Reservoir-cap rock assemblage and sedimentary characteristics of Cretaceous-Paleogene in southwestern Tarim Basin

The sedimentary studies of Mesozoic-Cenozoic in the southwestern Tarim Basin indicated that the Early Cretaceous inherited the sedimentary features of the Jurassic, mainly dominated by continental deposits. As the Tethys ocean expanded from the Late Cretaceous to Oligocene, sea water flooded several times to Kashi, Yecheng and Hetian from west to east, forming the West Tarim Bay. Marine facies sedimentation developed from the Upper Cretaceous to the Eocene Wulagen Formation in the southwestern Tarim influenced by large scale transgressions when global sea level rose. After that, continental sedimentation took the leading position from the Eocene to the Oligocene Bashibulake Formation due to global sea level decrease. Sea level changed frequently from the Cretaceous to the Paleogene in the southwestern Tarim, resulting in the preservation of carbonate, gypsum salt and mud rocks of littoral and neritic lagoon facies. There are two sets of reservoir and cap rock assemblages, the one is carbonate rocks in the Upper Cretaceous Yigeziya Formation and gypsum salt rocks in the Paleogene Aertashi Formation, the other is carbonate rocks in the Paleogene Kalataer Formation and dark mudstones in the uplying Wulagen Formation. These two assemblages are important for oil and gas exploration in the southwestern Tarim Basin.