Fault Systems And Reservoir-Controlling Mechanisms In Western Qaidam Basin

Faults are well developed in western Qaidam basin and they can be divided into 3 orders. According to bending shapes of second-order faults, combination relations among second-order faults, and the relations among second-order faults, third-order faults and local structures, the combination mode of faults in the plane can be divided into single fault combination and multistrip fault combination, and it can be futher subdivided into 5 types: single regular fault-sinistral transpression, single right step bending fault-sinistral transpression, right step fault system-sinistral transpression, parallel fault-sinistral transpression and thrusting and cross fault-sinistral transpression and thrusting. There has experienced four stages of tectonic evolution in western Qaidam basin: extensional rift basin stage (J1-J2), compressional uplifting deformation stage (J3-K2),"detachment extension-pull part basin"stage (E1+2-N21) and compressional strike-slip depression basin stage (N23-Q), and there has formed synsedimentary normal faults in early and middle stage, synsedimentary strike slip-thrust faults in middle-stage, basement-involved type and cover-decollement type strike slip-thrust faults in late-stage, long-lived positive inversion faults and inherited strike slip-thrust faults-a total of 6 sets of fault systems. Fault-related traps in western Qaidam basin can be divided into 3 types according to the effects of faults in hydrocarbon migration and accumulation: the first is self-trap whose efficiency is not controlled by lateral sealing of fault; second is fault trap whose efficiency is determined by lateral sealing of fault; the third is combination trap whose efficiency is controlled not only by lateral sealing of fault but also by matching relations between faults and sandbodies. In northern region of western Qaidam basin the critical SGR value of fault lateral sealing is 0.4, and risk fault throw is generally from 10m to 250m. Tault throws of typical fault-related traps are generally greater than 200m. The risk of leakage of fault-related traps is relatively small. The role of fault in hydrocarbon accumulation is analyzed according to the coupling relationship between fault active periods and hydrocarbon accumulation periods:â‘ In the first hydrocarbon accumulation period, active faults outside of hydrocarbon kitchen are changing layer fault and non-active faults are mainly accumulation faults.â‘¡In the second hydrocarbon accumulation period, synsedimentary normal faults in early and middle stage and synsedimentary strike slip-thrust faults in middle-stage are accumulation faults; positive inversion faults are damage faults; inherited strike slip-thrust faults are readjust and dissipation faults; basement-involved type faults in late-stage are oil source faults or dissipation faults; cover-decollement type faults in late-stage are oil source faults or readjust or dissipation faults. Accordingly, combining with hydrocarbon accumulation characteristics in western Qaidam basin, reservoir-controlling mode of faults can be divided into 3 types: early hydrocarbon migrating laterally and accumulated by barrier faults-late preservation type, early hydrocarbon migrating laterally and accumulated by barrier faults-late reactive adjustment type and late hydrocarbon vertical migration anticline accumulation type.