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Authigenic chlorite plays an important controlling role in reservoir quality, and is widely distributed in the tight sandstone reservoirs of the Triassic Yanchang Formation in the Southwestern Ordos Basin, China. The occurrence and origins of authigenic chlorite were investigated using an integrated approach of optical microscopy, scanning electron microscopy, X-ray diffraction, electron probe microanalysis, and fluid inclusion analysis. The sandstones are fine-grained lithic arkoses and feldspathic litharenite with 11.5% volcanic rock fragments. Authigenic chlorite occurs in two morphotypes, including pore-lining and pore-filling chlorite. Pore-lining chlorite occurs as coatings on the framework grains and covers the entire fining-upward sandstones. Pore-filling chlorite is restricted to the middle to upper part of the fining-upward sandstones and appears as discrete flaky plates or rosettes in the pores or attaches to the surfaces of pore-lining chlorite. Pore-lining chlorite originated from smectite precursors that were derived from volcanic material alteration in the early stage of diagenesis, with temperatures primarily about 90 °C. In contrast, pore-filling chlorite was precipitated in pore spaces with higher concentrations of Mg2+ and Fe2+ released from volcanic rock fragments and mica debris dissolution. Moreover, the chemical composition of pore-filling chlorite was affected by early calcite cementation which consumed some ions at temperatures lower than about 110 °C. The thin and continuous pore-lining chlorite is the only chlorite type with a positive effect on reservoir quality, whereas the very thick pore-lining chlorite (>6.7 μm) or pore-filling chlorite significantly reduce the porosity and permeability. Systematic studies of authigenic chlorite can provide important insights into the reservoir heterogeneity and are applicable to the exploration and development of tight oil. • Performed a detailed investigation on types and distribution of authigenic chlorite. • Clarified the origin of two generations of authigenic chlorite. • Determined the impact of authigenic chlorite on reservoir heterogeneity. [ABSTRACT FROM AUTHOR]

Reservoir quality is the key parameter controlling shale oil enrichment. However, the complicated lithofacies, which control diagenesis, result in reservoir heterogeneity in the shale strata, creating challenges for reservoir prediction. Combining with the detail analysis of inorganic and organic geochemistry, characteristics of the main lithofacies and diagenetic alteration were investigated in Permian Lucaogou Formation, Jimusar sag, Junggar Basin. Laminated rocks with laminae combinations containing pyroclastic and terrigenous clastic laminae, terrigenous clastic and dolomite laminae, and pyroclastic and carbonate laminae were identified. Massive rocks mainly includes dolomicrite, dolomitic tuff, silt dolostone, dolomitic siltstone, and tuffaceous siltstone. Feldspar dissolution and carbonate cementation were the two dominant diagenetic types, further causing heterogeneity of the shale oil reservoirs. Feldspar dissolution generally occurred in laminated rocks containing pyroclastic laminae with low content of carbonate minerals (PL1), as well as in their adjacent interbedded siltstones. Carbonate cementation generally occurred in laminated rocks containing carbonate laminae, as well as in their adjacent interbedded siltstones. Whether in laminated and massive rocks, sparry carbonate crystals were also precipitated adjacent to organic matter and in microfractures. In the burial processes, enriched transition metal elements of V and Ti in pyroclastic laminae promote catalytic pyrolysis of organic matter. Abundant CO 2 produced during the catalytic pyrolysis promoted feldspar dissolution. Dissolution of sedimentary carbonate minerals by organic acids and CO 2 provided the material sources for secondary carbonate cements. It promoted the recrystallization and re-precipitation of sparry carbonate crystals adjacent to organic matter and in the terrigenous clastic laminae or interbedded siltstones. Ca2+, Mg2+ Sr2+, Fe2+, and Mn2+ produced by the transformation of smectite to illite in the pyroclastic laminae also provided Ca source for secondary carbonate cements. External diagenetic fluids providing materials along microfractures is another genesis of carbonate cements in the Permian Lucaogou Formation. • Petrographic features in the shale strata in Permian Lucaogou Formation were identified based on in situ analytical method. • Differential feldspar dissolution and carbonate precipitation generally occurred in different lithofacies. • Catalytic pyrolysis of organic matter in presence of transition metal elements in pyroclastic laminae promoted dissolution. • Development of carbonate cements in the shale strata were mainly provided by three material sources. [ABSTRACT FROM AUTHOR]

