Fault-Caprock Dual Control Hydrocarbon Accumulation Regularity In Volcanic Rocks In Xujiaweizi Fault Depression

To recognize the gas forming rule of Xujiaweizi Fault Depression under the fault-cap rock dual control, based on the material of Xujiaweizi which includes three-dimensional seismic processing results,interpretation of logging curves,outcrop core observation and analytical test in laboratory, this paper analyzes the lithology, distribution and sand-sand connectivity of cap rock of Xujiaweizi Fault Depression, evaluates the sealing property of cap rock. Study on the internal structure of the fracture zone and sealing mechanism, establishing quantitative sealing evaluation method on corresponding fault, and quantitatively evaluating fault sealing in gas reservoir controlled by fault. Based on the damage degree of fault on the cap rock, analyzes the top sealing ability and vertical adjustment of gas. According to the correlativity between fault sealing and gas distribution, systematically summarize the forming models of gas controlled by different fault-cap rock passage system.Deep gas of Xujiaweizi Fault Depression mainly contains 3 types: mudstone、volcanic and argillaceous conglomerate. Mudstone mainly distributes in K1 d and K1 q, plays the role of regional cap rock of deep gas. When the thickness of mudstone and stratum ratio of cap rock is 50% or more, cap rock has the effect of sealing. Volcanic and argillaceous conglomerate mainly distributes in K1 yc, plays the role of direct cap rock of deep gas. When the thickness of cap rock is more than 20 m, it can seal 120 m or less of column height. Fault lithologic juxtaposition sealing is formed at the juxtaposition of the volcanic rock reservoir stratum and the overlying conglomerate bed and mudstone bed. The gas pools in the Xujiaweizi Depression are almost in the upside of faults: in the longitudinal, gas-water contract is almost at the most shallow position of volcanic juxtaposition and the horizontal distribution of gas pools around faults is just like the curve “sine” as the result of the “dolphin effect” of the Xuzhong strike fault. Controlled by the sealing of cap rock in K1 d, the natural gas reservoir is divided into upper and lower sets of gas systems. The lower sets of natural gas system are controlled by direct cap rock distributing at the top of volcanic weathering crust and fault which joint source rocks and reservoirs. The upper sets are controlled by adjust faults in reverse which break through cap rock in K1 d, in other words, is controlled by fault and cap rock. Statistical results showed that the faults whose cap rock thickness mines throw is less than 35 m is the channel through which natural gas can be adjusted to the shallow stratum.