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This paper discusses the effect of the source rock–reservoir assemblage and the assemblage–induced variability in hydrocarbon expulsion and the subsequent molecular composition as well as molecular thermal maturity within a hybrid shale system. The characterization work was conducted on lithofacies and microlamina scales using core samples from the Chang 7 3 sub-member of the Triassic Yanchang Formation in the Ordos Basin, China. Samples were collected from a narrow interval with a depth range of less than 15 m, and the main characterization work was performed by Rock-Eval pyrolysis and GC–MS analysis. The results show that chemical fractionation of preferential expulsion and migration of the saturated fraction exists in the source rock–reservoir assemblages at both the lithofacies and lamina scales. However, the molecular composition behaves differently at the lithofacies and lamina scale's source rock–reservoir assemblages, in which ∑C 21 − /∑C 22+ is higher in lamina scale reservoir but lower in lithofacies scale reservoir. It is assumed that the low-molecular weight n -alkanes also follow molecular fractionation. The lithofacies reservoir has a lower ∑C 21 − /∑C 22+ because of the strong storage capacity of the laminated micro-reservoir within shale, which prevents the newly generated lighter oil from being charged into the lithofacies reservoir. The variation trends of thermal maturity indices Ts/hopane, the relative pregnane content, and TA(I)/TA(I + II) ratios, which have the same chemical basis with ∑C 21 − /∑C 22+ , carry the same maturity signature as ∑C 21 − /∑C 22+. The above profile of the molecular composition and molecular-derived thermal maturity parameters indicate that within the short interval of a shale system where no differences in thermal maturity are expected, chain scission reactions and their derived thermal maturity indicators are very sensitive to source rock and reservoir. In addition, within a shale system, oil is more easily to expel out from the organic-rich lithofacies that are interbedded with organic-lean lithofacies. Oil expulsion may promote both chain cracking of oil and subsequently kerogen decomposition. This may provide geological evidence to explain why the frequent-stacking assemblage of source rock and reservoir lithofacies in a hybrid shale system is an ideal target for shale oil exploration. [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]

The upper fourth member of the Eocene Shahejie Formation (Es4s) is a typical example of a Bohai Bay Basin lacustrine shale. These sedimentary units are characterized by markedly different mineral compositions compared to marine siliciclastic shales. Thus, understanding the characteristics and sedimentary environment of lacustrine shales enables analysis of their origin, enrichment of organic matter (OM), distribution of organic rich shales, and exploration targets for shale oil and gas. Based on thin sections and field emission scanning electron microscope observations from well core samples, combined with X-ray diffraction and geochemistry, the mineralogy, lithofacies, organic and geochemical characteristics as well as the sedimentary environment of the Es4s shale are analyzed in this paper. The results show that, across the study area, the Es4s shale is mainly composed of calcite, clay minerals, and quartz, but also contains subordinate dolomite, feldspar, pyrite, and anhydrite. On the basis of mineral composition, total organic carbon (TOC) content and sedimentary structures, eight lithofacies are identified. The TOC content of samples ranges between 0.15% and 11.4%, with an average of 2.27%, while OM primarily consists of Type I kerogen. Based on paleoclimate and paleosalinity data as well as redox conditions and primary productivity, the sedimentary environment of the Es4s shale can be further divided into four intervals: Intervals I, II, III, and IV, and these four intervals have large differences, rather than persistently warm-damp climate and reducing condition. The evolution of these intervals is also analyzed in detail in this study, and the controls on lithofacies and OM accumulation are also discussed. The results of this study suggest that the development of mineral compositions and frequent vertical lithofacies associations are strongly controlled by the cyclic evolution of the sedimentary environment. Thus, the accumulation of OM occurs as the result of a number of inter-connected factors including primary productivity, terrestrial inputs, and variation in oxidizing-reducing conditions. Across a given interval, these factors can either work together, or one can play a leading role. [ABSTRACT FROM AUTHOR]