Regional Geological Setting for Shale Gas Exploration in Southern China – A Case Study from Lower Cambrian Formations in Sichuan Basin
The industry interest of shale gas is increasingly rapidly in China. Shale gas exploration or assessment have been initially conducted in Sichuan Basin and its surrounding area since the organic-rich Pre-Cambrian to Mesozoic shales are widely distributed in South China. This study investigated the sedimentary environment and the tectonic evolution of South China and focused on the lower Cambrian Qiongzhusi and its equivalent shales in Sichuan Basin. The results revealed that foreland period of Sichuan basin since late Triassic is inherited and developed from passive continental margin in Paleozoic period. The Paleozoic shales in the Sichuan Basin are mainly of marine origin and deposited in passive margin setting. Then Mesozoic-Cenozoic orogeny and its resulting overthrust faulting changed the basin regime to its current foreland setting. The basin margins were complicated by the thrust faulting and folding, which could cause the gas leaking of ancient shale gas reservoirs. The basin center is realtively stable and only slightly influenced by the orogeny. The lower cambrian Qiongzhusi and its equivalent formations are best known for potential shale gas plays regarding the thick organic rich shales with high organic matter content and high maturity. During the worldwide sea level transgression period (called ‘Niutitang/Badaowan Event’ period in Southern China),the Qiongzhusi and its equivalent formations were vastly developed in the shelf-slope-basin environment of the Ediacaran Yangtze block margin. The remaining thickness of cambrian Qiongzhusi and its equivalent shales range from 40 to 500 meters. The shales are mainly characterized by high content of brittle mineral compositon and type I-II1 Kerogen. The average TOC in shale is about 2-5% and average Ro is about 2-6%. The shales also have storage for both free gas and adsorbed gas. Therefore, the mediumly buried Qiongzhusi and its equivalent formations in the stable area of Sichuan Basin, are currently most prospective shale gas targets .
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California