ENRICHMENT MECHANISM OF ORGANIC MATTER IN SOURCE ROCKS OF THE 2ND MEMBER OF LIUSHAGANG FORMATION IN BEIBUWAN BASIN

Abstract

Introduction The Beibuwan Basin, located in the Beibu Gulf of the South China Sea, is a Cenozoic extensional sedimentary basin (Zhang et al., 2013). The 2nd member of the Eocene Liushagang Formation is the main source rock consisting of oil shales, mudstone and dark shales. This suite of source rocks has high organic matter abundance and good organic matter type (Fu et al., 2017), but the paleoenvironment, mechanism of the enrichment of organic matter and development model of source rock remain unclear (Huang et al., 2012). It is of great significance to define the enrichment mechanism of organic matter and control factors of this suite of source rocks, and to establish the development model of oil shale in the freshwater lake basin, which is also of guiding value to deepen the oil and gas exploration in the study area. In this work, the enrichment mechanism of organic matter and development model of source rock were studied by comprehensively applying seismic, drilling, logging and geochemical data, adopting geological, geochemical and geophysical research ideas. The data of conventional geochemistry, biomarker, organic maceral and palynology were analysed comprehensively. Based on the distribution characteristics of some nutrition elements(such as P, Fe and Mo) and geochemical parameters(such as 4-methyl sterance index, gammacerane index, S, Sr/Cu, Sr/Ca, V/(V+Ni), δCe, (La/Yb)N, VS-MnO (Sugisaki et al,1984)), combined with the La/Yb-∑REE chart (Allegre C J et al,1978), discrimination function chart of major elements and rare earth element partition curve chart, the geochemical characteristics and distribution features of oil shale, mudstone and dark shale of the 2nd member of Liushagang Formation were summarized. Meanwhile, the paleoenvironment of oil shale, mudstone and dark shale were reconstructed, source composition of organic matter was determined. Furthermore, the controlling effects of the provenance properties, paleoproductivity of lake, redox conditions and sedimentation rate on the accumulation of organic matter in oil shale and mudstone shale were analysed. Finally, the eutrophication mechanism of freshwater lake basin water was discussed, and the development model of oil shale in the study area was established.