Gas Generation as the Dominant Overpressure Mechanism in Jiaoshiba Shale Gas Region, Sichuan Basin, China

Abstract

Shale gas plays have been found in the Lower Paleozoic marine shales of Jiaoshiba region of Sichuan basin, China. The black shales from the Upper Ordovician Wufeng Group and the lower part of Lower Silurian Longmaxi Group are the gas reservoirs. The pressure coefficients of gas reservoirs range from 1.35 to 1.55, showing overpressure characteristics. Disequilibrium compaction, hydrocarbon generation, tectonic compaction and other overpressure generation mechanisms for the overpressured reservoirs are analyzed based on mudstone compaction characteristics, tectonic subsidence history, sediment burial history, hydrocarbon generation history, and it is found out that gas generation by kerogen and petroleum cracking during early Triassic to late Cretaceous is the controlling mechanism for overpressure. We tested the aspects of the overpressure caused by gas generation by reconstructing the timing of natural sealed fracture growth in overpressured reservoirs and associated temperature, pressure, and fluid-composition conditions using microthermometry and Raman microspectrometry of fluid inclusions trapped in fracture cement minerals that formed during fracture growth. Trapping temperatures of methane-saturated aqueous fluid inclusions range from 196.2 to 238.4°C. Compared trapping temperatures with burial and thermal maturity models, we infer that fracture formed between 145 and 85 Ma, which indicates the formation of fracture was concurrent with petroleum cracking gas generation close to the maximum burial depth (5600–6200m). Based on the Raman scatter peak v1 shift, calculated pore-fluid pressures for high density methane inclusions of 114.2 to 130.8 MPa during fracture opening and the presence of methane-rich inclusions in fracture cement indicate fracture growth was aided by abnormal high pore-fluid pressures during gas generation in organic-rich shales. Lack of systematic pore-fluid-pressure trends over time suggests dynamic pressure conditions require an active process of pressure generation during maximum burial conditions. Such a process is consistent with gas generation within the shale gas reservoirs and the presence of microfractures in the shale gas reservoirs may relate to overpressure dissipation or episodic expulsion of gas. Our results demonstrate that fracture growth in the shale gas reservoirs is the consequence of abnormal high pore-fluid pressures caused by gas generation, and the high pressure has been effectively preserved to date.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015