Gechemistry of Palaeozoic Black Shales in Northeast Sichuan Basin, China

Liang, Yingbo; Zhang, Shuichang; Zhu, Guangyou

Sichuan Basin is the most important gas region in China. Most of gases are originated from three sets of Paleozoic black shales, which are Qiongzhusi Formation of Lower Cambrian, Longmaxi Formation of Lower Silurian and Longtan Formation of Upper Permian. In the early periods of Cambrian and Silurian, the northeast of Sichuan Basin located in the passive margin of Yangtze Plate, the depression zones of which deposited the thick black shales. The black shale of Qiongzhusi Formation deposited in slope facies with abundant low-degree Homoneneae hydrobios. The shale of Longmaxi Formation deposited in confined basinal facies rich in various graptolites, the richness of graptolite fossils indicates the contents of organic matter. The Longtan Formation is composed mainly of thick-bedded coal measure shale and mudstone, the organic matter of which is originated from ancient nematophyta in swamp-tidal flat facies.

This study samples over 600 shale samples in 11 outcrop locations in northeast Sichuan Basin. Rock_Eval pyrolysis, marine vitrinite reflectance(Rv) and vitrinite reflectance(Ro) were carried out in most of the samples, some selected samples tested the carbon isotopic values of kerogen. Based on the data, the TOC contents of Qiongzhusi Formation black shale varies from 0.5 to 4.78%, an average of 1.88%, the maturity values (Rv) of it is 3.4%, in a over mature stage, the kerogen carbon isotopic values are from -32.1 ‰ to -33.0 ‰, mainly II type of organic matter. In Longmaxi Formatin, the TOC contents are between 0.5 to 8.65%, with an average of 2.82%, marine vitrinite reflectance Rv average of 2.2% in the over mature stage, kerogen carbon isotopes are from -29 ‰ to -31 ‰, mainly II type. In Longtan Formation, the TOC contents are between 0.5 to 12.7%, with an average of 4.3%, the maturity from 1.6 to 2.6%, in a mature and over mature stage, kerogen carbon isotope varies from -22 ‰ to -26 ‰, mainly type III organic matter. The three sets of shales are gas prone source rocks.

All the three black shale formations contained organic-rich (TOC> 2%) layers, the Qiongzhusi and Longmaxi marine shale are favorable to forming shales gas among them, whose maximal thickness reach 40m and 120m of TOC >2% respectively. The shale layers with high TOC contents widely distributed, showing a huge shale gas potential.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013