Effects of lacustrine depositional sequences on organic matter enrichment in the Chang 7 Shale, Ordos Basin, China

Abstract

Chang 7 Shale, characterized by lacustrine black organic-rich shales and interbedded siltstones, has become a target for shale oil exploration and development in the Upper Triassic Ordos Basin, China. However, the heterogeneous enrichment of organic matter (OM) has led to controversy regarding source rock evaluation and difficulty in accurate prediction of favorable zones. Sequence stratigraphy, as a tool for prediction of sedimentary deposits has encountered challenges in lacustrine deep-water environments because of rapid facies changes and limited lateral continuity of key beds. In this study, a novel method is proposed to predict OM enrichment based on parasequences division and calibration of well logging and core lithology through analyses of thin sections and geochemical data from 238 core samples, as well as available well-logging data from a selected well. Chang 7 shales were simplified into three facies in terms of sedimentary structure and mineral composition, combining geological genesis with lateral correlation. Argillaceous laminated shale (Facies 2), with the highest average total organic carbon and S1 + S2 values, is the most favorable facies for shale oil exploration. The Chang 7 Member was divided into 15 parasequences and six parasequence sets, within one transgressive-regressive sequence. Depositional sequences have a significant effect on the enrichment of OM, given 1) the influence of lake-level fluctuations on OM accumulation, 2) the OM enrichment determined by shale facies, and 3) the distribution of shale facies controlled by parasequences. In particular, the most favorable locations are the bases of parasequences (generally the lower third to half from the bottom) in the TST and early RST. The application of this method to a test well showed excellent performance based on verification from the organic geochemical data of 86 samples. Five favorable targets of Facies 2 were marked within fifteen favorable locations. By linking meter-level depositional sequences with centimeter-level OM enrichment, this study provides an alternative approach to determine the target area in a single well profile and realize isochronous inter-well prediction for deep-water shales.