Restoration of sedimentary environment and geochemical features of deep marine Longmaxi shale and its significance for shale gas: A case study of the Dingshan area in the Sichuan Basin, South China

Abstract

With the urgent need to increase shale gas production and reserves, deep shale gas has gradually become an important target for further exploration in China. However, there is still a lack in systematic research on the quality of different lithofacies types of shales in deep shale gas reservoirs. The overmature shales have a common problem, that is, the organic matter (OM) quality could not be clearly characterized by using conventional organic geochemical indicators, hindering the process of fine shale gas exploration. In this study, the deep marine Longmaxi (LMX) shale samples in the Dingshan area, southeastern Sichuan Basin were selected as a study case to investigate their lithofacies types, sedimentary environment and organic matter accumulation (OMA) mechanism, and restore the original OM attributes including original organic carbon content (TOCo) and original hydrogen index (HIo) for the different shale types to identify the favorable lithofacies for shale gas enrichment. The results show that the LMX shale can be divided into five different lithofacies, such as the OM-rich siliceous (RS) shale, OM-rich mixed (RM) shale, OM-lean siliceous (LS) shale, OM-lean mixed (LM) shale and OM-lean argillaceous (LA) shale, with a gradually decrease of their TOC contents. The TOC content of the LMX shale has a positive synergy with the indicators of paleoproductivity and redox conditions. However, since the studied area is close to the underwater highland with a high oxygen content in the bottom water and a great input of terrigenous debris, the OMA mainly depends on the level of high paleoproductivity. The analysis of sedimentary environment indicates that the OM amount and quality in various lithofacies shales are different. The RS shale at the lower part of the LMX shale has a better OM quality and a greater TOCo, which result in the greater effective organic carbon (TOCe), gas production potential, and retained gas content. It is followed by the RM, LS, and LM shales. In contrast, the LA shale lacks material base for shale gas enrichment. Combined with the available data from other deep blocks in the southern Sichuan Basin, the original OM attributes of RS and RM shales are always better than those of LM, LS, and LA shales. For deep and ultra-deep Longmaxi shale gas exploration, the RS shale should be the first choice, and then the RM shale.