Evaluation of Upper Triassic T_3x~5 Source Rocks (Western Sichuan Depression, Sichuan Basin) and their Hydrocarbon Generation and Expulsion Characteristics: Implication for Tight-Sand Gas and Shale Gas Accumulation Potential Assessment

摘要

An unconventional, continuous petroleum system consists of an accumulation of hydro carbons that is found in low-matrix-permeability rocks and contain large amounts of hydrocarbons. Tight-sand gas in the Jurassic and shale gas within the fifth member of Xuj-iahe Formation (T_3x~5) are currently regarded as the most prolific emerging unconventional gas plays in China. The conventional and systematical evaluation of T_3x~5 source rocks was carried out for the first time in the western Sichuan basin (WSD). Hydrocarbon generation and expulsion characteristics (including intensity, efficiency, and amount) of T_3x~5 source rocks were investigated. Results show that T_3x~5 source rocks are thick (generally >200 m), have high total organic content (TOC, ranging from 2.5 to 4.5 wt%), and dominated by III-type kerogen. These favorable characteristics result in a great hydrocarbon generating potential under the high thermal evolution history (R_o>1.2%) of the area. An improved hydrocarbon generation potential methodology was applied to well data from the area to unravel the hydrocarbon generation and expulsion characteristics of T_3x~5 source rocks in the WSD. Results indicate that the source rocks reached hydrocarbon expulsion threshold at 1.06% R_o and the comprehensive hydrocarbon expulsion efficiency was about 60%. The amount of generation and expulsion from T_3x~5 source rocks was 3.14×10~(10) and 1.86×10~(10) t, respectively, with a residual amount of 1.28 ×10~(10) t within the source rocks. Continuous-type tight-sand gas was predicted to develop in the Jurassic in the Chengdu Sag of the WSD because of the good source-reservoir configuration (i.e., the hydrocarbon generation and expulsion center was located in Chengdu Sag), the Jurassic sandstone reservoirs were tight, and the gas expelled from the T_3x~5 source rocks migrated for very short distances vertically and horizontally. The amount of gas accumulation in the Jurassic reservoirs derived from T_3x~5 source rocks is up to.9.3× 10~8 t. The T_3x~5 gas shale has good accumulation potential compared with several active US shale-gas plays. Volumetrically, the geological resource of shale gas is up to 1.05×10~(10) t. Small differences between the amounts calculated by volumetric method compared with that by hydrocarbon generation potential methodology may be due to other gas accumulations present within interbedded sands associated with the gas shales.