The Upper Triassic deep-lacustrine sedimentary gravity flow sandstones (Yanchang Formation, Southern Ordos Basin) form an important oil reservoir, however, it is considered as a low permeability to ...tight reservoir. This study combines petrographic, mineralogical and geochemical data in order to better constrain the controls on reservoir quality. Two major lithofacies types (lithofacies 1 - sandy debrite; lithofacies 2 - turbidite) were identified and their origin relating to deep-lacustrine gravity flows have been addressed. The gravity flow sandstones are significantly diagenetically altered, including mechanical compaction, authigenic clay formation, carbonate and quartz cementation and dissolution. In addition, volcanic tuffaceous dust alteration is one of the important processes during burial, which provided silica for quartz cementation and that easily transformed to authigenic smectite. It also formed poorly crystallised kaolinite and random mixed-layer I/S clays. The relatively negative δ13C (with an average of −2.3‰) and δ18O values (with an average of −17.0‰) of carbonate cements in the gravity flow sandstones reflect decarboxylation of organic matter as one of the important carbonate sources.
The factors controlling reservoir quality relate to depositional and diagenetic processes. Depositional factors (e.g. grain size, grain sorting) had a stronger influence on lithofacies 2 sandstones compared to lithofacies 1, which made lithofacies 2 sandstones more easily compact and lose most of its primary porosity. Samples with less intergranular volume (IGV) indicate that mechanical compaction plays an important role in reducing reservoir quality. However, cementation (clay minerals and carbonate cementation) affected sandstones of lithofacies 1 to a stronger degree, heavily destroying reservoir quality while in this lithofacies dissolution (e.g. feldspar, volcanic fragments and minor quartz) significantly improved reservoir characteristics in contrast to lithofacies 2 sandstones. Quartz cements could stabilize the framework and preserve the open porosity, however, they are present with relatively low contents and have limited effect in influencing the reservoir quality. In summary, lithofacies 1 sandstones possess a slightly better reservoir potential in the study area. All these factors have great implications for tight oil exploration and production of the Yanchang Formation in the Southern Ordos Basin which can serve as an example for other tight sandstone gravity flow reservoirs elsewhere in the world.
•Sandy debrite and turbidite were identified and their origin relating to lacustrine gravity flows have been addressed.•Volcanic tuffaceous dust was easily transformed to authigenic smectite, also providing silica for quartz cementation.•Decarboxylation of organic matter as one of the important carbonate sources.•The factors controlling reservoir quality relate to depositional and diagenetic processes.•Sandy debrites form the slightly better reservoirs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Recently, 15.1 billion tons of shale oil geological resources were discovered in the Qingshankou Formation of Gulong Sag, Songliao Basin, and this discovery has attracted considerable attention. Thus ...far, the sedimentary environment and diagenetic process of the Qingshankou Formation in Gulong Sag, particularly the reservoir space and accumulation mechanism of the shale oil, are unclear, which has seriously affected the exploration and development of shale oil. Based on detailed core and thin section observation, thin section analysis, mineral analysis, and geochemical analysis, thin layers and concretions of dolostone in Qingshankou Formation shale in Gulong Sag are studied. Three types of layers and nodules can be seen in the core and thin sections: the first is composed of very pure micrite, powdery calcite, or dolomite (dolomite greater than 90%); the second are argillaceous dolomite thin layers or nodules (dolomite content of approximately 75–90%); and the third is a highly impure dolomite thin layer (dolomite content between 50–75%). The chemical composition of three kinds of dolomite thin layers and nodules is different: the contents of CaO and MgO of the pure dolomite thin layer and nodules are more than 40%. The contents of CaO and MgO in the thin layer and nodules of argillaceous dolomite are between 30 and 40%. The content of CaO and MgO in the impure dolomite thin layers and nodules is less than 30%. The reservoir space is developed at the micron or nanometer scale in the thin dolomite layers and nodules, although most of these layers are filled with asphalt. The development of thin layers and nodules of dolomites is related to a dry and hot depositional climate and influenced by hydrothermal fluids. The findings presented here provide important information for exploiting the newly discovered shale oil resources. Many dolostone thin layers and nodules have curved silt veins, and the bending coefficient is 1.48, indicating that the dolostone thin layers and nodules have undergone compaction after formation.
