Study On Reservoir Characteristics And Charging Process In Central-north Structural Belt Of Xihu Sag

The Huagang Formation in the central structural of the Xihu Sag is an important gas reservoir,while reservoir quality plays an important role in the gas accumulation and enrichment.However,the cause of reservoir tightness of the Huagang Formation and its relationship with gas accumulation process is still unclear,which restricts the progress of further exploration and development.Based on the study of reservoir microscopic characteristics,reservoir evolution,diagenetic evolution,tectonic evolution,hydrocarbon generation evolution and fluid charging history,this paper analyzes the relationship between reservoir evolution and gas charging process to determine the accumulation process and post-accumulation evolution and provides an important basis for further understanding of the key factors of gas accumulation and enrichment.Through casting thin sections,scanning electron microscopy,high-pressure mercury injection,nuclear magnetic resonance and CT scanning,the pore radius calculation model to recalculate the pore radius data according to the cylinder model,and the comparative analysis of various experimental data is completed.The high-pressure mercury injection data can accurately characterize the pore distribution characteristics with pore radius less than 0.4 μm,the NMR data can be used to characterize the pore distribution characteristics with pore radius between 0.4 and 10μm,the pore distribution characteristics with pore radius larger than 10 μm can be characterized by CT scan data,and the thin section observation data can be used to characterize the pore distribution characteristics for samples without CT scan data.Thus,the quantitative characterization of full pore size of the reservoir is achieved.Studies on the formation and transformation of authigenic minerals and their relationship with diagenetic environment evolution in the Huagang Formation,central-northern tectonic belt show that authigenic minerals are widely developed,and different authigenic mineral combinations are developed at different depths,indicating different diagenetic environments.Early authigenic quartz-authigenic kaolinite-chlorite coating authigenic mineral assemblage,medium-term authigenic quartz-authigenic feldspar-chlorite coating-late illite/smectite authigenic mineral assemblage and the late-term authigenic quartz-authigenic illite-thick layer of chlorite coating authigenic mineral assemblage,respectively represent acid,acid-alkaline conversion and alkaline diagenetic environment.Strong compaction is the main cause of reservoir quality decline and the tightness of the Huagang Formation,combining with multi-stage carbonate cementation and multi-stage siliceous cementation.Organic acid dissolution is the main cause of the increase of porosity.At 11.5 Ma,the layers of the H5 member and below are generally dense under the combined action of strong compaction,tectonic compression and late carbonate cementation and siliceous cementation.The key factors of effective reservoir formation in the Huagang Formation are original deposition of coarse-grained sandstone with burial depth of less than 4000 m,the acid-acid to alkaline conversion diagenetic environment and ground temperature less than 150 ℃.There are three stages of fluid charging in the central-north structural belt.The first stage(16～14 Ma)is dominated by a small amount of mature-high mature oil charging before the formation of the trap,and the last two stages are the main gas accumulation stages.In the second stage(13～10 Ma),the reverse of furrow source fault and anticlinal trap caused by Longjing movement created conditions for massive gas charging.The untight H3 ～ H4 reservoir was the dominant accumulation layer of the gas reservoir,and the low abundance gas reservoir was formed when the H5 and lower reservoirs were charged alongside the tight reservoir.In the third stage(late 2.5 Ma ～ present),the gas reservoir was partially adjusted and destroyed due to the shallow active fault caused by the Okinawa movement.The gas of the lower Pinghu Formation is supplied vertically,the conducting system consists of the reverse fault and low permeability sandstone and the gas reservoir scale is mainly controlled by the reverse anticline trap.The reservoir quality is controlled by compaction degree and the formation of the cement,hydrocarbon supplement and damage affects the fullness of the gas reservoir in the late period.Effective reservoir H3～H4 have the advantage of gas accumulation,and the active fracturing in the late period causes damage and adjustments of the gas reservoir.Therefore,the development of effective reservoirs and the small damage caused by late activate faults are the keys to the formation and enrichment of gas reservoirs in the Huagang Formation.