Prediction of the gas-generating characteristics of the Qiongzhusi and Longmaxi Formations, Yangtze Platform, southern China, using analogues

China has been said to have the largest putative shale gas resources in the world. The highest potential occurs in the Sichuan Basin, with the overmature Qiongzhusi (Cambrian) and Long-maxi (Silurian) Formations as prime exploration targets. Here, the likelihood of late gas formation is examined using less mature equivalents from the Georgina Basin (Australia) and the Baltic Basin (Lithuania). We consider the respective functions of kerogen and polar bitumen in gas generation with reference to the Eagle Ford, Yanchang, Niobrara, and Vaca Muerta Formations. Both of the lower Paleozoic shales are bitumen poor in a geo-chemical sense, this being in stark contrast to the Mesozoic shales, which are bitumen rich. Kerogen is, therefore, the major gas precursor in the Cambrian and Silurian of the Sichuan Basin. Graptolites and solid bitumen are petrographically dominant. The solid bitumen exhibits flow structures and is deduced to be highly polar because it is insoluble in dichloromethane. Secondary cracking kinetics determined for the Arthur Creek are closely similar to source rocks containing predominantly paraffinic oil. Late gas generation from very stable refractory kerogen structures via alpha-cleavage reactions at maturities above 2% equivalent vitrinite reflectance (R-o) was verified, and importantly, the upper ceiling for late gas generation in Paleozoic shales of the Sichuan Basin is set at 3% R-o. As far as the Qiongzhusi shale is concerned, raising the prospective acreage to a 3% R-o limit brings an additional contribution of 40 mg HC/g total organic carbon of late gas charge. The same is true for the extensive fairway of the Longmaxi shale along the western flank of the basin, close to the subcropping erosional edge.

Automated summary

China is believed to have the largest shale gas resources globally, with the highest potential in the Sichuan Basin targeting the Cambrian and Silurian Formations. Lower Paleozoic shales in the basin are kerogen-rich and bitumen-poor, with late gas generation capped at 3% equivalent vitrinite reflectance. Late gas charge potential can be increased by raising the prospective acreage to the 3% R-o limit.