Stratigraphy, Origin and Significance of Oil and Gas Exploration of Cretaceous Breccia in Well PBX1, Southwestern Tarim Basin

Well PBX1 is located on Markit slope in the southwestern Tarim basin, and gets oil flow from the breccias upon the testing after oil show at the depth of 6 884-7 150 m.However, the stratigraphy age and formation reasons of the breccia still remains in dispute, which limits the exploration deployment and potential analysis in this area. Core observation and X-ray powder diffraction test show that the degree of breccia roundness and size classification are relatively poor. The compositions of breccia are white sparite psammitic composition (oolitic) limestone, light gray powder dolomite, grey powder dolomite, dark grey powder and a small amount of quartz and anhydrite, and its matrix mostly is cementation of carbonate gravels with fine grain. Many pollen microfossils are found in the lower part of objective formation, the age of which mainly is Cretaceous. Detrital zircon ages range from 2 529 to 127 Ma in the study area, and the age of sedimentation influenced by the terrigenous provenance is after 127 Ma. The comprehensive analyses of lithostratigraphy, biostratigraphy and isotope dating indicate that the age of breccia in Well PBX1 belongs to Yigeziya Formation of Late Cretaceous. The distribution scale of breccia is limited, which overlaps on the strata of Permian and is controlled by hot bottom split underground forming circular depression. Effected by the former caldera and later intrusive complex structure, Late Cretaceous platform facies carbonate fragmentate and deposit in the negative structures of north Pishan Mountain. The oil and gas resources of Well PSX1 were derived from Cambrian. The magmatic intrusion pierces the huge thickness of Paleozoic carbonate and clastic strata, forming a vertical unconformity surface which links up the source rock. The hydrocarbon migrates to the Mesozoic and Cenozoic reservoir-roof combination and gathering with the help of high angle faults, which were created by the volcanism, and form bottom-water structure formation oil reservoir. The Cenozoic of southwestern Tarim basin will be the next breakthrough of hydrocarbon exploration. Fault block traps around calderas edge and special traps which are sheltered by central arched-roof igneous-rock are the favorable targets of further hydrocarbon exploration.