Quantitative evaluation of fault-caprock sealing capacity: A case from Dabei-Kelasu structural belt in Kuqa Depression, Tarim Basin, NW China

Abstract

Based on the tests of rock mechanics characteristics and the anatomy of field outcrop, the paper analyzed the deformation mechanism of faults in anhydrite-salt caprock and glutenite reservoir in Dabei-Kelasu structural belt in Kuqa Depression, Tarim Basin, and studied the sealing mechanism of fault as well as established the comprehensive quantitative evaluation chart of gas preservation conditions. With the increase of depth, anhydrite-salt rock transforms from brittle stage to brittle-ductile or ductile stage, the depth for brittle to brittle-ductile transition of anhydrite-salt rock in Kuqa depression is about 1 740 m, brittle-ductile to ductile transition is about 3 400 m. Faults in brittle anhydrite-salt rock deform and form through-going faults, using caprock juxtaposition thickness by fault to characterize vertical sealing ability of the faults; faults deformed in brittle-ductile anhydrite-salt caprock form smear, the smear factor (SSF) of 3.5 is the critical value of anhydrite-salt smear from continuous to discontinuous status, which becomes a standard of determining vertical sealing of the faults in brittle-ductile anhydrite-salt rock; faults are generally hard to cut through ductile anhydrite-salt rock. Fault forms a fault zone of fault breccia type after deformation in dense glutenite, has no sealing capacity, the amount of gas gathered in the fault traps is controlled by the lithology juxtaposition amplitude of both walls of fault, the minimum juxtaposition amplitude within the trap determines the gas-water contact and gas column height of the fault trap. Comprehensively considering the four factors, the brittle-ductile transition depth of the anhydrite-salt rock, critical juxtaposition thickness of fault and SSF value as well as the minimum juxtaposition amplitude within the trap, the comprehensive quantitative evaluation chart of gas preservation conditions in Dabei-Kelasu structural belt, Kuqa depression was established, which provided a reasonable basis for fault trap risk assessment. Key words: Kuqa depression, anhydrite-salt caprock, brittle-ductile transition, caprock juxtaposition thickness by fault, shale smear factor, caprock sealing, fault sealing