Dissolution Of Feldspar Grains Research Articles

Diagenesis and Diagenetic Mineral Control on Reservoir Quality of Tight Sandstones in the Permian He8 Member, Southern Ningwu Basin

The sandstone reservoirs of the He8 member within the Lower Permian Shihezi Formation are important targets for oil and gas exploration in the southern Ningwu Basin. This study utilized thin-section identification, scanning electron microscopy, and X-ray diffraction analysis to examine the petrological features and reservoir characteristics, and evaluate the impact of the mineral composition and diagenesis type on the porosity of the sandstone reservoir. Additionally, a multiple linear regression prediction model was developed to predict the distribution of promising sandstone reservoirs in the study area. The results of the analysis revealed that the sandstone of the He8 member is mainly composed of feldspathic lithic sandstone, followed by lithic sandstone. The main reservoir type is characterized by secondarily dissolved pores and micropores within kaolinite aggregates. The low porosity (ranging from 0.2% to 10.7%) and permeability indicate that the He8 member is a tight sandstone reservoir. This reservoir has undergone compaction, cementation, and dissolution diagenesis, and is presently in the stage of mesodiagenesis B. The rigid framework of quartz, the dissolution of feldspar grains, and the intergranular pores of kaolinite are significant contributors to reservoir quality and the main drivers of porosity. In this study, a multivariate linear regression model was developed based on the mineral content of quartz, feldspar, carbonate minerals, kaolinite, smectite, and rock fragments, which accurately predicts the porosity of the studied reservoirs. Based on this model, it was predicted that the north of the Jingle South sub-depression contains a favorable reservoir space in the tight sandstone reservoir of the He8 member. The findings of this study hold significant reference value in the quantitative characterization of tight sandstone reservoirs with similar depositional and diagenetic characteristics, and improving the prediction effect of favorable reservoirs.

Main controlling factors and oil-bearing potential characteristics of a tight sandstone reservoir: A case study of southwest Ordos Basin

Clarifying the main controlling factors of reservoirs and their oil-bearing potential is vital for predicting tight sandstone reservoirs. The Chang 8 reservoir in the southwest of Ordos Basin is a typical tight sandstone reservoir and is widely distributed. Observation description and sampling analysis of cores, the grain size analysis, casting thin section, scanning electron microscope, mercury pressure, nuclear magnetic resonance, and conventional physical analysis are used to clarify the main controlling factors and oil-bearing potential characteristics of the Chang 8 reservoir in southwest Ordos Basin. The results show that delta front subfacies are mainly developed in Chang 8 member, including distributary channel, natural dike, estuary bar and distributary bay. The main rock type of reservoir is lithic feldspathic sandstone, followed by feldspathic lithic sandstone. The types of reservoir space are mainly intergranular pores and intragranular dissolved pores, with a small amount of clay-related pores and micro-fractures. The average porosity and permeability of the reservoir are 11.67% and 0.52 × 10−3μm2, respectively. The reservoirs with high oil saturation are mainly distributary channels and thicker mouth bar sand bodies. Compaction is the main factor of reservoir compaction (porosity loss rate is 55.73%), followed by cementation (porosity loss rate is 29.23%). The favorable diagenesis is the dissolution of feldspar grains and some cement. The Chang 8 tight reservoir contains various nano-scale pore-throat. For tight reservoirs with similar physical properties, the pore-throat structure controls the oil saturation of the tight reservoir. Favorable conditions for tight sandstone reservoirs oil saturation include favorable sedimentary environment (distributary channel or thick mouth bar) and suitable microscopic pore characteristics.

Quantitative evolution of pores in tight sandstone reservoirs: a case study of late Triassic Chang 6 member, Western Ordos Basin, China

In order to understand pore evolution in relation to sedimentary facies, a study has been conducted on the Chang 6 reservoir in Western Ordos Basin. Tight reservoir space comprises primary intergranular pores with an average original porosity is 36.29%, and secondary pores related to feldspar dissolution. Reservoir porosity is 8.03–11.43% and permeability is 0.12 × 10−3–1.31 × 10−3 μm2. Cementation and mechanical compaction are the main reasons for the quality of the reservoir. Dissolution of feldspar grains improved the reservoir quality. The reservoir has undergone four evolutionary stages: ① Syn-sedimentary compaction which decreased the porosity by 21.27%, to an average remaining porosity was 15.12%. ② Early diagenetic cementation which decreased the porosity by 6.23% to an average residual porosity was 8.89%. ③ Early stage of mesodiagenesis: involving dissolution and formation of micro-fractures that led to an increase in porosity by 5.47% resulting in an average porosity of 14.36%. ④ Compaction and cementation during the late mesodiagenetic stage resulting in an average porosity of 10.87%. Quantitative calculations reveal an average reservoir porosity is 10.68% with an error of 1.75%. The results contribute to a better understanding of the main controlling factors and pore evolution characteristics of tight reservoir development.

