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1.
This paper analyses the diagenetic evolution of sandstones belonging to the Bajo Barreal Formation (Cretaceous) in the Golfo de San Jorge Basin (Patagonia, Argentina). The Bajo Barreal Formation includes the main reservoirs, which are located along the western area of the basin and is composed of sandstones, conglomerates, mudstones, tuffaceous mudstones and some layers of tuffs. The principal reservoirs comprise medium-to coarse-grained sandstones, which are dominated by feldspathic litharenites and contain minor amounts of litharenites and lithic arkoses. The authigenic minerals include kaolinite, smectite, chlorite, quartz overgrowths, microquartz and calcite, with minor proportions of megaquartz, siderite, analcime, laumontite, feldspar overgrowths and illite/smectite and chlorite/smectite mixed layers. Secondary porosity is much more important than primary porosity and is produced by the dissolution of feldspar, lithic clasts and clay cements. The diagenetic history of the Bajo Barreal sandstones can be divided into seven diagenetic stages, each of which is characterized by a specific assemblage of authigenic minerals and diagenetic processes. Eogenetic conditions occur in stages 1, 2, 3 and 4. Stage 1 corresponds to shallow burial characterized by the physical reduction of primary porosity by compaction; during stage 2, rim clay cements of chlorite, smectite and clinoptilolite, as well as thin quartz overgrowths, were formed. The precipitation of pore-filling cements of kaolinite, chlorite and smectite occurred during stage 3, while stage 4 records the intense dissolution of feldspar, lithic fragments and kaolinite cements. Mesogenesis occurs in diagenetic stages 5 and 6. The former corresponds to a new phase of authigenic kaolinite, while the latter records the significant dissolution of feldspar, lithic clasts and previous cements, which produced the highest values of secondary porosity. Finally, stage 7 corresponds to the highest degree of diagenesis in the Bajo Barreal Formation (mesogenesis), which resulted in the precipitation of cements of zeolites and calcite, as well as quartz and plagioclase overgrowths. 相似文献
2.
A detailed laboratory study of 53 sandstone samples from 23 outcrops and 156 conventional core samples from the Maastrichtian-Paleocene Scollard-age fluvial strata in the Western Canada foredeep was undertaken to investigate the reservoir characteristics and to determine the effect of diagenesis on reservoir quality. The sandstones are predominantly litharenites and sublitharenites, which accumulated in a variety of fluvial environments. The porosity of the sandstones is both syn-depositional and diagenetic in origin. Laboratory analyses indicate that porosity in sandstones from outcrop samples with less than 5% calcite cement averages 14%, with a mean permeability of 16 mD. In contrast, sandstones with greater than 5% calcite cement average 7.9% porosity, with a mean permeability of 6.17 mD. The core porosity averages 17% with 41 mD permeability. Cementation coupled with compaction had an important effect in the destruction of porosity after sedimentation and burial. The reservoir quality of sandstones is also severely reduced where the pore-lining clays are abundant (>15%). The potential of a sandstone to serve as a reservoir for producible hydrocarbons is strongly related to the sandstone’s diagenetic history. Three diagenetic stages are identified: eodiagenesis before effective burial, mesodiagenesis during burial, and telodiagenesis during exposure after burial. Eodiagenesis resulted in mechanical compaction, calcite cementation, kaolinite and smectite formation, and dissolution of chemically unstable grains. Mesodiagenesis resulted in chemical compaction, precipitation of calcite cement, quartz overgrowths, and the formation of authigenic clays such as chlorite, dickite, and illite. Finally, telodiagenesis seems to have had less effect on reservoir properties, even though it resulted in the precipitation of some kaolinite and the partial dissolution of feldspar. 相似文献
3.
