Geochemical Characterization and Shale Gas Potential of the Longmaxi Shale Formation of Lower Silurian in JiaoShiba Area, Southeast Sichuan Basin, China

Abstract

The first shale gas field in China is located in Jiaoshiba area, southeast of Sichuan Basin, and the highest gas production of a well reaches one million cubic meters per day. During early Silurian, the depositional environment changed from deep-water continental shelf to shallow-water continental shelf and deposited Longmaxi formation (S1l), which is rich in organic matter and graptolite fossils. The upper part of the shale is composed of gray mudstone and argillaceous siltstone with an average thickness of 170m; the lower part is carbonaceous shale and siliceous shale with an average thickness of 90m. The porosity and permeability of S1l shale is mainly distributed in 1.2~8.0% and 0.001~355.2mD, averaging at 4.6% and 21.93mD, respectively. To figure out its organic geochemical characteristics and evaluate the resource potential, fifteen samples are analyzed by total organic carbon (TOC) content, vitrinite reflectance (Ro) and isothermal adsorption. The values of TOC range from 0.68 to 5.28 wt% with an average 2.99 wt% and increase gradually with the increasing depth. The lower part has higher TOC (3.50~5.29 wt%) compared to the upper units, which typically has less than 1 wt%. The shale is overmature with Ro varying from 2.2 to 3.2%. Organic maceral analysis reveals the organic kerogen type is type I. Gas carbon isotope data show ethaneδ13C values below −29‰ suggesting the gas may be generated by sapropel type organic matter. The isothermal adsorption shows that the adsorption capacity increases rapidly (from 60.74 to 166.43 scf/ton) with increasing TOC values (from 1.1 to 6.28 wt%), especially for the lower units reaching the maximum adsorption quantity of 166.43 scf/ton. In the whole research area, the middle part performs better than the rest area. Vertically, the lower parts would be the best target with most favorable geochemical characteristics and the adsorption quantity.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015