Mineralogical and geochemical studies of copper mineralization in the Paleozoic sedimentary section in southwestern Sinai, Egypt

The present study provides new insights into the distribution and characteristics of copper-bearing Paleozoic clastic-carbonate units, Araba and Um Bogma formations in southwestern Sinai, Egypt. Malachite, azurite, atacamite, chrysocolla, and cuprite occur as disseminations or stratiform, layer-parallel streaks, and lenticular bodies that are intercalated with vanadiferous sandstones in the middle unit of the Araba Formation. The vanadiferous sandstones are rich in mottramite, mimetite, duftite, beudantite, descloizite and vanadinite. The Um Bogma Formation is characterized by uncommon chalcanthite, antlerite, malachite, chrysocolla, covellite, and chalcocite in addition to traces of pyrite and barite. These ore minerals occur as acicular, fibrous, fracture-filling, disseminated, and pore-filling patches and are associated with Mn-minerals. Copper minerals form also thin laminae in the manganiferous and black organic-rich shales of the Um Bogma Formation.Copper and vanadium minerals are disseminated in the arkose, subarkose, sublitharenite, graywacke, shale, siltstone and dolostone members of the host formations. Elevated copper concentrations (up to 5.04 wt%) and vanadium (≤362.3 ppm) are associated with high CaO and MgO contents in the intergranular patches and discontinuous laminae. In the Um Bogma Formation, the mineralized layers show high contents of MnO (up to 84.27 wt%) and Fe2O3 (up to 72.1 wt%), Ba (0.03–2.86 wt%), S (up to 0.452 wt%) and Ni (0.012–0.041 wt%).Mineralogical and textural characteristics of the studied rocks indicate that copper and vanadium minerals in the Araba Formation are related to supergene processes and telodiagenesis after oxidation of sulfides and dissolution of feldspar grains and carbonate cement in the clastic rocks most probably during the Upper Devonian-Lower Carboniferous Hercynian event. In the Um Bogma Formation, copper mineralization is a supergene alteration of hydrothermal Fe-Mn minerals and associated primary Cu-Zn sulfides. Hydrothermal fluids likely mobilized several metals, i.e., Fe, Mn, and Cu, within dissolved and karstified carbonates in the Um Bogma Formation.

Diagenetic evolution and its implication for reservoir quality: a case study from the Eocene Es4 interval, Dongying Depression, Bohai Bay Basin, China

Petrographic analysis combined with thin section observation, fluid inclusions, burial history analysis, and core porosity and permeability, were used to determine diagenetic evolution and its effect on reservoir-quality of beach-bar sandstone reservoirs of Eocene Es4s interval in the Boxing sag, Bohai Bay Basin. Diagenesis is mainly characterized by strong compaction, undercompaction, cementation of carbonate, quartz, pyrite and clay minerals, dissolution of detrital grains and carbonate cements, lime-mud matrix recrystallization and hydrocarbon filling. Paragenetic sequence is determined as “early compaction / precipitation of calcite, dolomite and framboidal pyrite → dissolution of feldspar grains and early-formed carbonate cements/precipitation of quartz → hydrocarbon filling → precipitation of ferroan calcite and ankerite/lime-mud matrix recrystallization → dissolution of ferroan carbonate cements”. In the same diagenetic environment, diagenetic processes for different lithofacies significantly varied and subsequently resulted in great heterogeneity for reservoir quality during progressive burial. Lithofacies for both bar body and beach edge subfacies which are far from sandstone-mudstone contacts, are well sorted and have a strong ability to resist compaction. Abundant primary and secondary porosities are well developed because of modifications by acidic dissolution and hydrocarbon filling. The present physical properties are relatively well and numerous oil beds are developed through shallow to deep burial depth. Lithofacies for both bar body and beach edge subfacies which are adjacent to sandstone-mudstone contacts, are typically characterized by extensive cementation of calcite and dolomite. The present physical properties are bad and dry beds are extensively developed for sandstone reservoirs in both bar body and beach edge subfacies which are adjacent to sandstone-mudstone contacts. Lithofacies for both bar edge and beach mat subfacies were deposited in weak hydrodynamic conditions with high contents of limemud matrix. The reservoir porosity decreases continually due to strong compaction and weak dissolution. The present physical properties are extremely poor and dry beds are commonly developed through shallow to deep burial except for minor oil beds developed in the shallow burial.

