Modeling of Evolution Histories of Thermal Maturity and Hydrocarbon Generation for the Lower Paleozoic Black Marine Shales in Jiaoshiba Region of Sichuan Basin, China

Abstract

Jiaoshiba shale gas play is located on the southeast margin of Sichuan basin, China. The Lower Paleozoic black marine shales are about 40m thick and belong to the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, with the present buried depths of 2375~2413m and the present temperature of about 89°C. The shale strata experienced a long and complicated history of burial, thermal maturation, hydrocarbon generation and uplift. The overlying strata was continued to suffer massive uplift and erosion from about 85Ma (Late Cretaceous) to the Quaternary. Therefore, the understanding of shale maturities and stages of thermal evolution and hydrocarbon generation has become one of the basic problems for determining the enrichment conditions of shale gas. The experienced maximum paleotemperature of the black shales varies from 200°C to 230°C with an average value of about 213°C based on the rock thermo-acoustic emission data and the measured homogenization temperatures of fluid inclusions from the calcite veins in the black shales. According to paleotemperature, stratigraphic correlation, vitrinite reflectances from the Upper Paleozoic and Mesozoic strata, the uplift-erosion thicknesses of overlying strata during the Late Yanshan tectonic period and the Himalayan tectonic period are recovered to be about 3500~3800m. The evolution histories of burial, heating, thermal maturation and hydrocarbon generation were modeled and reconstructed by the BasinMod 1D software with a series of constraint conditions such as highest paleotemperature, erosion thickness and burial-uplift times etc. The studied results indicated that the thermal maturity (Ro) for the Wufeng-Longmaxi shales was reached 1.3% at 190Ma (Early Jurassic) and coincident with the end of the major oil generation stage, the phase of thermal cracking to shale gas from the kerogen and liquid hydrocarbon thermal cracking into shale gas in the Wufeng-Longmaxi shales occurred during 190~85Ma including the dry gas window at the period of 150~85Ma. The modeled results revealed that the shale strata experienced maximum burial depth and maximum paleotemperature and the thermal maturity (Ro) was up to 2.6~2.9% at 85Ma before the regional large-scale uplift and erosion. The produced dry gas from the Wufeng-Longmaxi shales was mainly generated in the stage of residue liquid hydrocarbon thermal cracking process, with the measured δ13C values of methane, ethane and propane appearing completely reversed.

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