渐新世花港组是东海陆架盆地西湖凹陷发育的最主要储层，基于普通薄片、铸体薄片、扫描电镜和荧光显微观察，结合同位素地球化学对东海陆架盆地西湖凹陷花港组砂岩储层的成岩作用、成岩序列及成岩流体演化进行了研究。结果表明，花港组砂岩储层目前处于中成岩阶段B期，主要经历了机械压实、绿泥石粘土摸、酸性及碱性溶蚀作用，石英次生加大，碳酸盐胶结和自生高岭石胶结等成岩作用。研究区发育有三期碳酸盐胶结物，早期菱铁矿胶结物，中期铁方解石和晚期铁白云石。根据碳酸盐胶结物的碳氧同位素特征分析认为早期碳酸盐胶结物是由过饱和的碱性湖水沉淀造成的，而晚期碳酸盐胶结物的形成与有机酸密切相关。研究区存在两类溶蚀作用，酸性溶蚀作用和碱性溶蚀作用，早期的酸性溶蚀作用主要是有机酸对长石、岩屑及早期碳酸盐胶结物的溶蚀，晚期的碱性溶蚀作用主要是发生于碱性环境下流体对石英及硅质胶结物的溶蚀。研究区发育有两期油气充注，早期发生于晚中新世，早期发生于晚中新世，早于中期碳酸盐胶结，晚于长石溶蚀和石英胶结充注，充注量较大，第四纪以来研究区发生了第二次充注，第二次充注发生于铁白云石胶结之后，此时储层已非常致密。 相似文献
4.
5.
鄂尔多斯盆地王窑地区上三叠统长6油层成岩作用研究 总被引：15，自引：0，他引：15
通过多项测试方法，对安塞油田王窑地区长6油层含油砂体的岩石学、成岩作用、储集物性和孔隙发育特征进行分析和研究。结果表明，该区储集砂体为成分成熟度较低的长石砂岩；主要自生矿物为绿泥石、浊沸石、方解石、石英、钾长石、伊利石和钠长石等；储层次生孔隙发育，主要孔隙类型为粒间孔隙、骨架颗粒溶孔和浊沸石溶孔。孔隙结构具有小孔、细喉的特点；储层成岩演化阶段处于晚成岩A亚期；储层性质明显地受到沉积微相和成岩作用的影响。沉积物粒度较粗、厚度较大的河道砂和河口砂坝砂的储集物性明显优于各种粒度较细、厚度较薄的席状砂体。 相似文献
6.
Composition, porosity, and reservoir potential of the Middle Jurassic Kashafrud Formation, northeast Iran 总被引：1，自引：0，他引：1
In the Kopet-Dagh Basin of Iran, deep-sea sandstones and shales of the Middle Jurassic Kashafrud Formation are disconformably overlain by hydrocarbon-bearing carbonates of Upper Jurassic and Cretaceous age. To explore the reservoir potential of the sandstones, we studied their burial history using more than 500 thin sections, supplemented by heavy mineral analysis, microprobe analysis, porosity and permeability determination, and vitrinite reflectance.The sandstones are arkosic and lithic arenites, rich in sedimentary and volcanic rock fragments. Quartz overgrowths and pore-filling carbonate cements (calcite, dolomite, siderite and ankerite) occluded most of the porosity during early to deep burial, assisted by early compaction that improved packing and fractured quartz grains. Iron oxides are prominent as alteration products of framework grains, probably reflecting source-area weathering prior to deposition, and locally as pore fills. Minor cements include pore-filling clays, pyrite, authigenic albite and K-feldspar, and barite. Existing porosity is secondary, resulting largely from dissolution of feldspars, micas, and rock fragments, with some fracture porosity. Porosity and permeability of six samples averages 3.2% and 0.0023 mD, respectively, and 150 thin-section point counts averaged 2.7% porosity. Reflectance of vitrinite in eight sandstone samples yielded values of 0.64-0.83%, in the early mature to mature stage of hydrocarbon generation, within the oil window.Kashafrud Formation petrographic trends were compared with trends from first-cycle basins elsewhere in the world. Inferred burial conditions accord with the maturation data, suggesting only a moderate thermal regime during burial. Some fractures, iron oxide cements, and dissolution may reflect Cenozoic tectonism and uplift that created the Kopet-Dagh Mountains. The low porosity and permeability levels of Kashafrud Formation sandstones suggest only a modest reservoir potential. For such tight sandstones, fractures may enhance the reservoir potential. 相似文献
7.
