Porosity Evolution of Overmature Marine Shale and Free Gas Volumes in the South China
Lower Silurian Longmaxi Formation shale and Lower Cambrian Qiongzhusi Formation shale are two most important shale plays in the South China, Petrologic and geochemical parameters suggest that maturity reach to 2.0-3.0%VRo and 2.5-5.5%VRo respectively, which raises critical questions regarding metagenetic gas generation and reservoir quality of the shales. 86 samples were selected from overmature Cambrian and Silurian marine shales from Sichuan Basin and marginal areas to analysis porosity with He-Hg method and FIB-SEM with ion milling and high resolution imaging analysis, these data indicate that porosity of overmature marine shale range 1.5% to 10%, the organic-rich(TOC 2-8%) lower Silurian Longmaxi Formation shale display the higher porosity with 5-10%, which indicate the organic-rich shales display higher porosity and gas saturation. Comparison of porosity data of three marine shale plays with three different maturity stage (1.5-1.8%,2.0-2.3%,2.5-3.1%) manifest porosity decrease significantly with increasing maturity in the overmature stage when VRo is more than 2.5%, which diminish free gas volume in shale. The typical porosity value of organic matter rich shales from Lower Silurian Longmaxi Formation(2.0-2.3%VRo) is about 6% with gas saturation 80-90%, in which free gas volume range 6.0-6.8 m3/t rock at the depth 3000m. With this correspondence, the typical porosity value of organic matter rich shale from Lower Cambrian Qiongzhusi Formation (2.8-3.1%VRo) is about 2.5% with gas saturation 50-60%, in which free gas volume range 1.2-2.3 m3/t rock at the depth 3000m. The difference in porosity of shale result in the significant variation of free gas volume, which will be help understand the shale gas initial production contrast of Lower Silurian Longmaxi Formation shale play and Lower Cambrian Qiongzhusi Formation shale play. We consider that commercial development value of shale gas are reduced or deteriorated when the maturity is more than 3.5%VRo, the geological cause lie in diminishing of free gas volume with the gas porosity were decreased.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014