Brittle Ductile Deformation Characteristics Of Anhydrite-salt Rock In Kuqa Depression And Quantitative Evaluation Of Its Sealing Ability

As one of the most important cap rocks in the world-wide petroliferous basins, anhydrite-salt cap rock has high capillary pressure, so its sealing capacity is excellent. Both Kumugeliemu and Jidike group anhydrite-salt caprocks of Kuqa depression in Tarim basin has very high capillary pressure. The brittle ductile transition of anhydrite-salt cap rock with the increasing depth has been proved by the evidence from field observation and triaxial tests. In order to determine the deformation characteristics of anhydrite-salt rock and evaluate its sealing ability quantitatively, based on the field outcrop observation, triaxial compression tests and microscopic observation, the deformation characteristics of anhydrite-salt rock in different brittle ductile regions were summarized, with the aspect of the deformation mechanism, fracture pattern, mechanical behavior, acoustic emission, etc. Applying Mohr-coulomb criterion, Byerlee’s friction law and Goetze’s criterion, the confining pressure and depth of brittle-ductile transition of anhydrite have been determined. The studies show that(1) for brittle anhydrite-salt cap rock, hydraulic fracturing risk should be estimated quantitatively using retention force firstly: the smaller the retention force is, the greater the risk is. Meanwhile, juxtaposition thickness is used for making a quantitative evaluation of the vertical sealing of faults within it;(2) SSF is applied to determine the vertical sealing of smearing faults deformed in brittle-ductile anhydrite-salt cap rock, and the field fault anatomy shows that SSF=3.5-4 is the critical value to determine the continuity of anhydrite smearing;(3) ductile anhydrite-salt cap rock has flow characteristics which will flow along the thrust fault plane, where the faults are sealing vertically, but the “salt window” which is vacancy resulted from differential compaction will contribute to the oil and gas migration. In addition, the preservation of the gas reservoir also depends on the fault lateral sealing ability which the minimum abutting joint amplitude within the trap determines the extent of accumulation of natural gas. Taking the four factors, brittle-ductile transition depth of the anhydrite caprock, critical juxtaposition thickness of fault, critical SSF and minimum abutting joint amplitude into consideration, a comprehensive quantitative evaluation chart on gas reservoir preservation of Kuqa depression is established and the favorable traps are predicted.