盐城凹陷天然气储层为成分成熟度很低的砂砾岩、砾状砂岩、砂岩和少量粉砂岩,砂岩类型主要为长石岩屑砂岩和少量岩屑长石砂岩。成岩矿物主要类型有粘土矿物、石英、钠长石、碳酸盐、硬石膏和石膏等。储层以次生孔隙发育为特点,以粒间孔隙和骨架颗粒溶孔最为发育,溶解作用发育程度与泥质岩在成岩过程中粘土矿物和有机质的演化关系极其密切。盐城天然气进入储层发生在始新世———新近纪,主要成藏期发生在4.5~10Ma。天然气储层成岩演化阶段处于晚成岩A亚期。储层性质明显地受到沉积相和成岩作用的影响。沉积物粒度较粗、厚度较大的河道砂的储集物性明显优于各种粒度较细、厚度较薄的席状砂体。 相似文献
8.
The discovery of deep (20,000 ft) gas reservoirs in eolian sandstone of the Upper Jurassic Norphlet Formation in Mobile Bay and offshore Alabama in the late 1970s represents one of the most significant hydrocarbon discoveries in the nation during the past several decades. Estimated original proved gas from Norphlet reservoirs in the Alabama coastal waters and adjacent federal waters is 7.462 trillion ft3 (Tcf) (75% recovery factor). Fifteen fields have been established in the offshore Alabama area. Norphlet sediment was deposited in an arid environment in alluvial fans, alluvial plains, and wadis in updip areas. In downdip areas, the Norphlet was deposited in a broad desert plain, with erg development in some areas. Marine transgression, near the end of Norphlet deposition, resulted in reworking of the upper part of the Norphlet Formation. Norphlet reservoir sandstone is arkose and subarkose, consisting of a simple assemblage of three minerals, quartz, albite, and K-feldspar. The present framework grain assemblage of the Norphlet is dominantly diagenetic, owing to albitization and dissolution of feldspar. Despite the simple framework composition, the diagenetic character of the Norphlet is complex. Important authigenic minerals include carbon ate phases (calcite, dolomite, Fe-dolomite, and breunnerite), feldspar (albite and K-feldspar), evaporite minerals (anhydrite and halite), clay minerals (illite and chlorite), quartz, and pyrobitumen. The abundance and distribution of these miner als varies significantly between onshore and offshore regions of Norphlet production. The lack of sufficient internal sources of components for authigenic minerals, combined with unusual chemical compositions of chlorite (Mg-rich), breunnerite, and some minor authigenic minerals, suggests that Louann-derived fluids influenced Norphlet diagenesis. In offshore Alabama reservoirs, porosity is dominantly modified primary poros ity. Preservation of porosity in deep Norphlet reservoirs is due to a combination of factors, including a lack of sources of cement components and lack of pervasive early cement, so that fluid-flow pathways remained open during burial. Below the dominantly quartz-cemented tight zone near the top of the Norphlet, pyrobitumen is a major contributor to reduction in reservoir quality in offshore Alabama. The highest reservoir quality occurs in those wells where the present gas water contact is below the paleohydrocarbon water contact. This zone of highest reservoir quality is between the lowermost occurrence of pyrobitumen and the present gas water contact. 相似文献
9.