Fluid flow and related diagenetic processes in a rift basin: Evidence from the fourth member of the Eocene Shahejie Formation interval, Dongying depression, Bohai Bay Basin, China

The purpose of this paper is to relate diagenetic processes in deeply buried sandstones in the fourth member of the Eocene Shahejie Formation interval, Bohai Bay Basin, China, to pore-fluid flow changes with progressive burial. Based on petrographic, mineralogical, and geochemical analysis, distribution patterns of authigenic minerals are recognized that reflect (1) the sources and patterns of fluid flow and (2) fluid flow in an evolving open-to-closed system. Partial to extensive precipitation of calcite and dolomite at or near mudstone–sandstone contacts during eogenesis was a result of large-scale mass transfer between sandstones and adjacent mudstones. This process was driven by steep diffusion gradients from adjacent mudstones in a relatively open geochemical system on the local scale. Support for this model is provided by large sulfur isotope fractionation between framboidal pyrite and precursor gypsum. Dissolution of feldspar grains and dissolution of nonferroan carbonate cements during early mesogenesis are spatially associated with quartz and ferroan carbonate cementation, respectively. This process was related to organic carbon dioxide expelled from adjacent source rocks and indicates a relatively open system. During late mesogenesis, dissolution of evaporitic cements related to thermochemical sulfate reduction (TSR) generated ankerite and nodular pyrite cements in adjacent pores. A lack of sulfur isotope fractionation between parent anhydrite and late-stage, nodular pyrite during TSR supports a relatively closed fluid-flow system. Because the velocities of pore-fluid flow were low during mesogenesis, large-scale thermal convection and advection probably did not occur. Instead, diffusion over short distances is inferred as the predominant transport mechanism for dissolved solids that were precipitated as other phases either in situ or in adjacent pores.

Characteristics of diagenesis, isotopic relations and reservoir properties of the Middle Miocene sandstones in the Carpathian Foredeep (Poland and Ukraine)

The Upper Badenian and Sarmatian sandstones recognized from boreholes in southeastern Poland and western Ukraine are very fine to medium-grained subarkosic and sublithic arenites and wackes. The deposits underwent diagenesis well below 100°C, and their evolutionary pathways of diagenetic and related reservoir properties represent eo- and mesodiagenesis. Eodiagenesis here comprised mechanical compaction, development of chlorite, microcrystalline calcite, pyrite, siderite, kaolinite and quartz, and dissolution of feldspar and mica grains. Mesodiagenesis included quartz and K-feldspar overgrowths, albitisation, crystallisation of dolomite and ankerite and coarsely crystalline calcite, dissolution of feldspar grains and carbonate cement, and illite growth. The isotopic data of δ 18 O VPDB of carbonate cements suggest their crystallisation from porewater which is a mixture of marine and meteoric waters. The δ 13 C VPDB values suggest derivation of carbon from microbial methanogenesis of organic matter. The 87 Sr/ 86 Sr values in coarsely crystalline calcite are higher than those of Badenian seawater. The radiogenic strontium may have been supplied during diagenesis by meteoric waters draining the continental area. The Middle Miocene sandstones show better filtration abilities (good and very good) in the western part of the study area (Poland) than in the east. Primary intergranular porosity is more frequent than secondary intragranular and intercrystalline porosities.The primary porosity was diminished due to compaction and cementation from west to east. Some increase in porosity was caused by dissolution of detrital grains and by decay of soft parts of organisms.

Diagenetic evolution and reservoir potential of the Barremian–Cenomanian Debre Libanose Sandstone, Blue Nile (Abay) Basin, Ethiopia

The Blue Nile Basin is located in the Central Plateau of Ethiopia. The basin consists of Precambrian basement, Palaeozoic and Mesozoic sedimentary rocks and Tertiary volcanic rocks. The sedimentary successions and Debre Libanose Sandstone reach a maximum thickness of 3000 and 310 m, respectively. The Debre Libanose Sandstone is composed of mudstone, finely laminated siltstone, fine-to medium-grained sandstone, massive to crudely cross-bedded gravely sandstone and massive to crudely bedded conglomerate. In the Blue Nile Basin it was deposited in a fluviatile environment, predominantly in braided and meandering rivers. The sandstone has a complex diagenetic history and is cemented by silica, calcite, kaolinite, hematite with minor amounts of illite and chlorite. Depositional environment, burial and thermal history, pore water composition and circulation, and diagenetic processes are the major factors that control the porosity and permeability of the sandstone. Minor amounts of primary porosity are preserved owing to framework grain stability. Dissolution of feldspar grains and calcite cement created a certain amount of secondary porosity in the sandstones. The fine-to medium-grained sandstones are porous and have good reservoir potential.

