Petroleum Systems of the Hebaochang Gas Field in the Southern of Sichuan Basin, SW China

Abstract

The Hebaochang gas field is located in the Upper Triassic Xujiahe terrestrial formation of southern Sichuan Basin in China, with at least 374 billion cubic feet (tcf) of proved gas reserves. Because of long-term compression, the reservoirs become mostly tight reservoir with low porosity, low permeability and strong heterogeneity. Furthermore, the contact between gas and water is so complex that there is no uniform gas-water interface. It is unclear for the distribution and physical properties of gas reservoirs and for matching relations between reservoir digenetic evolution and porosity, which hinder the exploration of Xujiahe formation. In this paper, petroleum system has been studied to investigate petroleum accumulation condition and pattern. The conclusions are as follows: (1) The rocks are mainly ash grey-debris-quartz sandstone, ash grey-debris-feldspar quartz sandstone and few feldspar-debris quartz sandstone. The reservoir spaces are mainly remainder intergranular pore and secondary dissolved pore, and it belongs to small pore-thin throat type; (2) Excellent hydrocarbon sources are mainly mudstones and shales from member 1st, member 3rd and member 5th in the Upper Triassic Xujiahe formation. The organic matters mainly consist of I and III types, which are in the late stage of mature to the earlier stage of high mature, so the potential of gas-generation is strong; (3) Reservoirs in member 2nd, member 4th and member 6th of Xujiahe formation produce gas and a little condensate. It is identified that gases mainly come from its coal hydrocarbon source rock and the genesis of natural gas is humus type; (4) Mudstone beds in Jurassic formation are considered as the direct seal for the gas accumulation in Upper Trassic, and the thick mudstones in member 1st, member 3rd and member 5th of Xujiahe formation also as an indirect cap rocks for the reservoir in member 2nd, member 4th and member 6th of Xujiahe formation; (5) Physical property of Xujiahe formation shows that reservoir is so tight that the resistance of natural gas secondary migration is strong, and the buoyancy force is so weak that the driving force of migration is mainly gas expansion. Accumulation pattern of Hebaochang gas field is “near-source gas accumulation-forming model.”