The Basque–Cantabrian Basin (NE Spain) has been considered one of the most interesting areas for hydrocarbon exploration in the Iberian Peninsula since the 60th to 70th of last century. This basin is characterized by the presence of numerous outcrops of tar sands closely associated with fractures and Triassic diapirs. The aims of this work is to establish the diagenetic evolution of the Upper Cretaceous reservoir rocks with special emphasis in the emplacement of oil and their impact on reservoir quality. The studied rocks are constituted of carbonates and sandstones that contain massive quantities of bitumen filling vugs and fractures.Petrographic results indicate that the carbonate rocks from Maestu outcrops are bioclastic grainstones and wackestones, whereas the tar sandstones from Atauri and Loza outcrops are dominated by quartzarenites and subordinated subarkoses. The paragenetic sequence of the main diagenetic phases and processes include, pyrite, bladed and drusy calcite cement, calcite overgrowths, silicification of bioclasts and microcrystalline rhombic dolomite cement, and first stage of oil emplacement, blocky calcite cement, coarse crystalline calcite cement, calcitized dolomite, calcite veins, saddle dolomite and stylolites filled by the second phase of oil entrance. Together with the above mentioned diagenetic alterations, the arenites are affected by early kaolinitization of feldspars and the scarce formation of clay rim and epimatrix of illite. All sandstones and dolomitized carbonate rocks show high intercrystalline and intergranular porosity which is full by biodegraded hydrocarbons (solid bitumen). The biodegradation affects alkanes, isoprenoids and partially hopanes and steranes saturated hydrocarbons. Aromatics hydrocarbons, like naphthalenes, phenanthrenes, dibenzothiophenes and triaromatics are also affected by biodegradation. Results indicate that the first HC emplacement corresponds to early stage of calcite and dolomite cementation, and the second and more important emplacement is related to fracturation processes resulting in the formation of excellent reservoirs. 相似文献
10.
Diagenesis is an essential tool to reconstruct the development of reservoir rocks. Diagenetic processes - precipitation and dissolution - have an influence on pore space. The present paper aims to study the diagenetic history of deep-marine sandstones of the Austrian Alpine Foreland Basin. To reach that goal, sediment petrology and diagenetic features of more than 110 sandstone samples from water- and gas-bearing sections from gas fields within the Oligocene-Miocene Puchkirchen Group and Hall Formation has been investigated. Special emphasis was put on samples in the vicinity of the gas-water contact (GWC). The sediment petrography of sandstones of Puchkirchen Group and Hall Formation is similar; hence their diagenesis proceeded the same way. In fact, primary mineralogy was controlled by paleo-geography with increasing transport distance and diverse detrital input.Sediment petrographically, investigated sandstones from the water-bearing horizon seemed quite comparable to the gas-bearing sediments. In general, they can be classified as feldspatic litharenites to litharenites and display porosities of up to 30% and permeabilities of up to 1300 mD. The carbon and oxygen isotopic composition of bulk carbonate cements from these sandstones range from−3.8 to +2.2 and from −7.5 to +0.2‰ [VPDB]. However, near the Gas-Water Contact (GWC) a horizon with low porosities (<3%) and permeabilities (<0.1 mD) is present. This zone is completely cemented with calcite, which has a blocky/homogenous morphology. A slight, but significant negative shift in δ18O isotopy (−2.5‰) is evident.During early diagenesis the first carbonate generations formed. First a fibrous calcite and afterwards a micritic calcite precipitated. Further siliciclastic minerals, such as quartz and feldspar (K-feldspar and minor plagioclase), exhibit corroded grains. Occasionally, clay minerals (illite; smectite, chlorite) formed as rims around detrital grains. Late diagenesis is indicated by the formation of a low permeable zone at the GWC. 相似文献
11.