Diagenesis of the Cretaceous-Tertiary Willow Creek sandstones, southwestern region of Alberta

Abstract Seventy sandstone samples of the Late Maastrichtian? Early Paleocene strata from the Willow Creek Formation of the southwestern region of Alberta were selected in order to characterize factors that control the sandstone’s properties and to investigate diagenetic processes related to porosity evolution. The sandstones are fine- to medium-grained, moderate- to well-sorted litharenite and have undergone a complicated diagenetic history. The diagenetic regimes of the sandstones combine several processes related to three stages of diagenesis: eodiagenesis after deposition, mesodiagenesis during burial, and telodiagenesis after uplift. The diagenetic processes recognized during eogenesis include compaction, cementation (calcite, clay minerals), and the dissolution of feldspar grains. Extensive occurrences of early calcite cement suggest that calcite was a main control on the porosity. Abundant calcite cements precipitated from pore water with a significant meteoric component, as shown by the strong negative oxygen isotopic values (−19.546 to −3.019). However, the calcite cements inhibited the later mechanical compaction of the sandstones, thus preserving an unfilled part of the primary porosity. The most important mesogenesis processes that controlled reservoir quality were the illitization and chloritization of smectite followed by dickite formation. The pore-filling illite and chlorite resulted in a considerable loss of porosity, whereas the pore-lining chlorite may have helped in retaining the porosity by preventing the precipitation of quartz overgrowths. During telogenesis dissolution of calcite cement created secondary porosity, whereas the precipitation of kaolinite caused a reduction in porosity. This study may enhance the understanding of the role of diagenesis on reservoir quality in similar aged fluvial sandstone deposits in the subsurface such as the Scollard Formation in the central Alberta plains and the Coalspur Formation in the southwestern Alberta foothills.

Relationship Between Deep Diagenetic Quartz Cementation and Sedimentary Facies in a Late Ordovician Glacial Environment (Sbaa Basin, Algeria)

Abstract Production of hydrocarbons from sandstone reservoirs is complicated by their diagenetic history. Although many aspects of sandstone diagenesis are well studied, the character and processes of glacial-associated diagenesis remain unknown. To address this problem, the origin, conditions, and timing of quartz cementation in glacial Ordovician sandstones of the Sbaa Basin, Algeria, were constrained on the basis of fluid-inclusion microthermometry, electron-microprobe data, and quantification by image analysis combined with optical and cathodoluminescence microscopic studies. Samples of the Cambro-Ordovician formations in the Sbaa Basin were investigated from four wells at burial depths between 2.0 and 2.5 km and at ambient temperatures between 110 and 130°C. Quartz cements range in abundance from 1 to 27% of bulk rock volume in all wells. Three separate phases of quartz cementation are readily distinguished by cathodoluminescence microscopy. The main silica sources are interpreted to be: dissolution of feldspar grains, intergranular quartz dissolution, stylolitization, and illitization of smectite. Fluid-inclusion data depths indicate that quartz overgrowths precipitated between 100 to 160°C. This range of temperature corresponds to the Visean–Namurian burial phase, according to the basin thermal history. Image analysis demonstrates intergranular quartz dissolution (also called “pressure dissolution”) in the presence of thin illite coatings around detrital quartz grains. These clay coatings were mechanically emplaced below the Ordovician ice sheet by pressurized meltwater circulation and subsequent fluid expulsion within sandy subglacial soft bedrock that was initially porous and permeable. Therefore, the subglacial bedrock lithology directly influenced quartz diagenesis because an increase in the content of clay matrix or the presence of an argillaceous layer can block the circulation of fluids flowing below the ice sheet, which prevents the formation of clay coatings. Quartz cements in this environment are favored in less compacted areas without clay coatings and with high porosity. The results of this study clearly illustrate the relationships among sedimentary facies, ice dynamics, and diagenetic architecture of the formation. These types of processes may control the porosity–permeability distribution in other glacial sandstone successions through quartz cementation and chemical compaction.

