Insights from a comparison of hydrocarbon accumulation conditions of Sinian-Cambrian between the Tarim and the Sichuan basins

The Tarim and the Sichuan basins with burial depths of over 6000 m are the key areas for marine deep oil and gas exploration in China. Many large and medium-sized deep and ultra-deep oil and gas fields have been discovered, and a number of key exploration wells have good oil and gas indicators confirming that there is still great exploration potential in this field. In this study, the hydrocarbon accumulation conditions of Sinian-Cambrian in the Tarim and the Sichuan basins are compared from the perspective of sedimentary formation and later structural transformation. Based on the recent exploration and research progresses, the deep to ultra-deep hydrocarbon exploration proposals are set forth. The results show that the hydrocarbon accumulation conditions of the deep Sinian-Cambrian in the Tarim and the Sichuan basins have the characteristics of "similar formation and different transformation". The two basins had similar tectonic settings during Sinian-Cambrian and were affected by the same global paleoceanographic, paleoclimatic and paleontological evolution. Both experienced the transformation from intra-continental rift basin to craton basin, and developed high-quality source rocks of the lower Cambrian with wide coverage, high-quality reservoirs of microbial mound dolomite of the upper Sinian and intra-platform shoal dolomite of the lower Cambrian dolomite, and gypsum-salt cap rocks of the middle Cambrian. After Cambrian, especially since Mesozoic and Cenozoic, there are great differences in the structural transformation of the two basins, mainly reflected by the basin geothermal system and sedimentary burial process. The paleo and present heat flow of the Tarim basin is relatively low, with an average of 35~60 mW/m2, while that of the Sichuan basin is relatively high, with an average of 55~80 mW/m2. The Tarim basin is characterized by slow subsidence before the Himalayan Period, and rapid subsidence afterwards. The Sichuan basin is characterized by rapid subsidence in the early Indosinian-Yanshanian Period and uplift after the late Yanshanian Period. Affected by the process of differential transformation, the late transformation of deep and ultra-deep reservoirs in the Tarim basin is weak, mainly developing primary reservoirs, with oil as the main phase and multiple coexisting phases. While the late transformation in the Sichuan basin is strong, mainly developing transformation or adjustment type gas reservoirs, there are almost no liquid hydrocarbon reservoirs. According to the discovered oil and gas accumulation in the Sichuan basin, the Sinian-Cambrian oil and gas reservoirs are characterized by "near source fault controlled transport, lithologic trap controlled reservoir, and local structure controlled enrichment". Inspired by this, the deep Sinian-Cambrian exploration in the Tarim basin should put emphasis on the description of lithofacies and paleogeography, and on the basis of the implementing the source rocks of lower Cambrian Yurtus Formation, the future exploration focus should be on finding lithologic traps controlled by high energy facies belts. © 2022, Science Press. All right reserved.