Characteristics,formation mechanism and geological implications of high water-cut shale gas reservoirs in western Chongqing area
The deep Longmaxi Formation shale gas reservoirs in the western Chongqing area is the main area of reserves increase and marine shale gas production in the Sichuan Basin.However,local high water cut in the shale gas reservoirs poses challenges to the exploration and development of deep shale gas in this area.After analyzing basic geology,geochemistry,pore structure,communication,fault activity period,shale gas accumulation adjustment period,high angle calcite vein of high water-cut shale and strontium isotope of overlying and underlying limestone strata,this paper studies the key factors in the formation of high water-cut shale gas reservoirs,the source and path of water in shale,the lower limit of water saturation in high water-cut shale gas reservoirs and the plane boundary between high water-cut shale gas reservoirs and low water-cut shale gas reservoirs,and clarifies the formation mechanism and geological implications of high water-cut shale gas reservoirs through numerical simulation.And the following research results are obtained.First,the fault that cuts through the Longmaxi Formation and terminates in the Permian karst reservoir is the key factor of the local high water cut.Second,the fault activity period is matched with the shale gas accumulation adjustment period.The shale gas is lost first,and then the formation water at the bottom of the Lower Permian Qixia Formation flows into the Longmaxi Formation shale along the fault to form local high water cut.Third,the lower limit of water saturation of high water-cut shale reservoir is 40％.When the external water supply near the fault is sufficient,the boundary between the high water-cut shale gas reservoir and the low water-cut shale gas reservoir is 5 km away from the fractured zone.When the external water supply is not sufficient,it can be extrapolated by 0.78-5.10 km from the high water-cut well,which is a favorable exploration area of shale gas.In conclusion,the Permian karst reservoir in the western Chongqing area and the active fault cutting through the Longmaxi Formation and terminating in the Permian are the factors leading to local shale gas reservoirs high water cut.The research results provide theoretical guidance for avoiding deep high water-cut shale gas reservoirs in the western Chongqing,determining the boundaries of favorable shale gas areas and deploying appraisal wells.
Sichuan BasinWestern Chongqing areaLongmaxi FormationDeepShale gas reservoirHigh water saturationFormation mechanismGeological significance
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