Exploration Tries To Answer Questions About the Lower Silurian Longmaxi Shale in China

Abstract

This article, written by Editorial Manager Adam Wilson, contains highlights of paper IPTC 14487, ’Sichuan Basin Shale Gas, China: Exploring the Lower Silurian Longmaxi Shale,’ by Claudia Hackbarth, SPE, Danny Soo, and Navpreet Singh, SPE, Shell, prepared for the 2011 International Petroleum Technology Conference, Bangkok, Thailand, rescheduled to 7-9 February 2012. The paper has not been peer reviewed. Shale-gas exploration in China is already under way and rapidly increasing in activity level. The Sichuan basin, one of the most prolific petroleum basins in China, is at the forefront. However, exploration and assessment of an untested shale-gas play is typically lengthy and expensive. Compared with conventional oil and gas projects, shale-gas projects may require a number of early wells to prove the existence and distribution of the gas but then many more wells to test and optimize engineering techniques until a commercially viable combination is found. This process could be summarized into two overarching questions: “Is there a material amount of gas?” and, if yes, “Can it be produced economically?” Sichuan Basin Structure The topographic Sichuan basin today is a remnant of the originally much larger Upper Yangtze cratonic sedimentary basin (Fig. 1). In the southern Sichuan basin, structures are characterized by thin-skinned faulting and thrusting, often above an Upper Cambrian detachment surface, likely an evaporitic ductile layer that now cores the folds. The Sichuan-area stratigraphic column contains several potential source-rock intervals. Industry has so far focused attention on the Lower Cambrian Qiongzhusi and the Ordovician-Silurian boundary as potential shale-gas reservoirs. The Upper Ordovician Wufeng shale and Lower Silurian Longmaxi formation are generally regarded as belonging to the same black-shale succession, but an erosional surface and regional disconformity separate the two intervals. The Longmaxi formation is not as widespread as the Wufeng formation and is restricted to the Yangtze region. From the bottom up, the Ordovician/Silurian succession consists of A lower graptolite-bearing, black or dark-gray, carbonaceous/siliceous shale (Wufeng formation), generally several meters thick The Guoyinchao limestone or calcareous siltstone with abundant Hirnantian fauna and other shelly fossils, typically 0.5–3 m thick The Lower Longmaxi black, graptolite-bearing, carbonaceous/siliceous shale with cherty slate The Upper Longmaxi gray calcareous mudstones and siltstones, interbedded with an increasing number of thin limestone layers toward the top of the formation Regional studies indicate that Wufeng and Longmaxi shales were buried deeply enough over a large area of the Sichuan basin to have reached the gas-generation window.