Growth and Distribution of Coal-Measure Source Rocks in Mixed Platform: A Case Study of Carboniferous in Bamai Area, Southwest Tarim Basin, China

Abstract

Coal-measure source rocks are generally developed in marsh facies under a humid climate and are rarely reported in a carbonate platform or a mixed platform. Carboniferous seawater intruded from west to east in the Tarim Basin, and mixed platform deposits of interbedded mudstone and carbonate developed in the southwest of the basin. In recent years, with the deepening of the exploration, nearly 20 m coal seams and carbonaceous mudstone source rocks have been found in the Carboniferous lagoon’s tidal-flat background. The hydrocarbon generation potential, development, and distribution of these coal-measure source rocks have become an important issue for oil and gas exploration. Coal seams and carbonaceous mudstones were found in the Carboniferous formation of wells BT5, BT10, and Lx2. The hydrocarbon prospect, development, and distribution characteristics of these coal formations have become an important research topic. The authors conducted organic geochemical tests and analyses of core and samples drill cuttings from multiple wells in the study area, combined with research focused on the identification and distribution of coal seams, dark mudstones, and depositional facies via logging cross plots of different lithology and 3D seismic inversion. The results show that coal-measure source rocks in the BT5 well are related to the set of delta-lagoon sedimentary systems widely developed in the Carboniferous Karashayi Formation. The maximum cumulative thickness of coal-measure source rocks is about 20 m, with total organic carbon (TOC) contents of 0.15–60%, kerogen types II2-III, and vitrinite reflectance (Ro) values of 0.78–1.65%. The rocks have generally low maturity in the northwestern area and high maturity in the southeastern area, and the maturity changes as the burial depth changes. The effective hydrocarbon source rocks such as coal, carbonaceous mudstone, and dark mudstone all show acoustic time (AC) greater than 300 µs/m, and density (DEN) less than 2.3 g/cm3, but possess different gamma ray (GR) values. The GR value is less than 75 API for coal, between 75–100 API for carbonaceous mudstone, and greater than 100 API for dark mudstones. The distribution of source rocks can be identified in the area between the wells according to a 3D seismic inversion impedance (IMP) of less than 7333 m/s·g/cm3. The development and controlled factors of coal-measure source rocks of delta facies in the mixed platform have a significant role for oil and gas exploration of Upper Paleozoic in this area. The coal measure and sandstones of delta in the Carboniferous are expected to form self-generation and self-storage pools in this area.