¹³C_VPDB=?4.7‰ to +3.7‰; δ¹⁸O_VPDB=?21.8‰ to ?13.4‰). Oil emplacement (95 Ma) retarded cementation by mesogenetic quartz and carbonate but had little influence on dickite, illite and chlorite formation. Retardation of quartz cementation was also due to the presence of chlorite fringes around detrital quartz grains. Dickitization of eogenetic kaolinite together with the short residence time at maximum burial temperatures (105–124°C) has retarded the albitization of K‐feldspars and illite formation and hence prevented severe permeability destruction. Telodiagenesis, which occurred after uplift (Eocene to end‐Neogene), caused slight dissolution and kaolinitization of feldspars. This study demonstrates that despite complex patterns of diagenetic modifications in the Triassic and Jurassic successions, depositional porosity and permeability are better preserved in fluvial meandering channel sandstones than in deltaic sandstones. These results should be important for modelling of reservoir‐quality distribution and exploration risk evaluation in the basin.     

DIAGENETIC-GEOCHEMICAL PATTERNS AND FLUID EVOLUTION HISTORY OF A LOWER JURASSIC PETROLEUM SOURCE ROCK, MIDDLE ATLAS, MOROCCO

Pliensbachian to earliest Toarcian marls and argillaceous limestones exposed at the surface near Aït Moussa (Boulemane Province, Middle Atlas) include the only examples of effective petroleum source rocks so far known in the Moroccan Atlas rift basins. The outcrop interval includes hemipelagic, peri‐Tethyan low latitude source rocks with Type II kerogen (total thickness of 2.5 m with mean TOC of around 3.2%). Early diagenetic, anoxic remineralisation of sedimentary organic matter resulted in hydraulic fractures, calcite cementation, a negative shift of carbon and oxygen stable isotopes relative to marine values (Δδ¹³C = ‐1.1; Δδ¹⁸O = ‐2.0), framboidal pyrite, and relative enrichment of the middle weight rare earth elements (REE). In combination, these attributes of early diagenesis may assist in the identification of other source rock intervals of similar age and setting. Progressive burial produced three generations of Fe‐calcite cemented veins, followed by three generations of replacive dolomite and concluded by ankerite replacing dolomite. Compaction fluids initially caused a slight positive shift of δ¹³C values (Δδ¹³C =+0.4), a flattening of the REE distribution pattern and an increase in REE content, together related to the dewatering of clay. Dolomitizing fluids (dol‐2 and dol‐3) record a positive shift of δ¹³C values (Δδ¹³C =+0.9) suggesting the effects of methanogenesis or an uptake of heavy δ¹³C from underlying rock formations during fluid migration. Dol‐3 is an Fe‐bearing saddle dolomite that carries a positive Eu‐anomaly (Eu/Eu*= 8.1) best explained by ascending hydrothermal fluids which are presumably of Middle Eocene age. A first migration of bitumen is recorded as fluorescent inclusions in dol‐2 (Late Jurassic ‐ Cretaceous), but bituminous fluids remained normally pressured until the establishment of inclined stylolites during Late Eocene tectonic compression. Comparative organic‐geochemical analyses (GC, NMR of inclusions, non‐expelled and expelled bitumen) indicate that thermal maturation advanced significantly after the onset of migration. Differences in terms of Pr/n‐C₁₇, Ph/n‐C₁₈ ratios and aromaticity corroborate the effects of differential expulsion. An exploration strategy should consider both secondary migration via opened tectonic stylolites in association with late‐diagenetic fractures and a persisting tightness that then could have created an unconventional oil reservoir.     

Diagenesis and Its Effect on Reservoir Quality of Silurian Sandstones,Tabei Area,Tarim Basin,China

The diagenetic processes of the Tabei sandstones in the Tarim Basin include compaction, cementation (quartz overgrowths, calcite, clay minerals and a minor amount of pyrite), and dissolution of the feldspar and calcite cement. Porosity was reduced by compaction from an assumed original 40% to about 22.1%. Cementation reduced porosity to 26.6%. The Tabei sandstones lost a little more porosity by compaction than by cementation. Quartz cementation, especially syntaxial quartz overgrowth, is a major cause of porosity-loss in many reservoirs in moderately to deeply buried sandstone. Calcite cementation played a key role in the porosity evolution of sandstones. At the early stage of burial, the early calcite cement occupied most of the pore spaces resulting in significant porosity. On the other hand, some primary porosity has been preserved due to incomplete filling or the presence of scattered patches of calcite cement. In addition to calcite, several clay minerals, including illite and chlorite occurred as pore-filling and pore-lining cements. The pore-lining chlorite may have helped in retaining the porosity by preventing the precipitation of syntaxial quartz overgrowths. Illite, which largely occurred as hair-like rims around the grains and bridges on the pore throats, caused a substantial deterioration of penetrability of the reservoir. Calcite cement dissolution was extensive and contributed significantly to the development of secondary porosity.     