Understanding reservoir heterogeneity and associated diagenesis, have a paramount importance for efficient petroleum exploration and development strategies. Diagenetic variations and factors that control these processes have, therefore, been investigated in this study using petrographic and geochemical methods, including cathodoluminescence, scanning electron microscopy, x-ray fluorescence, electron probe microanalyses and stable isotope analysis. Mechanical compaction and chemical diagenesis trends have obviously varied as a function of rock types and geological locations. The conglomerate reservoirs with high content of tuffaceous matrix and characterized by poor sorting showed stronger compaction. The tuffaceous matrix displays a large degree of dissolution during the progressive burial diagenesis due to its thermodynamic instability. The tuffaceous matrix in Na-rich conglomerate reservoir rocks mainly transformed into analcite, while it transformed into illite in K-rich conglomerate reservoir rocks. In most conglomerate reservoirs far away from deep-rooted faults, calcite cements are indicated to have been sourced from the decarboxylation of organic matter in the interbedded mudstones. Carbonate cements in conglomerate reservoirs near the deep-rooted faults possibly formed from hydrothermal fluids or thermochemical oxidation of pre-existing methane accumulations. This study can serve us an excellent example for the control of sediment composition and paleo-fluid evolution on diagenetic variations in fan-delta conglomerate reservoirs. • Compaction and dissolution in conglomerate reservoirs were controlled by sediment composition and texture. • Authigenic silicoaluminate minerals formation in conglomerate reservoirs were controlled by sediment composition. • Carbonate cements formation in conglomerate reservoirs were controlled by paleo-fluids evolution. [ABSTRACT FROM AUTHOR]

Considering the complex factors controlling volcanic reservoirs, the Carboniferous strata in the eastern slope area of the Mahu Sag (ESMS) in the northwestern Junggar Basin (NJB) were investigated using rock cores, thin sections, scanning electron microscope (SEM), physical properties, major elements, X-ray fluorescence (XRF), well logging, and seismic data. The volcanic rocks revealed by drilling are mostly weathering crust reservoirs (WCRs), the formation of which in and around the study area is significantly controlled by weathering and leaching (WL). Most types of volcanic rock can be improved by long-term weathering. Favorable reservoirs in the ESMS are often developed within 150 m below the tectonic unconformity boundary at the top of the Carboniferous. The longer the weathering duration, the better are the overall quality of the WCRs. Weathering duration of about 40 Ma is probably an important threshold in the NJB. Ultra-long leaching of atmospheric water and strong late dissolution of acidic fluids before oil and gas accumulations are important for reservoir development and petroleum accumulation in volcanic strata filled with authigenic minerals, especially calcite. The early regional tectonic movement affected the volcanic eruption and controlled the lithofacies distribution. The linear density of fractures was negatively correlated with the distance from the main controlling fault. Owing to the relatively weak filling, high-angle fractures contribute significantly to the reservoir permeability. The physical properties of volcanic breccia are better than those of tuff, and the porosity, permeability, and fracture density of andesite are higher than those of basalt. The physical properties of near-source facies belts of a volcanic edifice are better than those of far-source facies ones. Favorable exploration areas are the structural highs and fault zones where the duration of WL is more than 40 Ma, explosive facies and effusive facies near the crater are developed, or the inherited ancient buried hills transformed by faults and fractures near excellent source rocks, where the dissolution of atmospheric water and organic acidic fluids are strong. • Weathering duration controls degree of weathering, affects volcanic reservoir quality. • Most of favorable reservoirs developed within 150 m below tectonic unconformity. • Weathering duration of ∼40 Ma is probably an important threshold in NW Junggar Basin. • Coupling relationship between weathering-leaching and calcite filling plays a key role. [ABSTRACT FROM AUTHOR]