RESERVOIR SANDSTONES OF THE CRETACEOUS NAPO FORMATION U AND T MEMBERS IN THE ORIENTE BASIN, ECUADOR: LINKS BETWEEN DIAGENESIS AND SEQUENCE STRATIGRAPHY

This paper investigates whether diagenetic alterations in sandstones and resulting changes in reservoir quality are influenced by depositional environments and sequence stratigraphy. The study focusses on the Cretaceous U and T sandstone members of the Napo Formation in the Oriente Basin of Ecuador. The sandstones were deposited in fluvial, transitional and marine environments, and comprise Lowstand (LST), Transgressive (TST) and Highstand Systems Tract (HST) deposits. The data were obtained by detailed petrographic observations supported by microprobe, stable isotope, and fluid inclusion analyses. The sandstones consist of fine‐ to medium‐grained quartzarenites and subarkoses. Diagenetic events include cementation by chlorite, early and late kaolinite/dickite, early and late carbonates (siderite, Fe‐dolomite/ankerite), and quartz.Early (eogenetic) processes included formation of chlorite grain coatings, kaolinite pore filling, and siderite (SI) cementation. Chlorite is absent in TST sandstones but was found frequently in LST‐HST sandstones. Early kaolinite is not present in LST sandstones but occurred frequently in LST‐HST sandstones.The distribution of mesogenetic cements relative to sequence stratigraphy is different in the U and T units. In the U sandstones, calcite is frequent in LST deposits and absent in the LST‐HST. Fe‐dolomite/ankerite is abundant only in the TST. S2 siderite is present in the TST and LST, but absent in the LST‐HST. Quartz cement and kaolinite/dickite are equally distributed in all systems tracts. In the T sandstones Fe‐dolomite/ankerite is only abundant in the TST, whilst calcite, quartz and dickite have similar distributions in all the systems tracts.The distribution of kaolinite cement is interpreted to be the result of relatively more intense meteoric‐water flux occurring during sea‐level fall, whereas chlorite cement may have formed through burial diagenetic transformation of precursor clays e.g. berthierine which was precipitated in mixed marine‐meteoric waters in tidal channel and estuarine environments.Chlorite cement in the T and U sandstones appears to have retarded development of quartz overgrowths, and 12–13% primary porosity is retained. The T sandstones (LST‐HST) contain up to 4% chlorite cement. Little evidence for chemical compaction was found with the exception of occasional concave‐convex grain contacts.Eogenetic siderite appears to have helped to preserve reservoir quality through supporting the sandstone framework against further compaction, but mesogenetic calcite has considerably reduced primary porosity. Eogenetic siderite (SI) was partly replaced by later carbonate cements such as late siderite (S2) and Fe‐dolomite. Although there appears to be a relationship in the Napo Formation between the occurrence of siderite SI and sequence stratigraphy, the relationship may change when original volumes of siderite are considered. There is likewise partial replacement of early kaolinite and recrystalization to dickite which masks the amount of original early kaolinite. Since the amount of early kaolinite could not be confirmed, the relationship to sequence stratigraphy is tentative. Only chlorite seems to have a clear relationship to sequence stratigraphic framework in the Napo Formation.The high intergranular volume (IGV) of the sandstones indicates that cementation played a more important role than mechanical and chemical compaction in both Napo Formation sandstone members. Later dissolution of feldspar grains and siderite cements was the main process of secondary porosity development (up to 11% in the U sandstones).

田上花こう岩中のアルカリ長石の微細組織とその形成史

Alkali feldspars in the Tanakami granite, southwest Japan, contain a complicated fine-scale mixture of various micro-textures; lamellar crypto- and micro-perthites and micropore-rich patch perthites. Mineralogical examination mainly by TEM and SEM disclosed that there are two formational stages of these micro-textures, the subsolidus cooling stage and hydrothermal alteration stage. In the subsolidus cooling stage, lamellar crypto- and micro-perthite were formed by exsolution. Then nearly regularly arranged dislocations were formed at interfaces of Ab-rich lamellae and Or-rich feldspars. Hydrothermal alteration caused the dissolution of feldspar grains and re-precipitation of new feldspar grains mainly around the dislocations. These processes result in neo-formation of patch perthites that comprise irregular microcline, tweed orthoclase, numerous micropores and albite grains. Bulk compositions of cryptoperthite have excessive Or content (around Or90) in comparison to those expected for primary alkali feldspars. It is estimated that retrograde inter-crystalline Na-K exchange between alkali feldspars and plagioclases occurs in the subsolidus stage. On the contrary, bulk compositions of patch perthite have poor Or content (Or64-71) from those of the cryptoperthite. This feature indicates that the chemical compositions of alkali feldspars changed drastically as well as the micro-textures in the hydrothermal alteration stage. Coexisting plagioclases also underwent chemical change from the subsolidus cooling stage through the hydrothermal alteration stage. Especially in the hydrothermal alteration stage, alteration by percolation of hydrothermal water occurred in the internal areas of the plagioclase grains. These processes result in lower Or content (max Or2.7) and higher Ab content (>Ab90) of the plagioclases, and neo-formation of patchy zoning including micropores and fluorite subgrains. Summarizing all of these features, the relation between micro-textural development and chemical compositional changes in feldspars was discussed taking into consideration their formation processes such as inter-crystalline Na-K exchange and hydrothermal reaction.