The Flemish Pass Basin is a deep-water basin located offshore on the continental passive margin of the Grand Banks, eastern Newfoundland, which is currently a hydrocarbon exploration target. The current study investigates the petrographic characteristics and origin of carbonate cements in the Ti-3 Member, a primary clastic reservoir interval of the Bodhrán Formation (Upper Jurassic) in the Flemish Pass Basin.The Ti-3 sandstones with average Q86.0F3.1R10.9 contain various diagenetic minerals, including calcite, pyrite, quartz overgrowth, dolomite and siderite. Based on the volume of calcite cement, the investigated sandstones can be classified into (1) calcite-cemented intervals (>20% calcite), and (2) poorly calcite-cemented intervals (porous). Petrographic analysis shows that the dominant cement is intergranular poikilotopic (300–500 μm) calcite, which stared to form extensively at early diagenesis. The precipitation of calcite occured after feldspar leaching and was followed by corrosion of quartz grains. Intergranular calcite cement hosts all-liquid inclusions mainly in the crystal core, but rare primary two-phase (liquid and vapor) fluid inclusions in the rims ((with mean homogenization temperature (Th) of 70.2 ± 4.9 °C and salinity estimates of 8.8 ± 1.2 eq. wt.% NaCl). The mean δ18O and δ13C isotopic compositions of the intergranular calcite are −8.3 ± 1.2‰, VPDB and −3.0 ± 1.3‰, VPDB, respectively; whereas, fracture-filling calcite has more depleted δ18O but similar δ13C values. The shale normalized rare earth element (REESN) patterns of calcite are generally parallel and exhibit slightly negative Ce anomalies and positive Eu anomalies. Fluid-inclusion gas ratios (CO2/CH4 and N2/Ar) of calcite cement further confirms that diagenetic fluids originated from modified seawater. Combined evidence from petrographic, microthermometric and geochemical analyses suggest that (1) the intergranular calcite cement precipitated from diagenetic fluids of mixed marine and meteoric (riverine) waters in suboxic conditions; (2)the cement was sourced from the oxidation of organic matters and the dissolution of biogenic marine carbonates within sandstone beds or adjacent silty mudstones; and (3) the late phases of the intergranular and fracture-filling calcite cements were deposited from hot circulated basinal fluids.Calcite cementation acts as a main controlling factor on the reservoir quality in the Flemish Pass reservoir sandstones. Over 75% of initial porosity was lost due to the early calcite cementation. The development of secondary porosity (mostly enlarged, moldic pores) and throats by later calcite dissolution due to maturation of organic matters (e.g., hydrocarbon and coals), was the key process in improving the reservoir quality. 相似文献
12.
The tight sandstones of the Cretaceous Quantou formation are the main exploration target for hydrocarbons in the southern Songliao basin. Authigenic quartz is a significant cementing material in these sandstones, significantly reducing porosity and permeability. For efficient predicting and extrapolating the petrophysical properties within these tight sandstones, the quartz cement and its origin need to be better understood. The tight sandstones have been examined by a variety of methods. The sandstones are mostly lithic arkoses and feldspathic litharenites, compositionally immature with an average framework composition of Q43F26L31, which are characterized by abundant volcanic rock fragments. Mixed-layer illite/smectite (I/S) ordered interstratified with R = 1 and R = 3 is the dominating clay mineral in the studied sandstone reservoirs. Two different types of quartz cementation modes, namely quartz grain overgrowth and pore-filling authigenic quartz, have been identified through petrographic observations, CL and SEM analysis. Homogenization temperatures of the aqueous fluid inclusions indicate that both quartz overgrowths and pore-filling authigenic quartz formed with a continuous process from about 70 °C to 130 °C. Sources for quartz cement produced are the conversion of volcanic fragments, smectite to illite reaction and pressure solution at micro stylolites. Potassium for the illitization of smectite has been sourced from K-feldspar dissolution and albitization. Silica sourced from K-feldspars dissolution and kaolinite to illite conversion is probably only minor amount and volumetrically insignificant. The internal supplied silica precipitate within a closed system where the transport mechanism is diffusion. The quartz cementation can destroy both porosity and permeability, but strengthen the rock framework and increase the rock brittleness effectively at the same time. 相似文献
13.
