断层-盖层封闭性定量评价——以塔里木盆地库车坳陷大北—克拉苏构造带为例

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

基于岩石力学特征测试和野外露头解剖,分析塔里木盆地库车坳陷大北—克拉苏构造带断裂在膏盐岩盖层和砂砾岩储集层中的变形机制,研究断层封闭机理并建立天然气保存综合定量评价图版。随埋深增加,膏盐岩从脆性阶段向脆-塑性、塑性阶段转变,库车坳陷膏盐岩从脆性向脆-塑性转化的深度约为1 740 m,从脆-塑性向塑性转化的深度约为3 400 m。断裂在脆性膏盐岩中变形形成贯通性断裂,可用断接厚度表征断层垂向封闭性;断裂在脆-塑性膏盐岩盖层中变形形成涂抹,涂抹因子(SSF)值3.5为膏盐岩涂抹连续与否的临界值,为判断脆-塑性膏盐岩中断裂垂向封闭性的标准;断裂一般不断穿塑性膏盐岩。断裂在砂砾岩中变形形成断层角砾岩型断裂带,不具封闭能力,天然气在断层圈闭中聚集数量取决于控圈断裂两盘岩性对接幅度,圈闭范围内最小对接幅度决定断层圈闭气水界面和气柱高度。综合考虑膏盐岩脆塑性转化深度、临界断接厚度、SSF值、圈闭范围内最小对接幅度4因素,建立了大北—克拉苏构造带天然气保存综合定量评价图版,为断层圈闭风险评价提供了依据。

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.