DIAGENETIC EVOLUTION OF INCISED CHANNEL SANDSTONES: IMPLICATIONS FOR RESERVOIR CHARACTERISATION OF THE LOWER CARBONIFEROUS MARAR FORMATION, GHADAMES BASIN, WESTERN LIBYA

This paper assesses the diagenetic history of potential fluvial hydrocarbon reservoir rocks deposited within incised valley systems of the Lower Carboniferous Marar Formation in western Libya. Outcrop data were collected in the Tinedhan Anticline, located at the southern margin of the Ghadames Basin. Four discrete intervals with channelized sandstones were identified in a section dominated by alternating offshore mudstones and shallow-marine clastics. The incised channels were cut during major sea-level lowstands, and filled by fluvial sandstone packages up to 50 m thick. Fifty-eight samples from four different localities, representing three lowstand systems tracts, were analysed to obtain a statistically meaningful mineralogical and compositional dataset. In addition to burial compaction, three main diagenetic events influenced the reservoir quality of the sandstones. Firstly, early eodiagenesis involved kaolinitization of plagioclase grains. This began before subsequent calcite cementation, probably as a result of flushing by meteoric pore-waters. The deformation of kaolinite during later compaction resulted in the formation of pseudomatrix which further reduced porosity and permeability. Kaolinite is commonly transformed to illite at temperatures above 140°C in the presence of K-feldspar. Although K-feldspar was recorded in the samples, no illite was observed, suggesting that the Lower Carboniferous strata in the study area were not buried in excess of approximately 3.5 km. The second diagenetic phase was the precipitation of calcite cement, present either dispersed throughout the sandbodies or as concretions up to 2 m across, in both cases reducing reservoir quality. The high intergranular volumes (IGV) of calcite-cemented sandstones (ranging between 35% and 40%) suggest that cementation occurred at burial depths of <500 m. Sandstones without calcite cement have lower IGV of between 17% and 25% as a result of mechanical and chemical compaction. Stable C and O isotope analysis of the calcite cement also supports precipitation at shallow burial depths, indicating a meteoric pore-water source for the calcite. The third and final diagenetic stage was partial chloritisation of kaolinite during meso-diagenesis. The elevated temperatures required for this transformation indicate burial to a minimum depth of approximately 2.5 km, which is consistent with the compaction data. Despite these diagenetic effects, the fluvial sandstones have an average porosity of 12%, with a range from 0.5% up to 25%. Permeability measurements on four sandstone samples indicate that the development of pseudomatrix did not reduce permeability significantly.     

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.

     

塔里木盆地英南2气藏成藏机理

探讨了塔里木盆地东部英南2凝析气藏的非常规盖层———致密钙质胶结砂岩的形成机理。这种致密砂岩孔隙中钙质胶结物共有两期,早期方解石阴极发光下发出桔红色光,晚期发出桔黄色光,早期方解石被晚期交代。碳酸盐岩胶结物微区和全岩碳、氧同位素分析表明,钙质胶结物碳同位素组成表现出与生物成因有关的负值,δ¹³C分布在-7.17‰～-9.06‰,氧同位素组成具有淡水渗流成岩的特点,δ¹⁸O分布在-12.44‰～-21.85‰,这说明致密砂岩是"与烃类相关的成岩作用带(HRDZs)"的产物。HRDZs的形成是烃类缓慢渗漏遭受生物降解的过程,它使气藏的天然气组分的成分和同位素组成产生了分异。该气藏是特殊的下生上储、早期散失、晚期聚集成藏的次生凝析气藏,成藏过程伴随着圈闭致密性的逐渐加强,气藏的成藏过程是有机无机相互作用的结果。     

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.     

湖北宜昌地区下奥陶统碳酸盐岩成岩作用

湖北宜昌地区下奥陶统以碳酸盐岩沉积为主,成岩作用主要有胶结、压实、泥晶化、新生变形、硅化、溶解和白云石化作用,以胶结作用和白云石化作用最为广泛。成岩环境为淡水渗流带、淡水潜流带、淡水-海水混合带、海水潜流带与埋藏带。大多数成岩变化发生于淡水潜流带和混合带。     

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.     

