Rift-uplift-depression accumulation-controlling models of the large gas province in Cambrian pre-salt, Tarim Basin, China

The Cambrian pre-salt has the accumulation conditions for forming a compound large gas province and was the strategic replacement area with the lowest exploration level and the largest exploration potential in the Tarim Basin. However, no significant discoveries have been found in the exploration for many years. The key problem is that the unclear controlling factors and spatial distribution of the hydrocarbon generation center and scale beach reservoir result in the ambiguity of favorable exploration directions and zones. Given this, a comprehensive application of seismic, drilling, well-logging, mud-logging, and geochemical data has been carried out to study the source-controlling, reservoir-controlling, caprock-controlling, and accumulation-controlling of the Cambrian pre-salt area, constructing the accumulation-controlling model and defining the favorable zones and exploration breakthrough direction. The following points have been gained: (1) Paleo-rifts controlled hydrocarbon generation center. The rift-depression structure formed in the Nanhua System to Sinian System provided the structural background for the development of the Lower Cambrian and controlled the deposition of the Cambrian Yuertusi Formation. In the negative tectonic area controlled by rifting, the thickness of the Yuertusi Formation was large and formed the hydrocarbon generation center. (2) The paleo-uplifts controlled the development of beach facies dolomite reservoir. The paleo-uplifts formed simultaneously with the Nanhua System paleo-rifts controlled the development and distribution of high-energy beaches in the Xiaerbulake Formation, and large-scale dolomitic beach facies reservoirs were formed on both sides of the Tanan paleo-uplift. (3) The paleo-depression controlled the development of gypsum-salt caprock. In the Middle and Late Cambrian, the Taxi platform has closed and formed a strong evaporation environment, while a wide-covering gypsum-salt caprock was deposited in the unified depression within the platform due to the development of the Luntai-Gucheng near the north-south rimmed platform margin. (4) The time-space matching of rift-uplift-depression controlled the formation and distribution of oil and gas. Three types of accumulation-controlling models, the stable rift-uplift-depression, the active rift-uplift-depression, and the transitional rift-uplift-depression, were established, indicating the stable and transitional areas as the main exploration directions, and the Aman low uplift between Awati Sag and Manxi Sag as the preferred target area for the large gas province in Cambrian pre-salt.