Tight-gas reservoirs, characterized by low porosity and low permeability, are widely considered to be the product of post-depositional, diagenetic processes associated with progressive burial. This study utilizes a combination of thin section petrography, scanning electron microscopy, microprobe and back scatter electron analysis, stable isotope geochemistry and fluid inclusion analysis to compare the diagenetic history, including porosity formation, within sandstones of the second member of Carboniferous Taiyuan Formation (C3t2) and the first member of Permian Xiashihezi Formation (P1x1) in the Ordos Basin in central China.In the P1x1 member, relatively high abundances of metamorphic rock fragments coupled with a braided river and lacustrine delta environment of deposition, produced more smectite for transformation to illite (50–120 °C). This reaction was driven by dissolution of unstable minerals (K-feldspar and rock fragments) during the early to middle stages of mesodiagenesis and consumed all K-feldspar. Abundant intragranular porosity (average values of 2.8%) and microporosity in kaolinite (average values of 1.5%) formed at these burial depths with chlorite and calcite developed as by-products.In the C3t2 member, relatively low abundances of metamorphic rock fragments coupled with an incised valley-coastal plain environment of deposition resulted in less smectite for transformation to illite. High K+/H+ ratios in the early pore waters related to a marine sedimentary environment of deposition promoted this reaction. Under these conditions, K-feldspar was partially preserved. During the middle to late stages of mesodiagenesis, K-feldspar breakdown produced secondary intragranular (average values of 1.4%) and intergranular pores (average values of 1.2%). Release of K+ ions promoted illitization of kaolinite with quartz overgrowths and ferrous carbonates developed as by-products.This study has demonstrated that whereas both members are typical tight-gas sandstones, they are characterized by quite different diagenetic histories controlled by the primary detrital composition, especially during mesodiagenesis. Types of secondary porosity vary between the two members and developed at different stages of progressive burial. The content of unstable detrital components, notably feldspar, was the key factor that determined the abundance of secondary porosity. 相似文献
14.
A. Förster R. Schöner H.-J. Förster B. Norden A.-W. Blaschke J. Luckert G. Beutler R. Gaupp D. Rhede 《Marine and Petroleum Geology》2010
Ketzin, in the Northeast German Basin (NEGB), is the site for pilot injection of CO2 (CO2SINK project) into a saline aquifer (the Upper Triassic Stuttgart Formation) situated at a depth of about 630–700 m. This paper reports the baseline characterization of the reservoir formation based on new core material and well-logs obtained from one injection well and two observations wells, drilled at a distance from 50 m to 100 m from each other. The reservoir is lithologically heterogeneous and made up by fluvial sandstones and siltstones interbedded with mudstones showing remarkable differences in porosity. The thickest sandstone units are associated with channel sandstone, whose thickness varies over short lateral distances. In-depth petrographic, mineralogical, mineral-chemical, and whole-rock geochemical analysis were performed focusing on the sandstone intervals, which display the best reservoir properties for CO2 injection. The dominantly fine-grained and well to moderately-well sorted, immature sandstones classify as feldspathic litharenites and lithic arkoses. Quartz (22–43 wt.%), plagioclase (19–32 wt.%), and K-feldspar (5–13 wt.%) predominate mineralogically. Muscovite plus illite and mixed-layer minerals are omnipresent (4–13 wt.%). Quartz, feldspar, as well as meta-sedimentary and volcanic rock fragments comprise the most abundant detrital components, which often are rimmed by thin, early diagenetic coatings of ferric oxides, and locally of clay minerals. Feldspar grains may be unaltered and optically clear, partially to completely dissolved, partially altered to sheet silicates (mainly illite), or albitized. Analcime and anhydrite constitute the most widespread, often spatially associated pore-filling cement minerals. Authigenic dolomite, barite, and coelestine is minor. The percentage of cements ranges in total from about 5 vol.% to 32 vol.%. Except of samples intensely cemented by anhydrite and analcime, total porosities of the sandstones range from 13% to 26%. The fraction of intergranular porosity varies between 12% and 21%. About 1–5% porosity has been generated by dissolution of detrital plagioclase, K-feldspar, and volcanic rock fragments. The comparatively large modal abundance of feldspars, micas, chlorite, clay minerals, Fe–Ti-oxides, and analcime account for the richness in Ti, Al, Fe, Mg, Na, and K, and the paucity in Si, of the Stuttgart sandstones relative to mature sandstones. Altogether, these sandstones are comparatively rich in minerals that may potentially react with the injected CO2. 相似文献
15.