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Shale oil has become a global hotspot of unconventional exploration and development. In this study, the latest drill core and experiment analyses of the Qingshankou Formation in the northern Songliao ...Basin were used to evaluate its lithofacies classification, sedimentary environment, pore types, pore-throat structure characterization, and shale oil potential. Lithofacies classification was determined according to the total organic carbon (TOC) content, sedimentary structure, and rock mineral content. Laminae genesis and micro-sedimentary structures indicate the deposition of fine-grained sedimentary rocks (FGSRs) in a semi-deep to deep lacustrine environment; however, evidence also suggests partial reworking by storm events and bottom current flows. FGSRs mostly comprise type I kerogen, with small amounts of type II1. The average vitrinite reflectance of the FGSRs was 1.37%, indicating middle to high stages of thermal maturation within the oil generation window. The N2 adsorption experiment indicated that silty mudstone (SM), silty fine mixed sedimentary rock (SFMR), and argillaceous fine mixed sedimentary rock (AFMR) had ink-bottle-shaped and slit-shaped pores, and the lithofacies were dominated by mesopores, accounting for 77.4%, 71.9%, and 80.8% of the total pore volume, respectively. Mercury injection capillary pressure analysis indicated that SM and SFMR had an average pore-throat radius of 0.01–0.04 μm, whereas AFMR and CM were dominated by nanopores, mainly distributed in the range of 0.004–0.0063 μm. Based on the comprehensive studies of TOC content, pore development, and brittleness, we concluded that organic-rich laminated SM and SFMR should be the focus of shale oil exploration of the Qingshankou Formation in the northern Songliao Basin, followed by organic-rich or organic-moderate laminated and layered AFMR, as well as calcareous fine mixed sedimentary rocks.
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The detailed characteristics and formation mechanisms of organic-rich clasts (ORCs) in the Upper Paleozoic tight sandstone in the northeastern margin of the Ordos Basin were analyzed through ...818-m-long drilling cores and logging data from 28 wells. In general, compared with soft-sediment clasts documented in other sedimentary environments, organic-rich clasts in coal-bearing tight sandstone have not been adequately investigated in the literature. ORCs are widely developed in various sedimentary environments of coal-bearing sandstone, including fluvial channels, crevasse splays, tidal channels, sand flats, and subaqueous debris flow deposits. In addition to being controlled by the water flow energy and transportation processes, the fragmentation degree and morphology of ORCs are also related to their content of higher plants organic matter. The change in water flow energy during transportation makes the ORCs show obvious mechanical depositional differentiation. Four main types of ORC can be recognized in the deposits: diamictic organic-rich clasts, floating organic-rich clasts, loaded lamellar organic-rich clasts, and thin interlayer organic-rich clasts. The relationship between energy variation and ORCs deposition continuity is rarely studied so far. Based on the different handling processes under the control of water flow energy changes, we propose two ORCs formation mechanisms: the long-term altering of continuous water flow and the short-term water flow acting triggered by sudden events.
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The Qingshankou Formation in the Gulong Sag has attracted significant attention due to its rich shale oil reserves. However, several fundamental geological problems associated with shale from the ...Qingshankou Formation remain unresolved. The Qingshankou Formation in the Gulong Sag is a set of deep-lake and semi-deep-lake fine-grained deposits primarily composed of mudstone and shale. Recent studies have shown that tempestites and storm deposits commonly occur in the Qingshankou Formation. Fine liquefaction veins often occur at the bottom of the tempestite, indicating that storm-shaking liquefaction is expected in the swale structure. Meanwhile, the mudstone and shales primarily have a sand structure with fine grain size, indicating that the environment is exposed to an undercurrent activity, with a water flow velocity of 20–55 cm s
−1
. Unlike previous studies, we posit that the ancient water depth was only 20–30 m or shallower. These phenomena have significance for understanding the formation environment and diagenetic process of shale in the first member of the Qingshankou Formation, which provides a reference for oil and gas exploration and development.
Tight sandstone gas is on the first position of unconventional natural gas sources, which can be developed under today’s technical conditions. In recent years, tight sandstone gas reservoirs have ...been found in several wells in the Linxing area, eastern margin of Ordos Basin, China. In this article, a variety of methods, including cast thin sections, X-ray diffraction analysis, scanning electron microscope, and drill core data were used to study the petrological characteristics and their influences on tight sandstone reservoir in coal-bearing strata of the Linxing area. Based on the analysis of thin section, it can be concluded that the sandstone reservoir is essentially constituted of lithic sandstone as well as lithic arkose and feldspathic litharenite. Cement types are complicated, including carbonate minerals, clay minerals, and quartz overgrowth. Illite, kaolinite, chlorite, illite–smectite mixed layer, and chlorite–smectite mixed layer are found in clay minerals. Compared with other clay minerals, illite is in the dominant position. Pores can be divided into residual intergranular pore, intragranular dissolution pore, intergranular dissolution pore, cement dissolution pore, intercrystalline pore, and microcrack in sandstone reservoir of the Linxing area. Quartz has an average content of 68% with the feature of low compositional maturity and plays a major role in increasing porosity due to dissolution and protecting of quartz. Feldspar dissolution plays a role in decreasing porosity because the by-product materials of feldspar dissolution remain in original place, instead of being transported to other areas. Dissolution pores are 2–20 μm and may be filled with kaolinite, illite, or halite. It is worth mentioning that grain-coating chlorite may be of sufficient thickness to protect reservoirs along with the increasing content of chlorite, which is testified by the crossplot between the chlorite and porosity when the absolute content of chlorite is less than 1.5%.
