Shale Gas Accumulation Characteristics of Wufeng Formation-Longmaxi Formation in Luzhou Area
To find favorable areas for shale gas accumulation in the Wufeng formation-Longmaxi formation in the Luzhou area of the Sich-uan basin,fluid inclusion detection,shale micropore observation,and gas bubble-pore evolution simulation were performed.On this basis,the tectonic burial process and the hydrocarbon generation and thermal evolution process in the Luzhou area were investigated,and the characteristics and patterns of shale gas accumulation in the Luzhou area were summarized.The results show that the Wufeng formation-Longmaxi formation in the Luzhou area has a set of thick organic-rich shales.During the Permian to Early Triassic,oil was generated from the shale which then experienced two tectonic uplift events during the Middle Triassic and the Yanshanian-Himalayan movement.The gen-tle uplifting of the formation in the Middle Triassic did not induce extensive hydrocarbon loss,while the uplifting during the Yanshanian-Himalayan movement was later than that of the structure in the southeastern part of Sichuan basin,with a small magnitude,which was favor-able for shale gas preservation.The organic pores in the shales were developed as a result of the fact that the uplifting with short duration and low intensity in the Middle Triassic did not cause massive hydrocarbon expulsion,leaving a large quantity of liquid hydrocarbons in the reservoirs and allowing well preservation of organic pores during the Late Triassic to Middle Cretaceous,when liquid hydrocarbons were cracked to gas along with deep burial,and the strata were universally under overpressure.Although the Triassic uplifting in the Luzhou ar-ea was short-lived and of low intensity,the simulation results suggest that it led to crude oil thickening and gas pore formation,which was beneficial for shale gas accumulation.The uplifting in the Indosinian period resulted in crude oil thickening,facilitating shale gas accumu-lation.The late strata uplifting together with a short period of shale gas loss,developed low-angle bedding fractures,and fewer vertical frac-tures all contribute to the formation of overpressured shale gas accumulation zones in the Luzhou area.The proposed patterns of shale gas accumulation in the Wufeng formation-Longmaxi formation in the Luzhou area are of great reference significance in guiding oil and gas ex-ploration for similar reservoirs.
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