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Maturity and Overpressure Evolution of the Lower Cambrian in the Central Paleo-Uplift, Sichuan Basin

Abstract

Sichuan basin is a prime gas producing basin in China. The Central Paleo-Uplift, located in the middle of the basin, is always an important region for gas exploration. After Weiyuan gas field was discovered in 1960s, another large gas field has been ascertained in the lower Cambrian recently (CNPC). Simultaneously, the lower Cambrian is promising for shale gas exploration. In this paper, maturity history of source rock and pore pressure evolution have been simulated, which are helpful for further gas exploration. The present geothermal gradient of the Central Paleo-Uplift is 24∼30°C/km. According to the burial history and thermal history, maturity history of the Lower Cambrian source rock has been simulated by using Easy%Ro model in this paper. The source rock in the southern part became mature with RO>1.0% in the early Caledonian (∼420Ma), but was still immature in other regions. After a sluggish development caused by tectonic uplift during S∼C, the thermal maturity increased again in mid-Permian (∼270Ma), and Ro reached the peak of 2%∼4% in late Cretaceous (∼80Ma). The maturity evolution stopped as the regional uplift during Yanshan and Himalayan movement. The Central Paleo-Uplift has two overpressure systems: Permian-Triassic sequences and lower Cambrian. Based on the DST measurement, the overpressure coefficient is 2.0 in the upper system; and it is 1.5∼1.75 in the lower. Anomalously high mudstone acoustic time in the Permian and Triassic reveals that disequilibrium compaction is the mainly mechanism for the overpressure. However, the combination of the normal acoustic time and geologic history analysis indicates that overpressure in the lower Cambrian is mainly resulted from fluid expansion, most like gas generation. 1D and 2D geopressure models demonstrate that surplus pressure in the lower Cambrian appeared during Triassic, and reached the maximum at about 80Ma. In conclusion, the lower Cambrian source rock in the Central Paleo-Uplift experienced twice hydrocarbon generation, and oil cracking is during 220∼100Ma. Evolution of the pore pressure in the lower Cambrian includes three phases – normal pressure, overpressure and pressure releasing. Overpressure provides the motivation for hydrocarbon migrate laterally and downward into Sinian series. Furthermore, pressure releasing contributes to gas release from formation water.