Development characteristics and exploration potential of the Lower Carboniferous black shale in the Guizhong Depression

Abstract

The Carboniferous shale in the Guizhong Depression of central Guangxi presents good exploration potential of shale gas, but its exploration effectiveness is restricted by the unclear distribution, quality and potential of high-quality shale. In this paper, the Lower Carboniferous Luzhai shale was taken as the research object in order to guide the exploration of Carboniferous shale gas in the Guizhong Depression. Based on the field outcrops, well drilling, well logging and test data of the Guizhong Depression and its surrounding areas, the shale formation was studied systematically based on the characteristics of distribution, organic geochemistry and reservoir, and the preservation conditions. Then, the development characteristics of black shale were confirmed and the distribution laws of high-quality shale were summarized. Finally, shale gas exploration was analyzed and the favorable exploration areas were defined. And the following research results were obtained. First, the Luzhai shale in the Guizhong Depression is mainly carbonaceous shale and siliceous shale of deepwater continental shelf facies. Second, the high-quality Luzhai shale is 20–60 m thick and it is characterized by high total organic carbon, good type of organic matters, moderate maturity of organic matter, high content of brittle minerals, good reservoir property and developed pores of organic matters. Third, regional caprocks and roofs & floors are well developed in the Liucheng Slope and the Yishan Sag. They are weakly deformed, suffer from less uplift and denudation, and are far away from the strong strike–slip reverse fault, so they have good preservation conditions. Fourth, shallow wells which were drilled in the surface structures of the Guizhong Depression in the early stage present abundant gas shows, and most of them produce gas flow, which reveals a promising prospect of shale gas exploration in this area. In conclusion, the wide and gentle synclines with large burial depth in the Liucheng Slope and the Yishan Sag are the favorable areas for shale gas exploration in the Guizhong Depression.