松辽盆地梨树断陷十屋油田区营城组、沙河子组储层成岩作用分析

通过大量钻井岩心铸体薄片、扫描电镜、压汞、阴极发光、X-衍射、包裹体等分析,对松辽盆地梨树断陷十屋油田区沙河子组、营城组储层的成岩作用、成岩序列及孔隙演化进行了深入研究.该区主要成岩作用有压实作用、胶结作用、溶蚀作用及构造破裂作用,其中对储层物性具建设性的作用主要有溶蚀作用和构造破裂作用,而具破坏性作用的主要有压实作用和胶结作用.砂岩储层孔隙类型主要为原生粒间孔、粒间溶蚀扩大孔及少量裂缝的组合,以溶蚀孔隙为主.建立了砂岩储层的成岩序列,即机械压实→石英、长石次生加大→构造破裂→长石等铝硅酸盐溶蚀→粒状方解石胶结→早期进油→破裂→第二期溶蚀→连晶方解石胶结→晚期油气充注.研究认为,影响本区储层储集性能的主控因素为储层的沉积环境、成岩作用、构造作用及油气充注等因素.      

柴北缘腹部砂岩中碳酸盐胶结物特征及成因探讨

利用岩心观察和描述、镜下鉴定、碳氧同位素分析等手段,结合研究区沉积环境和构造背景,对研究区侏罗系—新近系储层砂岩中碳酸盐胶结物的类型、期次、分布形态、地球化学特征、成因机制等方面进行了详细的研究。结果显示：柴北缘腹部地区中新生界砂岩主要胶结物类型为碳酸盐胶结,胶结物分为早、中、晚3期,早期碳酸盐胶结物包括方解石、白云石、菱铁矿,中期主要是方解石和含铁方解石,晚期为含铁白云石。侏罗纪柴北缘腹部地区经历了由封闭咸湖向开放型湖盆的转变过程,δ¹³C值为-16.77‰~8.01‰和δ¹⁸O值为-18.52‰~-8.34‰,古盐度值（Z）和古温度分别为86.50~133.72 ℃和67.81~142.19 ℃,成岩阶段处于早成岩阶段B期—中成岩阶段B期。古近系（E₁₊₂、E₃¹、E₃²）形成于开放水体环境,δ¹³C值为-8.71‰~-2.40‰,δ¹⁸O值为-16.62‰~-8.77‰,古盐度和古温度分别为102.85~116.16 ℃和63.03~117.28 ℃,成岩阶段为早成岩阶段B期—中成岩阶段A期。新近系上干柴沟组（N₁）和下油砂山组（N₂¹）也形成于开放水体环境,δ¹³C值为 -6.81‰~ -3.80‰和δ¹⁸O值为-12.73‰~-6.13‰,古盐度和古温度值分别为107.65~114.89 ℃和25.54~88.93 ℃,成岩演化阶段处于早成岩阶段A期—早成岩阶段B期。不同层位碳、氧同位素含量基本呈现出相同的变化趋势,但是仍存在个别点呈现相反的趋势,可能与大气淡水有关。其碳酸盐胶结物成因机制既包括有机成因也包括无机成因,早期主要受大气淡水淋滤作用,中期主要是有机质热解脱羧作用提供碳源,晚期主要为生物成因碳酸盐,同时也含有少量有机碳源。     

鄂尔多斯盆地红河油田长8致密砂岩储层成岩特征

综合利用铸体薄片、扫描电镜和电子探针等分析化验资料,对鄂尔多斯盆地红河油田三叠系延长组长8致密砂岩储层的成岩作用类型、分布特征及主控因素进行了系统研究。研究区长8储层目前处于中成岩A阶段,主要成岩类型为压实作用、溶解作用及2期方解石、2期高岭石、3期绿泥石胶结作用。独立型薄层砂体分布在河道边部和河道上游,其塑性矿物含量高,以强压实作用为特征,溶解作用和胶结作用较弱,发育强压实成岩相。复合型厚层砂体分布在河道中心,其中顶部和底部砂体以强方解石胶结为特征;中部砂体根据砂体岩相类型存在多个成岩序列和成岩相,其中富含刚性矿物中-细砂岩岩相主要发育绿泥石胶结中溶解成岩相,过渡型细砂岩岩相由于早期方解石胶结程度差异,可发育方解石中胶结成岩相或高岭石中胶结成岩相。