U.S. Geological Survey Assessment of Paleozoic Shale Gas Resources in the Sichuan Basin, China

Abstract

The U.S. Geological Survey (USGS) applied a geology-based assessment methodology to estimate quantities of technically recoverable shale gas resources in the Paleozoic of the Sichuan Basin of China. The USGS assessment methodology consists of a well-performance approach that recognizes the geologic variability within assessed reservoirs. Shale gas accumulations and related data for mean estimated ultimate recoveries and mean drainage areas of directionally drilled wells in the United States were used as analogs in this assessment. Six assessment units (AUs) were quantitatively assessed: Cambrian Qiongzhusi Platform and Foldbelt AUs, Silurian Longmaxi Platform and Foldbelt AUs (including uppermost Ordovician Wufeng Formation), and Permian Longtan Platform and Foldbelt AUs. These AUs encompass organic-rich shales that are the principal source rocks for conventional oil and gas in the Sichuan Basin. Since 2008, there have been multiple tests of gas potential in Cambrian, uppermost Ordovician, and lower Silurian shales in the Sichuan Basin. Commercial production of shale gas from the Longmaxi Formation in the southeastern foldbelt began in late 2012. The early Paleozoic organic-rich shales of the Qiongzhusi, Wufeng, and Longmaxi Formations were deposited in deep marine depocenters on the continental shelf off the South China continental block near the early Paleozoic Gondwana margin. During the Paleozoic, the shale formations were deeply buried and generated oil. After the collision of the North and South China continental blocks in Triassic time, rapid and voluminous continental sedimentation accelerated the burial of shale formations and resulted in increased thermal maturity and cracking of oil to gas. Oil-prone, organic-rich shale in the Permian Longtan Formation also accumulated in discrete basinal lows and generated oil and gas during Mesozoic continental sedimentation. Oil and gas from each of these Paleozoic source rocks migrated into shallower conventional traps; however, substantial volumes of gas remained in the shale formations to form the present shale gas accumulations. Leakage along fault and fracture systems and along unconformities could have compromised gas retention in some of these shale reservoirs.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016