This paper investigates the reservoir potential of deeply-buried Eocene sublacustrine fan sandstones in the Bohai Bay Basin, China by evaluating the link between depositional lithofacies that controlled primary sediment compositions, and diagenetic processes that involved dissolution, precipitation and transformation of minerals. This petrographic, mineralogical, and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones. Eogenetic alterations of the sandstones include: 1) mechanical compaction; and 2) partial to extensive non-ferroan carbonate and gypsum cementation. Typical mesogenetic alterations include: (1) dissolution of feldspar, non-ferroan carbonate cements, gypsum and anhydrite; (2) precipitation of quartz, kaolinite and ferroan carbonate cements; (3) transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite. This study demonstrates that: 1) depositional lithofacies critically influenced diagenesis, which resulted in good reservoir quality of the better-sorted, middle-fan, but poor reservoir quality in the inner- and outer-fan lithofacies; 2) formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality; and 3) oil emplacement during early mesodiagenesis (temperatures > 70 °C) protected reservoirs from cementation and compaction. 相似文献
16.
P.J. Armitage R.H. Worden D.R. Faulkner A.C. Aplin A.R. Butcher J. Iliffe 《Marine and Petroleum Geology》2010
Fine-grained siliciclastic lithologies commonly act as sealing caprocks to both petroleum fields and host reservoirs for carbon capture (CO2 sequestration) projects. Fine-grained lithologies are thus of great importance in controlling fluid flow and storage in the subsurface. However, fine-grained rocks are rarely characterised in terms of primary sedimentary characteristics, diagenesis and how these relate to their flow properties (i.e. sealing or caprock quality). Seventeen samples from Lower Carboniferous estuarine caprock to a gas field (also to be used as a carbon capture site), have been analysed using a range of petrological and petrophysical techniques. The rock unit that represents the caprock to this gas field was found to be predominantly silt grade with porosity values as low as 1.8%. In these rocks, caprock quality (porosity) is controlled by intrinsic and extrinsic factors linked to primary mineralogy and diagenetic processes. Depositional mineralogy was dominated by quartz, detrital mica, detrital clay (likely Fe-rich 7Å clay and illite–smectite) with minor feldspar and oxide phases. Diagenetic processes included compaction, minor feldspar dissolution and kaolinite growth and the more important processes of chlorite, siderite and quartz cementation, as well as the likely transformation of smectite into illite. Caprock quality is controlled by the primary quantity of illite-muscovite in the sediment and also by the localised extent of chlorite and quartz cementation. Deposition in an estuarine environment led to highly heterogeneous distribution of primary and diagenetic minerals, and thus caprock quality, between and within the samples. 相似文献
17.
Rapid supply and deposition of 1000's of meters of Miocene and Pliocene sediment tend to lead to a different set of controls on reservoir quality than older, more slowly buried sandstones. Here we have studied Miocene fluvial-deltaic Bhuban Formation sandstones, from the Surma Group, Bengal Basin, buried to >3,000 m and >110 °C, using a combination of petrographic, geochemical and petrophysical methods in order to understand the controls on Miocene sandstone reservoir quality to facilitate improved prediction of porosity and permeability. The main conclusions of the study are that mechanical compaction processes are the dominant control on porosity-loss although early calcite growth has led to locally-negligible porosity in some sandstones. Mechanical compaction occurred by grain rearrangement, ductile grain compaction and brittle grain fracturing. Calcite cement, occupying up to 41% intergranular volume, was derived from a combination of dissolved and recrystallized bioclasts, an influx of organic-derived carbon dioxide and plagioclase alteration. Clay minerals present include smectite-illite, kaolinite and chlorite. The smectitic clay was probably restricted to low energy depositional environments and it locally diminishes permeability disproportionate to the degree of porosity-loss. Kaolinite is probably the result of feldspar alteration resulting from the influx of organic-derived carbon dioxide. Quartz cement is present in small amounts, despite the relatively high temperature, due to a combination of limited time available in these young sandstones, grain-coating chlorite and low water saturations in these gas-bearing reservoir sandstones. Reservoir quality can now be predicted by considering primary sediment supply and primary depositional environment, the magnitude of the detrital bioclast fraction and the influx of organic-derived carbon dioxide. 相似文献
18.
