Late Jurassic Stratigraphic Correlation And Palaeogeography In The Coqen Basin And Its Adjacent Areas

Jurassic reservoirs combinations in the Tethys tectonic domain are rich in oil and gas resources.The Qinghai-Tibet Plateau,which is located at the eastern part of Tethys,is not only the most ideal area for Jurassic marine sediments study,but also a favorable area for oil-gas exploration.Stratigraphy is both the basis and important prerequisite for the petroleum geological survey,thus scientific and perfect stratigraphic and paleo-geographic frameworks are strongly needed for the petroleum geology.However,research on the Upper Jurassic in the Coqen Basin is relatively weak and stratigraphic correlation still remains unclear.Based on the latest stratigraphic data,the past tectonic framework of "three continental blocks and two troughs" gradually shows its limitations,now therefore the author intends to do some research on the paleontology and sedimentology characteristics of Late Jurassic in order to reconstruct the sedimentary environment,paleogeographic framework and build a foundation for the subsequent study of petroleum geology in the Coqen Basin and its adjacent areas.In this study,we carry out field investigations at the Baoji,Baila,Pengyanco,Lhaze areas of Tibet,collect and analyz stratigraphy,paleontology,sedimentology and petroleum geology materials in the Coqen Basin and its adjacent areas.The Upper Jurassic Sabozhibule and Tukari formations are discovered from the Baoji County,and the latter is divided into the upper and lower members.The recognition of the Tukari Formation and Suowa Formation from the Baila County and Pengyanco sections indicates that carbonate sedimentation of Upper Jurassic is still distributed in the Bangonghu-Nujiang and South Qiangtang stratigraphic regions.Besides that,they are comparable to the Tukari Formation in the Gangdese stratigraphic region.This paper still uses the previous stratigraphic frameworks of Late Jurassic in the Himalayan,Yarlung Zangbo and the Northern Qiangtang stratigraphic regions.The sedimentary facies analysis of Late Jurassic typical sections suggests the Himalayas passive continental margin and mature Neo-Tethys oceanic basin environments for the Himalayan and Yarlung Zangbo stratigraphic regions respectively based on the previous inrepretation,while the cognition for Gangdese,Bangonghu-Nujiang and Qiangtang Basin stratigraphic regions has changed.There is no old land but a slope at the south Gangdese stratigraphic region,and its northern area is in a broad carbonate platform rather than a shallow-sea environment.The Bangonghu-Nujiang and Qiangtang Basin stratigraphic regions are located in the northern extension of this carbonate platform.Besides,the environment of Qiangtang Basin changes from the lagoon-shore to the limited platform-evaporation platform from north to south until the stable open platform appears in the southern margin.This study verifies the macroscopic geological understanding of “two continental blocks and one basin” for the Qinghai-Tibet Plateau.In this paper,the Baoji Strata,which has potential value of oil-gas exploration,is comparable to the source-reservoir-cap combinations,trap and sedimentary facies belts in the Arab Basin and Amu Darya Basin.It is suggested that the petroleum geology study of Upper Jurassic in the Coqen Basin should be strengthened.