Shale gas accumulation in the Upper Permian Leping Formation: a case study of well Ganfengdi-1 in the central Pingle Depression, Jiangxi Province

The Upper Permian Leping coal measures are widely developed in the Pingle Depression, Jiangxi Pro-vince, which is a key area for shale gas exploration in the Lower Yangtze region. The core data of well Ganfengdi-1, the first geological survey well in the Pingle Depression, which drilled through the main shale of Leping Formation, were studied in order to determine the shale gas accumulation conditions and exploration potential in the central Pingle Depression. More than 200 core samples were analyzed using non-uniform sampling. By means of thin section observations, organic geochemical analysis, X-ray diffraction analysis of whole rock and clay minerals, argon ion polishing scanning electron microscopy and in situ analysis of shale, it was determined that a large thickness of the black carbonaceous shale of the Leping Formation in the central Pingle Depression has a high content of organic carbon. The type of organic matter is good, and the stage of high maturity gas generation is favorable for shale gas generation and accumulation. In addition, the content of brittle minerals is high. The brittle index I is 26%-53.5%, averaging 37.3%, while the brittle index Ⅱ is 23.3%-52.6%, averaging 39.0%, which indicates that the shale formation has a good compressibility. Organic pores and intragranular pores in clay minerals are well-developed. The internal connectivity of the pores is good, and the specific surface area of the minerals is high, which is of great significance for the occurrence of shale gas. The micro-cracks mainly play a role of communication and diversion to improve the permeability of the shale. By analyzing the exploration potential parameter indexes of the 4 sections of the Leping Formation (Guanshan, Laoshan, Shizishan and Wangpanli), it is considered that the Wangpanli and Laoshan shale have more outstanding gas show, thickness, TOC content, organic matter pore and mineral composition, which can be regarded as favorable shale gas exploration targets in the Leping coal-bearing system in the central Pingle Depression.