Pore structure characteristics of marine Silurian shales in the Sichuan Basin, China: Insights to reserve assessment and production design

Abstract

Due to shortage of conventional resources, tight/shale oil/gas reservoirs have become increasingly important in balancing the energy consumption in China. As the most important shale reservoirs in China, marine Silurian shales in the Sichuan Basin have received intensive investigation during recent years in the area of pore structure characterization and reserve assessment. However, no work has been conducted to compare the China shale plays with other mature shale plays in the North America using multiple experimental methods based on the mineralogy analysis and organic geochemistry. In this paper, pore structure, formation porosity, total organic carbon (TOC), thermal maturity, and mineralogy of 36 samples in Longmaxi shale and 7 samples in Qiongzhusi shale have been investigated using multiple experimental methods. FIB-SEM imaging results show that Longmaxi shale gas reservoir include primary pores, residual intergranular pores, dissolution pores, and intergranular pores. The majority of the pores are distributed in residual intergranular pores, secondary dissolution pores, and intercrystalline pores, but less in primary intergranular pores. X-ray diffraction analysis shows that the clay minerals of Longmaxi shale reservoir mainly consist of illite and kaolinite, then followed by chlorite. The content of kaolinite is from 7.2% to 16.87%, and the average is 12.21%. The content of illite is from 8.03% to 26.8%, with the average of 16.1%. TOC tests show that most of the samples have their TOC more than 0.5%, which means Longmaxi formation has a potential to produce shale gas. Clay minerals mainly include quartz, K-feldspar, calcite, dolomite, anorthose, and pyrite. The quartz has the highest content value, followed by calcite, which are 29% and 22% average respectively. The content of pyrite and anorthose are less than 10%. The average content of brittle minerals of Longmaxi formation samples is 66% (>50%), which is suitable for hydraulic fracturing. Thermal maturity tests show that Vitrinite reflectance of each sample is more than 2.0%, which indicates that it is high enough to generate shale gas in the formation. Comparing with other mature shale plays, the Longmaxi and Qiongzhusi shale plays have lower porosity and lower TOC than that of the U.S. shale plays, and the quartz content and thermal maturity are similar. The Qiongzhusi shale and Longmaxi shale are over-mature (Ro>2.5%) compared with U.S. shale plays. The results of this study will contribute to the formation characteristics understanding and reserve assessment of China shale plays. Also, it will facilitate the development of suitable and efficient production plan for China shale plays.