关键词: 四川盆地, 高石梯地区, 走滑断裂, 断裂活动, 二叠系栖霞组, 白云岩成因, 构造-热液白云石化作用

Abstract: Based on the development of Permian Qixia Formation dolomite in Gaoshiti area, it is found that the distribution of dolomite is obviously related to the distribution of main strike-slip faults and multi-stage tectonic activities, but their coupling mechanism remains unclear. The periods of fault activities can be determined based on the geometric and kinematic characteristics of the strike-slip faults, as well as dating data from fracture cements and fracture-filling minerals in the fault zones. Through the comparison with classical diagenetic models, the analysis of elemental geochemical characteristics, and the restoration of geothermal history, it has been confirmed that the Qixia Formation dolomite is of tectonic-hydrothermal origin. The control and significance of strike-slip faults on the development of dolomite reservoirs in Qixia Formation are explored from the perspective of genetic mechanism. (1)The main strike-slip faults extending from the Sinian to the Lower Paleozoic into the Permian in Gaoshiti area were formed through multiple phases of activities along the pre-existing basement faults. These faults mainly experienced three periods of tectonic superimposition, i.e., initial development in the Pre-Sinian period, intense activities during the Late Sinian to Early Caledonian and Late Caledonian to Early Hercynian periods, and localized reactivation in the Late Permian. (2)The distribution pattern, CaO-MgO correlation, order degree, geochemical characteristics of rare earth elements, and paleo-heat flow evidences of dolomites had confirmed that the formation of dolomite reservoirs of Qixia Formation in Gaoshiti area were influenced by the multi-phase activation of main strike-slip faults and the Late Permian Emeishan rifting movement, which was caused by the hydrothermal fluid action related to widespread thermal events. (3)The dolomite reservoirs of the Member 2 of Qixia Formation were horizontally distributed in banded and lenticular patterns near four primary strike-slip fault zones, i.e., F_I-8, F_I-9, F_I-10, and F_I-11. With the Emeishan rifting movement occurring in the Late Permian to end of Early Triassic periods, the deep basement strike-slip fault was reactivated, the fault communicated with the deep thick clastic hydrothermal reservoirs of Chengjiang Formation, and the tight shale of Longtan Formation overburdening Qixia Formation formed a sealing layer, which provided favorable conditions for the development of hydrothermal dolomite reservoir facies, including heat source, fluid channels, and effective sealing. (4)The fault-hydrothermal interaction exerted a significantly greater influence on carbonate rocks on the active side of the fault rather than that on the passive side of the fault. The passive side of the fault might serve as the barrier for the intrusion and penetration of deep upwelling hydrothermal fluid, while the deep Mg-rich fluid would preferentially enter the beach granular limestone with good porosity on the active side of the fault when entering the Qixia Formation along the deep fault, thus resulting in hydrothermal dolomitization.

Key words: Sichuan Basin, Gaoshiti area, strike-slip faults, fault activity, Permian Qixia Formation, dolomite genesis, tectonic-hydrothermal dolomitization