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Organic matter (OM) type critically controls the hydrocarbon generation potential and organic pore development in black shales. However, maceral variation in lacustrine shales and its control on hydrocarbon generation potential and organic pore development are not yet well understood. In this study, 15 Chang 7 Member shale samples of the Yanchang Formation, Ordos Basin, were investigated with organic petrography, Rock-Eval pyrolysis, and a scanning electron microscope to study the maceral composition, hydrocarbon generation potential, and organic pores in this black shale succession. The results show that the studied shales are in the oil window (Ro~0.70%). OM belongs to Type I and Type III kerogen, as demonstrated by Rock-Eval pyrolysis. Macerals in the Chang 7 Member shales are composed of amorphous OM, alginite, sporinite, liptodetrinite, vitrinite, inertinite, and solid bitumen. Amorphous OM and alginite are major hydrocarbon-generating macerals, and their content determines the hydrocarbon potential of shales. Secondary organic pores were not observed in the studied Chang 7 Member shales due to either a low thermal maturity or a dominance of terrigenous OM. Maceral variation can affect the reliability of using Rock-Eval Tmax as a thermal maturity indicator. This study provides important insights into maceral control on hydrocarbon generation and organic pore development in black shales, calling for a critical evaluation of OM in black shale successions with organic petrography. [ABSTRACT FROM AUTHOR]

The pore structure characteristics of shales are controlled by their mineral and organic matter compositions. However, the contributions of different components to the pore structure characteristics of lacustrine shales remain poorly understood. In this study, fifteen Chang 7 Member shales of the Yanchang Formation, Ordos Basin, were investigated through total organic carbon (TOC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and low-pressure N2 and CO2 adsorption analyses to study the control of shale composition on the pore structure characteristics of lacustrine shales. The results show that the average TOC content of the Chang 7 Member shales is 9.63 wt.%. XRD analysis shows that minerals in the Chang 7 Member shales consist of quartz, feldspars, clay minerals, and pyrite. The clay minerals were dominated by illite, chlorite, and interstratified illite/smectite. The mesopore characteristics of the Chang 7 Member shales and micropore characteristics of organic-lean shales are mainly controlled by clay minerals, whereas the micropore characteristics of organic-rich samples are controlled by both clay minerals and organic matter. SEM observations show that the phyllosilicate framework pores are the main pore type in the Chang 7 Member shales. The results of this study provide important insights into compositional control on the pore structure characteristics of lacustrine shales. [ABSTRACT FROM AUTHOR]

The fractured-vuggy carbonate reservoirs display strong heterogeneity and need to be classified into different types for specific characterization. In this study, a total of 134 cores from six drilled wells and six outcrops of the Deng #2 and Deng #4 members of the Dengying Formation (Sichuan Basin, Southwest China) were selected to investigate the petrographic characteristics of void spaces in the fractured-vuggy carbonate reservoirs. Four void space types (VSTs) were observed, namely the solution-filling type (SFT), cement-reducing type (CRT), solution-filling breccia type (SFBT) and solution-enlarging fractures and vugs type (SEFVT). The CRT void spaces presented the largest porosity and permeability, followed by the SEFVT, SFBT and SFT. The VSTs presented various logging responses and values, and based on these, an identification method of VSTs using Bayes discriminant analysis (BDA) was proposed. Two test wells were employed for the validation of the identification method, and the results show that there is good agreement between the identification results and core description. The vertical distribution of VSTs indicates that the SFT and SEFVT are well distributed in both the Deng #2 and Deng #4 members. The CRT is mainly found in the Deng #2 member, and the SFBT occurs in the top and middle of the Deng #4 member. [ABSTRACT FROM AUTHOR]