Transformation Of Critical Tectonic Regimes And Its Mechanisms In Eastern Tarim Basin:Implications For Tectonic Controls On Oil And Gas Occurrence

Tarim Basin is an important superimposed basin in northwest China, and it experienced several periods of tectonic transformations during the geological history. The structure deformation, temperature-fluid pressure field and stress field during tectonic transformation periods control hydrocarbon accumulation in the basin. Based on seismic, well data, outcrop and analytic data, we identified critical tectonic transformation periods in eastern Tarim Basin and discussed the structural deformations and genetic mechanism in each period. Numerical simulation of energy field was conducted in order to analyze their roles in hydrocarbon accumulation. This work has important guiding implications for the oil and gas exploration in eastern Tarim Basin.There are significant differences in distribution, superposition and erosion thickness of the nine unconformities in the different units of eastern Tarim Basin. In combination with fracture features, we defined the timing of four periods of critical tectonic transformations in the study area, including mid-Caledonian, early Hercynian, late Hercynian and Indosinian. Their formations are closely related to the deep dynamic mechanism of the basin and the tectonic evolution of its peripheral trench-arc systems.Numerical simulation has been considered as an effective method for analyzing the evolution of energy field. The results suggest that the geothermal gradient experienced a change of “decrease-increase-decrease” during mid-Caledonian to Indosinian. Abnormal pressure was mainly distributed in the Manjiaer depression and Tangguzibasi depression. Both of the distribution and value of the excess fluid pressure increased gradually. The southern region of eastern Tarim Basin was in a compressional setting in mid-Caledonian and early Hercynian, followed by late Hercynian extension. In contrast, the northern region displays the opposite evolutionary trend from mid-Caledonian to late Hercynian. The entire eastern Tarim Basin was dominated by compression stress field both in its northern and southern region in Indosinian.The temperature-pressure-stress coupling in the critical tectonic transformation periods play significant control on the generation, migration and accumulation of hydrocarbon in eastern Tarim Basin. In mid-Caledonian, the stratum temperature of the Cambrian-Ordovician source rocks is higher than 60℃ in eastern Tarim Basin, when the hydrocarbon started to generate. At the same time, the Tazhong low uplift and Tabei uplift began to develop and became the main areas of oil and gas migration under the influence of compression in the south. In early Hercynian, with the stratum temperature increased, excess fluid pressure resulting from hydrocarbon generation became a main dynamic condition for hydrocarbon migration to Tazhong low uplift and Tabei uplift. In late Hercynian, both of value and distribution of excess fluid pressure increased, which promoted the hydrocarbon expulsion. The uplifting of Tabei area due to compassion led it to be a favorable area of hydrocarbon migration. In Indosinian, the excess fluid pressure had been extended to the Upper Ordovician formation. Meanwhile, S-N trending compression not only resulted in the formation of abundant fractures that provided the benefit pathways for fluid migration, but also led the uplift region to be more close to the center of source rocks, advantageous to migration and accumulation of hydrocarbon.