Geochemical characteristics and organic matter accumulation of argillaceous dolomite in a saline lacustrine basin: A case study from the paleogene xingouzui formation, Jianghan Basin, China

Abstract

Shale oil resources have been discovered in argillaceous dolomite in the Lower Member of Paleogene Xingouzui Formation (LXF) of the Jianghan Basin, opening up a new field of shale oil exploration and development. Previous studies have mainly focused on the evaluation of reservoir quality, but the research on the enrichment mechanisms of organic matter (OM) in argillaceous dolomite is very limited or unknown. On the basis of organic geochemical and mineralogical data, combined with sedimentological analyses, the factors controlling the differential enrichment of OM in the LXF Member II oil Bed argillaceous dolomite are studied. The results show that the average TOC content of the samples is greater than 1.0%, which could be regarded as good source rocks. The kerogen type is mainly type II, with a small amount of type I and type III, and most of the samples are in the low-mature stage. Vertically, the development of the LXF Member II oil Bed argillaceous dolomite can be divided into three stages: the lower section (LS) has TOC content ranging from 0.57 to 4.95%, the middle section (MS) has a higher TOC content, between 0.55 and 9.52%, and the TOC content of the upper section (US) is relatively low, between 0.41 and 3.49%. The distribution of biomarkers showed that both higher plants and aquatic organisms contributed to the OM accumulation. The parameters of biomarkers such as pristane/phytane (Pr/Ph), gammacerane/C30 hopane (G/H), extended tricyclic terpane ratio (ETR) and their degree of dispersion, as well as sedimentological characteristics, indicate that the LS was deposited in a stratified deep waterbody with anoxic bottom water, the MS was deposited in a saline stratified moderate-deep waterbody with anoxic bottom water, and the US was deposited in a disturbed saline waterbody. Different sedimentary environments controlled the source of OM, the effectiveness of nutrients recycling and preservation conditions, which jointly affect the enrichment of OM. The stratified moderate-deep waterbody with anoxic bottom water and no physical disturbances, coupled with low terrigenous organic matter (TOM) inputs and the contribution of algae and other phytoplankton, are the best conditions for the enrichment of OM in argillaceous dolomite.