Characteristics of coal-measure source rock and gas accumulation belts in marine-continental transitional facies fault basins: A case study of the Oligocene deposits in the Qiongdongnan Basin located in the northern region of the South China Sea

This study carried out the development and distribution prediction research on coal-formed gas accumulation belts from the perspectives of the development and distribution of the coal measure source rock and the thermal evolution degrees of those rock masses in marine-continental transitional facies fault basins. The purpose of the current study was to improve the accuracy of gas reservoir explorations in the sea areas. The methods used in this study were based on the systematic analyses of the drilling, logging, geochemistry, and seismic data of the Oligocene deposits in the Qiongdongnan Basin of the northern region of the South China Sea. It had been previously established that the sedimentary processes of the fault basin were mainly controlled by faulting, and (fan) deltas had often developed in the fault bending sections, tectonic transformation sections, and fault ends. The distribution along the fault zone was observed to be beaded. During the early Oligocene period of the Qiongdongnan Basin, there were strong extensional actions, and the marine and continental transitional facies strata had developed forming the northern fault depression and the central fault depression. Three (fan) delta belts were formed in the slope belt in the northern fault depression and also in the slope belts located in the north and south of the central fault depression. Due to the superposition of the humid and hot climate during that period, the majority of the coal measure source rocks were developed in each (fan) delta. Also, three macroscopic coal measure source rock belts were formed. The types of source rock had mainly included autochthonous coal, allochthonous coal, and terrigenous marine mudstone, with terrigenous higher plant organic matter accounting for more than 50%. After the deposition of the lower Oligocene period, the basin continued to sink, and the coal measure source rock masses continued to be heated. During that period, the maturity of the organic matter had continued to rise. In the northern slope belt of the northern fault depression, the maturity of the organic matter in the coal measure source rock belt were low and the gas generation abilities were weak, which was advantageous to the formation of potential gas accumulation belts. The coal measure source rock belt on the northern side of the central fault depression had a high maturity of organic matter and strong gas generation ability, forming a macroscopic gas accumulation belt. In addition, the Yacheng 13-1 large coal-formed gas field had been discovered. The maturity of the organic matter had varied greatly in the coal measure source rock belt on the southern side of the central fault depression. The coal measure source rock masses with high maturity had strong gas generation abilities and had also formed a macroscopic gas accumulation belt, and three large and medium-sized coal-formed gas fields (such as Lingshui 17-2) had been discovered. The gas accumulation belts on the southern and northern sides of the central fault depression were favorable areas for coal-formed gas explorations. Furthermore, the trap zones in and near the coal measure source rock belts with mature thermal evolution were determined to be the most favorable areas for coal-formed gas explorations.