Allogenic and authigenic clays of the Lower Palæozoic sandstones of the Naqus Formation at Gebel Gunna, central Sinai, Egypt: their recognition and geological significance

The Lower Palæozoic Naqus Formation of Gebel Gunna in the Sinai Peninsula is conformably underlain by the Araba Formation and unconformably overlain by the Cenomanian Malha Formation. It consists mainly of fine- to medium-grained pebbly sandstones with a few siltstone and granulestone interbeds. Petrographical, X-ray diffraction, scanning electron microscope and chemical analyses of the sandstones revealed that they are mainly quartzarenite, containing allogenic and authigenic clays. The allogenic clays were found in small amounts. Such clays exhibit some of the characteristic features of infiltration clay coats. The clays coat a few grain surfaces and form meniscus-shaped pore bridges at points of grain contact. In addition, the clays were observed on the surfaces of crystalline authigenic minerals and in-filled elongated pores of partially dissolved feldspar grains. The recorded authigenic clays are mainly kaolinite with a minor amount of illite. The kaolinite exhibits three morphological habits: vermicular, blocky and fan-shaped. The vermicular kaolinite is dominant and was interpreted to have formed by dissolution of feldspar grains. The blocky kaolinite was observed with a textural relationship, indicating that it was neomorphosed after vermicular kaolinite. The fan-shaped kaolinite was found to be a result of mica alteration. Study of both allogenic and authigenic clays has helped in understanding the sedimentological history of the studied sandstones. The sandstones were deposited in a braided stream, buried at depth of about 1–3 km, and afterwards subjected to surface exposure.

Origin of Quartz Cement in the Tirrawarra Sandstone, Southern Cooper Basin, South Australia

ABSTRACT Quartz cement is the most abundant authigenic mineral in the fluvio-deltaic Tirrawarra Sandstone and plays an important role in controlling reservoir quality. Quartz cement ranges from 0 to 19% and is controlled by the original sandstone composition. Petrographic, fluid inclusion, electron microprobe and cathodoluminescence (CL) data from the quartz cement indicate multiple stages of cementation at different temperatures and suggest more than one silica source. CL observations indicate up to six stages of quartz cement in some samples. The stages of quartz cement can be classified into three zones: an innermost zone of brown-luminescing cement (Z1), a middle zone of bright blue-luminescing cement (Z2) and an outer zone of brown-luminescing cement (Z3). Dead oil or bitumen is trapped b tween Z2 and Z3, indicating that Z3 formed after oil migration commenced. Measurements of homogenization temperatures from fluid inclusions in quartz overgrowths indicate that quartz cement precipitated over a temperature range of 65 to 130°C. Z1 quartz cement formed between 65 and 80°C and Z2 cement was precipitated between 80°C and 100°C. Z3 quartz cement formed later at temperatures around 130°C. Microprobe analysis shows a consistent variation in aluminum between each quartz cement zone. The average aluminum content for Z1, Z2, and Z3 is respectively 237, 538, and 58 ppm. Fluid-inclusion precipitation temperatures and aluminum content have been used to help identify the silica sources for different zones of cement. A likely source of silica for Z1 quartz cement is early alteration of feldspar grains. The bright blue CL color in the Z2 quartz cement is related to its high aluminum content. The source of silica for this cement is likely to have been late dissolution of feldspar grains by acidic fluids generated during kerogen maturation. Late quartz cement (Z3) has the lowest aluminum content, which is similar to that of detrital quartz grains in the Tirrawarra Sandstone. Consi ering the temperature of precipitation, very low aluminum content, and the presence of Z3 cement in facies prone to stylolitization, the silica source for the cement is likely to have been pressure solution of detrital quartz at stylolites and grain contacts.

