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Diagenesis plays a crucial role in shale oil reservoir formation. However, complex organic-inorganic interactions result in pore system formation and heterogeneity changes at the centimeter, or even micrometer scale, which controls the evolution of shale oil reservoirs. In this paper, an integrated approach including core observation, thin section observation, cathodoluminescence thin section observation, scanning electron microscope, μ-XRF analysis, electron microprobe analysis, AMICS analysis, LA-ICP-MS analysis, and isotope analysis was used to clarify the laminae combination carbonate-rich shale, diagenetic alterations in different types of shale laminae and the controls on shale oil reservoir formation. According to the lamina combination, the carbonate-rich shale in the Permian Lucaogou Formation can be divided into three types. They are shale consists of dolomitic lamina and terrigenous felsic lamina (Type A shale), shale consists of calcite-rich tufaceous lamina and terrigenous felsic lamina (Type B shale) and shale consists of calcareous lamina and tufaceous lamina (Type C shale). Dolomite cementation is the major diagenesis in terrigenous felsic lamina of Type A shale. The dolomite can be identified as the early stage forming from recrystallization of micrite dolomite, and the late stage precipitate from adjacent dolomitic lamina. In the terrigenous felsic lamina of Type B shale, however, the calcite cementation is the dominated diagenesis, which was sourced from adjacent calcite-rich tufaceous lamina. Also, some feldspar dissolutions occurred in terrigenous felsic lamina during organic matter evolution. For the Type C shale, the main diagenetic alteration is micrite calcite recrystallization in calcareous lamina. The dissolution and precipitation of carbonate minerals between different laminae of carbonate-rich shale reveal that the organic matter evolution significantly influence the diagenetic alterations of inorganic minerals. Particularly, the diagenetic mass transfer and redistribution at the micro-scale in a relatively diagenetic system were confirmed by organic-inorganic interactions in laminated shale. In this process, dissolution pores and recrystallization pores have provided considerable reservoir pores for shale oil. • The laminae combination and diagenetic alterations of carbonate-rich shale were identified. • The diagenetic mass transfer at the micro-scale during organic-inorganic interactions in laminated shale were clarified. • The pores induced by diagenetic mass transfer within a closed system is crucial for shale reservoir formation. [ABSTRACT FROM AUTHOR]

The Triassic climate has been a relatively neglected topic and conventionally considered a "hot-house" world. Using detailed petrographic characterization, in situ geochemical testing, and high-resolution cycle analysis, the Triassic lacustrine shale in the Ordos Basin was studied to confirm that periodic oscillations of the paleo-environment occurred at different timescales during the Triassic. On the millennial scale, increased obliquity induced enhanced transfer of heat and moisture to high latitudes, which then recharged continental aquifers, resulting in an increase in temperature, humidity, and levels of lakes in the mid-latitudes of the Northern Hemisphere. In contrast, decreased obliquity induced temperature and humidity reduction, followed by lake-level decline. Variations in eccentricity further regulated the magnitude of obliquity-forced paleo-environmental evolution. On the decadal–centennial scale, solar activity, including the 360–500 yr cycle I, 81–110 yr cycle II, and 30–57 yr cycle III, caused high-frequency oscillations of humidity and lake level. In the mid-latitudes of the Northern Hemisphere, intense (weak) solar activity promoted the decline (rise) in humidity and lake level. On the interannual scale, in the coastal area of western Panthalassa, El Niño-Southern Oscillation (ENSO)-like cycles also induced significant fluctuations in humidity and reducibility on a 2–8 yr scale. Whether complex paleo-environmental oscillations at different timescales occurred worldwide during the Triassic remains to be verified at other places. It is also noteworthy that high-frequency paleo-environmental oscillations in the Triassic stimulated algal blooms over a short period. The increase in rainfall caused by solar activity and ENSO-like cycles led to a frequent influx of nutrients into the Ordos Basin. The 30–57 yr cycle III induced red algae enrichment at intervals of decades. ENSO-like cycles also promoted algal bloom on 2–8 yr scale to form organic layers in lacustrine shale. • Periodic paleo-environmental oscillation occurred on multi-timescales in the Triassic. • Solar activity and ENSO-like cycles were identified in the Triassic for the first time. • Orbital, solar activity and ENSO-like cycles induced complex climatic changes in the Triassic. • High frequency paleo-environmental evolution stimulated the algal blooms within a brief period. [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]