The Upper Cretaceous Mesaverde Group in the Piceance Basin, western Colorado, contains thick sections of low porosity, low permeability sandstones that are reservoirs for large accumulations of hydrocarbon gas. The Mesaverde sandstones are lithic arkoses and feldspathic litharenites, containing quartz, plagioclase, variable K-feldspar, chert, and volcanic rock fragments. Important diagenetic processes that have affected the sandstones include compaction, dissolution of feldspar and/or alteration to clay, precipitation of carbonate and quartz cements, precipitation of illite, and alteration of detrital clays (mixed-layer illite–smectite). Porosity is relatively constant; a decrease in primary porosity downward is approximately balanced by an increase in secondary porosity, with significant microporosity in authigenic and diagenetic clays. K-feldspar is almost completely absent below 5500 feet (1675 m). Fibrous illite is relatively abundant above this depth and variable in abundance below. The K–Ar ages of the clays increase with depth, from 40 m.y. at the top of the sampled section to 55 m.y. at the base, indicating illitization of a precursor smectite at approximately the 100 °C isotherm. Samples with abundant fibrous illite have relatively smaller age values. Mass balance calculations indicate that dissolution of K-feldspar, illitization of smectite and precipitation of fibrous illite were coupled. These reactions suggest the transport of dissolved potassium upward hundreds of feet (100–300 m) in the section, possibly associated with water driven vertically by the migration of gas. 相似文献
19.
The Jiaolai Basin (Fig. 1) is an under-explored rift basin that has produced minor oil from Lower Cretaceous lacustrine deltaic sandstones. The reservoir quality is highly heterogeneous and is an important exploratory unknown in the basin. This study investigates how reservoir porosity and permeability vary with diagenetic minerals and burial history, particularly the effects of fracturing on the diagenesis and reservoir deliverability. The Laiyang sandstones are tight reservoirs with low porosity and permeability (Φ < 10% and K < 1 mD). Spatial variations in detrital supply and burial history significantly affected the diagenetic alterations during burial. In the western Laiyang Sag, the rocks are primarily feldspathic litharenites that underwent progressive burial, and thus, the primary porosity was partially to completely eliminated as a result of significant mechanical compaction of ductile grains. In contrast, in the eastern Laiyang Sag, the rocks are lithic arkoses that were uplifted to the surface and extensively eroded, which resulted in less porosity reduction by compaction. The tectonic uplift could promote leaching by meteoric water and the dissolution of remaining feldspars and calcite cement. Relatively high-quality reservoirs are preferentially developed in distributary channel and mouth-bar sandstones with chlorite rims on detrital quartz grains, which are also the locations of aqueous fluid flow that produced secondary porosity. The fold-related fractures are primarily developed in the silt–sandstones of Longwangzhuang and Shuinan members in the eastern Laiyang Sag. Quartz is the most prevalent fracture filling mineral in the Laiyang sandstones, and most of the small-aperture fractures are completely sealed, whereas the large-aperture fractures in a given set may be only partially sealed. The greatest fracture density is in the silt–sandstones containing more brittle minerals such as calcite and quartz cement. The wide apertures are crucial to preservation of the fracture porosity, and the great variation in the distribution of fracture-filling cements presents an opportunity for targeting fractures that contribute to fluid flow. 相似文献