RESERVOIR POTENTIAL OF THE MAASTRICHTIAN PAB SANDSTONE IN THE EASTERN SULAIMAN FOLD‐BELT, PAKISTAN

The hard, resistant, generally cliff‐forming Pab Sandstone Formation of Maastrichtian age is well exposed in parts of the eastern Sulaiman Fold‐Belt. Thin‐section, XRD and SEM studies of 66 samples from seven outcrops extending 200 km along strike were made in order to assess the reservoir quality of this formation. Petrographic studies indicated that the sandstones, classified as quartz arenites (locally subarkose), are texturally and mineralogically mature. The proportion of potassium feldspar varied from trace to 16%, and this mineral was confined to certain stratigraphic zones. Alteration of feldspars to clay minerals was common, particularly in the south of the study area. Plagioclase, heavy minerals, and lithic fragments were present in minor amounts.Diagenetic kaolinite, illite and chlorite were common, with kaolinite being the most abundant. Quartz overgrowths constitute the dominant cement. The reservoir quality of the formation largely depends on the composition of the sandstone, and its susceptibility to chemical alteration. Most of the porosity (up to 16%) results from the dissolution of feldspar grains; dissolution was restricted to the south of the study area, and the feldspars in the north show little alteration and no dissolution.Reservoir potential has also been influenced by the substantial input of silica‐rich solutions that completely destroyed the primary porosity of the sandstone. The observed porosity is mostly secondary, commonly formed by dissolution of feldspar grains, and occasionally cement/matrix.The sequence of diagenetic events is, from oldest to youngest: formation of pyrite; formation of early‐stage calcite; formation of dolomite; silica cementation; formation of clays; precipitation of late‐stage calcite; feldspar dissolution; and formation of iron oxides. The textural and mineralogical maturity, coupled with local well‐developed secondary porosity, may make the Pab Sandstone Formation a good reservoir rock.

Aux Vases Sandstone Diagenesis: Implications for Hydrocarbon Recovery from Southern Illinois Reservoirs

The Aux Vases Sandstone (Mississippian) is a problematic yet productive reservoir in the Illinois basin. The Aux Vases Formation was deposited in a mixed siliciclastic-carbonate offshore environment. Hydrocarbon reservoirs are dominantly elongate sandstone bodies interpreted as subtidal facies within a prograding tidally influenced deposystem. Oil saturated zones, in most cases, are composed of fine-grained, cross-bedded, friable, feldspathic quartz sandstone. Resistivity logs, even from productive wells, are typically characterized by unusually low resistivities (2-4 ohms) that lead to high calculated water saturations. X-ray diffraction, SEM/EDS, and thin section studies were used to analyze diagenetic processes that may affect hydrocarbon production in several Aux Vases reservoirs. Diagenetic processes common to the field studied include: (1) early calcite cementation occluding primary porosity and inhibiting compaction, (2) dissolution of feldspar grains to form authigenic clay, (3) dissolution of early calcite cement, (4) diagenesis of detrital clay minerals into new authigenic clay minerals, and (5) multiple stages of quartz overgrowths that reduce porosity. Commonly used drilling and completion practices may adversely affect reservoir quality by reacting with diagenetic products. Loosening of the fine mineral fraction that causes migration of fines and swelling of expandable clay minerals are examples of this type of damage. Knowledge ofmore » reservoir mineralogy and diagenesis may influence drilling and completion practices and affect selection of secondary and tertiary recovery methods.« less

Secondary Porosity in Sandstones and Acid-Producing Reactions in Adjacent Shales, Oligocene, South Texas: ABSTRACT

Frio and Vicksburg sandstones in the McAllen Ranch and Hinde fields in south Texas show two depth-related zones of secondary porosity. Anomalously high porosity in the upper zone, from 4,000 to 6,500 ft (1,220 to 1,980 m), is related to the dissolution of analcime cement by acidic pore fluids released from adjacent shales. Much of this acid may be produced by the reduction of octahedral iron in smectite clay. Sodium released by analcime dissolution may be responsible for abnormally high salinities calculated for pore fluids in these sandstones. Calcite cementation below the upper zone may be related to fluids rich in calcium carbonate that have migrated up fault zones from great depth. The lower zone of secondary porosity extends from 8,400 to 10,000 ft (2,560 to 3,050 m) at Hinde and from 8,400 to 14,000 ft (2,560 to 4,270 m) at McAllen Ranch. The top of this zone also corresponds to the clay dehydration depth and the top of the abnormally pressured zone. At this depth and temperature, randomly interstratified illite and/or smectite clay becomes ordered and a large amount of water is expelled from the clay structure by potassium fixation. Smectite illitization produces minor amounts of kaolinite and chloritemore » and, more importantly, releases silica and acidic pore water. The acidic pore water migrates into adjacent sandstones and enhances the dissolution of feldspar grains and calcite, creating secondary porosity. The shales at McAllen Ranch have undergone more extensive illitization than those at Hinde, and more secondary porosity is observed in the McAllen Ranch sandstones. Data from detailed chemical and mineralogical analyses were used in mass-balance calculations to confirm that the amounts of acid produced by these reactions can account for the observed secondary porosity in these sandstones.« less

