The genesis of gas condensates and light oilsin the lower paleozoic of Tarim Basin, NW China: The exploration implications for ultra-deep petroleum

Abstract

Almost 8 × 108 tons marine condensate reserveshave been estimated to be present in the Paleozoic to Cenozoic reservoirs of the Tarim Basin. Thethermal maturity of condensate gases are estimated to be greater than 2.2%REo (vitrinite reflectance equivalent),which are higher than the calculated maturity of thecondensates <1.9%REo. Therefore, the formation of gas-condensates might include a mixture of later cracked gases with earlieroil accumulations. The mixture model indicates that the relative content of oil-cracking gases varies from 42% to 98% in condensate gases, and the mixing extent of kerogen and oil cracking gas with various maturities could also lead to the carbon isotope reversal of methane and ethane. The positive correlation between the gas-oil ratio (GOR) of gas condensates and the maturity of cracking gas shows that the cracking gas is much more enriched in the eastern Tabei uplift of the northern Tarim Basin thanthe Tazhong uplift ofcentral Tarim Basin. The relative abundance of the triaromatic dinosteranes in the condensates coincides with the feature of hydrocarbons derived from Cambrian source rocks Furthermore, based on the multi-stagethermal maturity of the Cambrian source rocks, the formation mechanism of secondary gas condensates has been demonstrated and reconstructed. Recent exploration discoveries in the ultra-deep Cambrian of the Tabei and Tazhong uplifts, involving ZS-1 and LT-1wells, confirmed the cracking gases were derived from the post-high mature Manjar Depression and that liquid hydrocarbons could be maintained in the ultra-deep of Tabei uplift. The findings are helpful toestimate the exploration potential of the deeper cracking gases in the high-temperature and high-pressurereservoirs of the Tarim Basin.