• Extracted bitumen and solid bitumen were characterized simultaneously. • Characterization was carried out at lamina scale and lithofacies scale. • Biomarkers reveal greater thermal maturity in source rocks than reservoirs. • Raman spectroscopy reveals greater thermal maturity in reservoirs than source rocks. • Organic–inorganic interactions increase the aromaticity of solid bitumen. Organic solvent extracted bitumen (EB) and microscopically observed solid bitumen (SB) carry many geological implications in unconventional source-rock reservoirs. EB is a commonly used term in organic geochemistry, and SB is normally used in organic petrology. Although both EB and SB are secondary organic matter initially formed from kerogen degradation and partially describe the same components, they are defined by different physical and chemical criteria and have specific applications, and thus are rarely comparatively investigated. In this study, by taking the shale of the seventh member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, as a case study, we performed an integrated characterization on the two types of bitumen. The characterization was carried out on source rocks and reservoir assemblages at two scales: lamina-scale (organic-rich lamina and silty lamina) and lithofacies-scale (shale and sandstone). Programmed temperature pyrolysis (Rock-Eval 7 pyrolysis), gas chromatography–mass spectrometry (GC–MS), and Raman spectroscopy were used in this study. The samples are distributed within a 12 m interval and therefore should have experienced the same degree of thermal stress. However, maturity parameters derived from GC–MS for EB and Raman spectroscopy for SB exhibit a different inferred thermal maturity between source rocks and reservoirs at both lithofacies- and lamina-scales. The side-chain scission reactions related (non-)biomarker parameters such as Ts/C 30 H, Σ n C 21− /Σ n C 22+ alkanes, relative C 21 + C 22 sterane content and TA(I)/TA(I + II) suggest higher thermal maturity in source rocks (shale and organic-rich lamina) than the corresponding reservoirs (sandstone and silty lamina), while Raman-derived parameters RBS and G-FWHM indicate higher maturity in reservoirs than the corresponding source rocks. It is speculated that the EB measured by GC–MS comprises saturated and aromatic components corresponding to relatively mobile hydrocarbons. The maturation of the source rock exerts greater control over this component than that of the reservoir. In comparison, the SB measured using Raman spectroscopy mainly consists of solid residue left behind after migration and/or decomposition of a once-liquid oil phase that is less readily able to move. It is more intensely altered by the organic–inorganic interactions (mineral dissolution–precipitation processes) in the reservoir than that in the source rock, resulting in a consolidated SB with higher aromaticity. The storage ability of silty lamina in shale may complicate the data interpretation of geochemical differences in EB- and SB-derived parameters. On a practical note, when assessing thermal maturity, taking into the account the lithology or rock texture, which affects the organic–inorganic interactions, as well as specific components detected by different techniques may provide helpful clues to explain some contradictory results. [ABSTRACT FROM AUTHOR]

Lamina, a common lithologic unit in shale, is the smallest unit for organic–inorganic fluid–rock interactions. The effects of lamina-scale diagenetic processes are superimposed to exert on lithofacies-scale diagenetic sequences, which further impose on the entire diagenetic system of a shale reservoir. Organic acids from kerogen maturation are indispensable components in diagenetic reaction. However, previous studies mainly focused on low molecular weight (LMW) water soluble acids, few studies have paid attention to the role of high molecular weight (HMW) carboxylic acids dissolved in fluid–rock reactions. Herein, by taking the shale of the seventh member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, as a case study, we investigate the mutual impacts between mineral alteration and HMW organic acid degradation. The characterization is carried out at two scales: lamina-scale (organic-rich clay-rich lamina and organic-lean silty lamina) and lithofacies-scale (shale and sandstone). Integrated analytical techniques (microdrill technique, thin section observation, X-ray diffraction, advanced mineral identification and characterization system, Rock-Eval pyrolysis, Fourier transform ion cyclotron resonance mass spectrometry) are performed. Results show that, in reservoirs of both silty lamina and sandstone, consumption of LMW aqueous acids through mineral transformation reactions promotes the forward evolution of HMW carboxylic acids in the bitumen phase toward decarboxylation or dehydration. During diagenesis, short chain fatty acids (SCFAs) are more easily broken down into small acids and, thus, more susceptible to be consumed. The lamina-scale reservoirs (silty lamina) have the privilege to receive abundant aqueous organic acids to maintain the micro-geochemical environment and progressively boost organic–inorganic fluid–rock interactions. The resultant dissolution pores not only accumulate plenty of bitumen but also an appreciable amount of bitumen dissolved acids. Because of the strong storage capacity of silty lamina within lithofacies-scale source rock (shale), fluids are less readily to expel out to lithofacies-scale reservoir (sandstone). As a result, the sandstone may not receive enough fluids from shale to maintain a consistent diagenetic environment, resulting in the observed discontinuous diagenesis of quartz precipitation transition to quartz dissolution. [ABSTRACT FROM AUTHOR]