Petrology and Diagenesis of Pocono-Price Big Injun Sandstone, Granny's Creek Field, West Virginia: ABSTRACT

Two cores of the Lower Mississippian Pocono-Price Big Injun sandstone were studied megascopically and petrographically to determine the effects of mineral content and diagenesis on porosity. Most of the samples studied are fine-grained sublitharenites. Quartz, polycrystalline quartz, and argillaceous rock fragments comprise up to 95% of the framework grains. Many samples also are characterized by abundant pseudomatrix resulting from the squeezing and degradation of mica flakes, argillaceous rock fragments, and feldspars. Initial porosity probably was very high because of the well-sorted and angular nature of the sand grains, although reservoir porosities generally range only up to 15%. Mechanical compaction, diagenetic cements, and degradation of sand grains have significantly reduced initial porosity. The general sequence of postdepositional events includes: (1) mechanical compaction, (2) illite and chlorite grain coatings, (3) quartz and barite pore filling and replacement, (4) calcite pore filling and replacement, (5) dolomite, siderite, and pyrite replacement, and (6) dissolution of feldspar grains. Chlorite coatings are abundant in some samples and, where present, help to preserve primary porosity by inhibiting the growth of quartz cement. Minor secondary porosity has resulted from dissolution of feldspar. Clean, high quartz sandstones with well-developed chlorite coatings form the best reservoirs in this area.

Geologic Factors Influencing Reservoir Performance at Texaco's Salem Tertiary Recovery Project, Marion County, Illinois: ABSTRACT

A detailed lithologic reservoir study was conducted to aid Texaco personnel in designing and monitoring an experimental surfactant-polymer flood in the Mississippian Benoist Sandstone, one of several producing formations in the Salem field of south-central Illinois. Twelve elongated five-spot patterns are distributed over the 60-acre (24 ha.) project area. The Benoist Sandstone averages about 49 ft (15 m) of net pay at a depth of about 1,800 ft (550 m). Cores from eight wells were studied in detail. Particular attention was paid to variations in sedimentary structures, lithology, and mineralogy that could influence reservoir performance. Techniques employed in this study included examination of slabbed cores, thin-section petrography, X-ray diffraction (XRD) mineralogy, a d scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). The Benoist Sandstone is one of several Late Mississippian deltaic sandstone units deposited in the subsiding Illinois basin. These sandstones are bounded above and below by fossiliferous marine limestone and shale. Delta-front sandstones, hereafter referred to as bar-finger sandstones, comprise the bulk of the formation. Channel-fill deposits are found near the base of the unit and nonreservoir, tidal-flat deposits near the top. The bar-finger deposits are moderately to well-sorted, fine to medium-grained sandstones with horizontal to inclined planar bedding and some ripple and planar cross-bedding. The planar bedding is accentuated by clay and mica-rich layers, one millimeter to several centimeters thick. These shale layers increase and thicken upward, and separate the bar-finger sa ds into several reservoir units. The Benoist sandstones are quartzose, containing 70 to 98% monocrystalline and polycrystalline quartz and small amounts of detrital feldspar and shale clasts. Cement is predominantly quartz in the form of syntaxial overgrowths, with minor calcite. Small amounts of clay occur as detrital laminae, authigenic pore fillings, and sand-grain coatings. The percentage of detrital and authigenic clay increases near the top of the bar-finger sandstone and significantly reduces permeability. Illite, the dominant clay, End_Page 445------------------------------ occurs as both grain coats and pore bridgings, kaolinite forms scattered pore fills, and Fe-chlorite coats a few grains. The channel-fill deposits are well to poorly sorted, very fine to medium-grained sandstones, commonly conglomeratic, and contain calcite-cemented zones. These deposits are mineralogically similar to the bar-finger deposits but contain abundant shale and fossil fragments at the base. Noncalcareous channel sandstones are characterized by scattered Fe-chlorite grain coatings and pore-filling illite. Intergranular porosity is well developed and has not been severely reduced by the pervasive quartz overgrowth cementation. The eastern part of the project area contains a higher quality reservoir section because of the sparsity of clay zones in the bar-finger sandstone and the thicker channel-fill deposits in this area. Secondary porosity, produced by the dissolution of feldspar grains, has slightly enhanced the overall quality of the Benoist Sandstone. End_of_Article - Last_Page 446------------