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Upon channel bars or point bars within the lows of the Yellow River, a new sedimentary structure, named 'silt mushroom', has been observed. The process of their formation is interpreted to be via the ...ice process. The name, the silt mushroom comes from their figurative form. This is because they look somewhat similar to mushroom's in size and shape; being in the range of 1 to 10 cm in diameter, with the medium 3-5 cm, and on average 10 cm in height, occuring generally in groups, and occasionally in isolation in relatively soft silt. They develop in the transition from winter to spring, and are convincingly related to ice processes. Ice-induced silt mushrooms are best examined in association with the many other newly discovered ice-induced sedimentary structures (over 20 kinds). Clearly, up to now, ice processes have been significantly underestimated. With the substantial discovery of the ice-induced silt mushroom, it opens up new questions. This is because its structure mirrors the same sedimentary structures found in rocks, questioning their genesis, and sedimentary environment analysis. This achievement is significant not only in sedimentology, but also in palaeogeography, palaeoclimate, geological engineering, hydraulics and fluviology.
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•Lithofacies decipher information about paleoclimate, hydrodynamic condition, and diagenesis.•Sedimentary environments and rate are the most important factors that influencing organic ...matter enrichment.•Macropores provided by intergranular pores between recrystallized calcite and microfractures.•Laminated organic-rich calcareous mudstone is the most promising shale oil lithofacies.
Recent economic production of shale oil in the lacustrine basin has prompted the reevaluation of fine-grained sedimentary rocks (FGSRs) in the Dongpu Sag, Bohai Bay Basin. In this study, FGSRs in the Eocene Shahejie Formation were examined using petrological, geochemical, and quantitative pore size characterization analyses to clarify lithofacies classification, sedimentary environment, controls of reservoir quality, and shale oil potential. The classification of lithofacies was based on total organic carbon content, sedimentary structure, and mineral content. The laminae formation processes and distribution indicate that the FGSRs were deposited in semi-deep to deep anoxic lacustrine environments with humid to semiarid climates. Laminated and layered FGSRs primarily comprise macropores and mesopores, accounting for an average of 68.02% and 27.33%, respectively, whereas micropores account for 4.65%. The macropores show a positive relationship with the carbonate mineral content and primarily comprise interparticle pores between recrystallized calcite and parallel interlayer microfractures. The mesopores are controlled by the content of clay and quartz-feldspathic minerals, revealing that they are primarily interparticle pores between rigid and soft grains. Micropores are dominated by crystalline pores in clay mineral. A low sedimentation rate is available for organic matter (OM) enrichment. The lithofacies containing recrystallized calcite laminae are high-quality shale oil reservoirs. The OM abundance, hydrocarbon generation potential, reservoir quality, and brittleness collectively highlight that OM-rich and moderately laminated calcareous mudstone and calcareous fine-grained mixed sedimentary rocks should be the focus of shale oil exploration. The results have important implications for shale oil development of the similar saline lacustrine basin worldwide.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Bedding-parallel calcite veins (BPCVs) occur widely in Upper Cretaceous lacustrine shales of the Qingshankou Formation in the Northern Songliao Basin, China. Research on the formation processes and ...geological significance of BPCVs has focused on the sources of fluid, timing of formation, initiation, dilation and growth mechanisms, and on relationships with hydrocarbon generation. In this study, we addressed these issues through a comprehensive study of drill cores containing BPCVs using petrographic observations, isotope geochemistry, rare earth element (REE) analysis, fluid inclusion analysis, in situ U–Pb dating, and basin modeling. BPCVs were observed in medium–high maturity shales with high organic matter contents with heterogeneous compositions. Similarities in δ13C, δ18O, 87Sr/86Sr, and REEs contents between BPCVs and the host shale indicate that locally derived formation fluids driven by chemical gradients were involved in BPCV formation. The in situ U–Pb age dating (70 ± 8.9 Ma and 70.6 ± 9.7 Ma) constrained BPCVs formation to the late Cretaceous, corresponding to a burial depth of ∼1600 m, indicating that the BPCVs formed in the oil window. The latter is also supported by fluid inclusions-derived temperatures (∼76–109 °C). BPCV formation included vein initiation and dilation, and the microstructures, e.g., domal and pincer structures, sigmoidal calcite veins, conjugate en-échelon calcite vein arrays, as well as mechanical crystal twins of the BPCVs, all indicate that tectonic compression influenced fracture development as also indicated in other basins where beef occur. Fluid overpressure caused by hydrocarbon generation, tectonic compression, and clay mineral transformation (mainly smectite to illite conversion) was responsible for vein dilatation. Antitaxial fibrous crystals with smooth crystal boundaries and high aspect ratios, containing sinusoidal solid inclusions as well as median planes, indicate that crystallization force promoted vein growth. Thus, BPCVs can be used as a good indicator of hydrocarbon generation, and horizontal tectonic compression.
•Bedding-parallel calcite veins (BPCVs) occur widely in medium–high maturity lacustrine shales of the Songliao Basin.•Locally derived formation fluids driven by chemical gradients were involved in BPCV formation.•The in situ U–Pb age dating indicates that the BPCVs formed in the oil window.•Tectonic compression influenced fracture development, and crystallization force promoted BPCV growth.•BPCVs can be used as a good indicator of hydrocarbon generation, and horizontal tectonic compression.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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