Evaluation of Cambrian caprock in the platform-basin area of Tarim Basin and optimization of favorable area for oil and gas caprock

This paper is the result of oil and gas exploration engineering.

Objective Gypsum-salt rocks, gypsum-bearing mudstone, and gypsum-bearing dolomite were extensively deposited in the platform-basin area of Tarim Basin during the Middle Cambrian. The study on the oil and gas sealing capacity and distribution of this set of strata can provide a basis for the exploration of the Cambrian subsalt field in the Tarim Basin.

Methods 44 core samples from 4 wells were studied under microscopic thin sections and tested for physical parameters and breakthrough pressure. Based on the relationship among physical parameters, the sealing ability of the Middle Cambrian rocks in the study area was quantitatively evaluated. Meanwhile, we studied the thicknesses of the Middle Cambrian gypsum-salt rocks, gypsum-bearing mudstone, and gypsum-bearing dolomite using two-dimensional seismic data and drilling data from 14 wells. The microscopic and macroscopic appraisals of Middle Cambrian successions are constrained by drilling data obtained during exploration. Finally, we proposed a comprehensive method to evaluate the sealing ability of the caprock.

Results The results suggest that gypsum-salt rocks have better sealing ability than gypsum-bearing mudstone and gypsum-bearing dolomite, but the faults and burial condition will affect the sealing ability of the caprock. It shows that the thicknesses of the gypsum caprocks are great in the Awati Depression, the north of the Tazhong Uplift, the south of the Manxi Low Uplift, and the middle and north of the Bachu Uplift. The thinness of gypsum is greatest in the Bachu Uplift and decreases unevenly towards the surrounding areas.

Conclusions The research results show that the Awati Depression, the West and south of Manxi Low Uplift, the north and west of Tazhong Uplift, and the middle of Tabei Uplift are favorable areas for the development of the caprock.