Characterization of the reservoir in Lower Silurian and Lower Cambrian shale of south Sichuan Basin, China

Abstract

Natural gas has become an important source of energy. In China, external dependency on natural gas reached 32.2%. China is rich in shale gas resources, and both government and oil companies have promoted the development of shale gas. Lower Silurian Longmaxi Formation (LSLF) shales and Lower Cambrian Qiongzhusi Formation (LCQF) shales are believed to be favorable shale gas targets in the Sichuan Basin. Oil companies have extracted gas from LSLF shale but not from LCQF shale in and around the Sichuan Basin. The total organic carbon content (TOC) increases with buried depth in LSLF shales and LCQF shales, but the organic matter (OM) in the LCQF shales is more compacted than LSLF shales. The mineral brittleness index of these shales means that they are favorable for hydraulic fracturing to generate the complex fracture geometry in both shales. The OM is rich in the pores and the surface porosity of OM is from 7.18 to 9.52% in the LSLF shale and LCQF shales with low TOC (TOC <1.0%). The LCQF shales with high TOC are poor in the OM host pores and surface porosity of OM is less than 0.1%. The gas contents of LSLF shales increase with the TOC contents, but there is a poor correlation in LCQF shales, especially in LCQF shales with TOC of greater than 3.0%. There was a linear correlation between gas content and VTOC in both shales, which was multiplied by the TOC and surface porosity. For the LCQF shales, the TOC and the surface porosity of OM are also important to the shale gas content and selection of favorable resource targets. The TOC of LCQF shales was less than 1.0% and greater than 3.0% in this paper, so the inflection point of surface porosity should be further examined in OM.