Organic Matter Characteristics in Silurian Marine Mudstone and Factors to Shale Gas Accumulation in Sichuan Basin, China

Zhang, Xiaolong; Li, Yanfang; Lv, Haigang; Yan, Jianping; Zhang, Tongwei

Shale gas is one kind of unconventional gas resources and stores in organic-rich shales and mudstones. The global endowment of shale gas resource is 456 trillion cubic meter, mainly distributed in North America, Asia, Europe and Africa. It is assumed that nearly 40% of this endowment would be economically recoverable. Recent preliminary studies show that shale gas reserves in Cambrian Qiongzhusi Formation and Silurian Longmaxi Formation in Southern Sichuan Basin, China is about 6.8-8.5 trillion cubic meter.

In order to examine the vertical changes in organic matter content of Silurian Longmaxi Fm. marine shale, two fresh outcrop profiles located in Southwest margin and South margin of Southern Sichuan Basin were selected for our study. The Shuanghe Town outcrop is East-West anticline, developed black or dark gray graptolite shale, carbonaceous shale, siliceous shale and silty shale with argillaceous from bottom to top. The Bowangshan Town outcrop is north inclined strata, lithofacies is consistent with Shuanghe Town one.

TOC content, Rock-Eval pyrolysis and petrological characterization of 124 Shale samples from the two outcrops were conducted. The results show that TOC content at the bottom of the Longmaxi Fm. ranges from 3.04% to 7.28%, and the thickness of high TOC content interval is about 30m. The value of Ro is more than 2.5%, indicating high to over thermal maturation favorable to gas generation. X-ray diffraction analysis shows that the brittle minerals content is about 37.1%-71.2%, in which quarts content is 24.3%-43.5%, potassium feldspar and plagioclase content is 4.3%-10.8%, calcite content is 8.5%-16.9%, clay mineral content is 37.4%-48.3%. Clay mineral is dominated by stable mineral, lack of swelling clay minerals like montmorillonite, in which illite content is 52.0%-80.0%, chlorite content 10.0%-20.0%, kaolinite content 0-6.0%. Natural fractures, cracks and pores developed in the Longmaxi Fm. provide spaces for shale gas storing. The vertical variation in TOC content in Longmaxi Fm. reflects regional transgression, deepwater reducing environment and slow sedimentary rate; as a result the lower part of Silurian marine mudstones in Sichuan basin is favorable target to shale gas exploration.

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