Abstract:

The Tarim Basin serves as the major area for deep and ultra-deep oil and gas exploration and development. A significant breakthrough has recently been achieved in the exploration of ancient buried hill-type oil and gas reservoirs within the Cambrian System of the Paleozoic strata in well Tuotan 1, Kuqa Depression, Tarim Basin. Due to the ancient geological age and complex reservoir geology, systematic studies on oil and gas accumulation process and fluid evolution in this area remain insufficient. In this study, methods including biomarker analysis, petrological analysis, in-situ micro-area trace element and strontium isotope analysis, and fluid inclusions were used to determine the fluid origins of vein formation and the timing of multiple-phase of oil and gas charging in the dolomite reservoirs of the Xiaqiulitage Formation. The results revealed that the dolomite reservoirs in Xiaqiulitage Formation, well Tuotan 1 primarily developed two phases of calcite veins that filled fractures and dissolution pores. The first phase of calcite originated from deep, strontium-rich fluids, while the second phase of calcite veins derived from seawater. Two phases of oil-bearing fluid inclusions were predominantly developed within the second-phase calcite veins, comprising secondary blue-white fluorescent oil inclusions and secondary green fluorescent oil inclusions. The integration of fluid inclusion thermometry with single-well burial history reconstruction revealed that the first-phase blue-white fluorescent inclusions recorded oil and gas accumulation during the deposition of the Neogene Jidike Formation (23-20 Ma), while the second-phase green oil inclusions recorded oil and gas accumulation during the deposition of the Neogene Kuqa Formation (5-3 Ma). Oil-source correlation analysis indicated that the two phases of crude oil in the reservoirs of Xiaqiulitage Formation were derived from mixed contributions of lacustrine source rocks in the Triassic Huangsanjie Formation and Jurassic Qakmak Formation. The new findings from well Tuotan 1 in Kuqa Depression demonstrate that ancient strata in the foreland region of the Tarim Basin still retain favorable conditions for large-scale oil and gas accumulation, making buried hill-type oil and gas reservoirs a promising frontier for increasing reserves and production in Kuqa Depression.

Key words: fluid inclusion, oil-source correlation, fluid evolution, Cambrian, Kuqa Depression