Depositional Environment Of Yanshiping Group In Longyala Region Of Eastern Qiangtang Basin And Its Feature In Petroleum Geology

Qiangtang Bas in is the la rge st ma rine ba sins in Qin ghai-Tibet Plateau, which has huge oil and gas resource potential, so it is regarded as the largest potential oil and gas resources in Qinghai-Tibet Plateau and the most hopeful to achieve breakthrough in the exploration of choice basin. Rese arc h on sedimentary environment and pe troleum geology feature is the basis of petroleum exploration, which has great significance for petroleum exploration. The thesis with "Depositional En vi r on m e n t of Ya n s hi p i ng gr o up in Lo ng y a l a Re gi o n of Ea s t e rn Qiangtang Bas in and its feature in Petroleum Geology" in the title,center with depositional environment and petroleum geology feature of the Yanshiping group for a more in-depth and comprehensive study.Based on the in-depth study of the basic geological characteristics of the pre vious data of Qian gtan g Bas in Yans hi ping grou p and the sedimentary environment of oil and gas, targeted on the Yanshiping gro u p fo r m a t i o n lo c a t e d in th e re s e a r c h ar e a of fi e l d ge o l o g i c a l investigation, collection and analysis of relevant system test samples,and the petrology, sedimentology, petrology and geochemistry, isotopic geochemistry, organic geochemistry were studied systematically. These researches not only proved the recovery and geological characteristics of sedimentary environment and oil Yanshiping strata in petroleum, but al so fo r the fu rth e r res e a rch of Ya ns h ipi ng st ra ta in the re g ion of petroleum, transport, storage, provides an important scientific basis cover rule. To sum up, main achievements of this paper are as follows:( 1) Measuring the Yanshiping group stratigraphic section,systematic collection of lithology, lithofacies, sedimentary structure and bivalve, Brachiopoda fossil data, dividing Yanshiping group into five lithostratigraphic units, from bottom to top, which is Quemocuo formation, Buqu formation, Xiali formation, Suowa formation and Xueshan formation, every groups are integrated contact relationship, on which are the Paleogene, Neogene or Quaternary angular unconformity coverage.( 2)Combined with the establishment of the biostratigraphic units,the chronostratigraphic division, Quemocuo formation determine the age group of Middle Jurassic Bajocian early Bathonian, Buqu formation era determine during Middle Jurassic to late Bathonian, Xiali formation determined for Middle Jurassic card Luo period, Suowa formation age determined for the base period in Late Jurassic- provided the early Tang, Xueshan formation determined to provide Tang period- white lias period.Detailed lithostratigrap,biostratigraphic and chronostratigraphic research for studying area on Jurassic Yanshiping group sedimentary environment analysis has laid a solid foundation.( 3)According to Yanshiping group of the sedimentary characteris-tics and tectonic features and sedimentary facies in the study area,combined with previous research results, the sedimentary environment was analyzed and divided into delta facies, tidal flat facies, carbonate ramp facies, facies and neritic shelf facies and clastic rock- carbonate rock mixed shelf facies, and the division of the sub facies and micro facies. The Quemocuo formation facies is a single of delta facies, many sedimentary facies of Buqu formation is carbonate ramp facies, neritic shelf facies, clastic rock and carbonate rock mixed shelf facies, the main sedimentary facies of Xiali formation is tidal flat facies,then delta facies, Suowa formation facies is also single,which is clastic rock-carbonate rock mixed shelf facies.( 4) Through of analysising the argillaceous rock about geochemi-cal characteristics in Yanshiping group, identifying the formation of elemental composition and distribution characteristics, determining the Yanshiping group stratum in the source region of the original material for acidic rocks, showing that the provenance area is a continental island arc tectonic background, revealing the source region may be a warm and humid climate conditions or as a relatively stable region, and has a moderately strong chemical weathering degree. Overall the Yanshiping group of sedimentary environment of redox conditions for weak oxidation environment(not strong anaerobic environment), and the area of macro geological characteristics of evidence is very consist-ent.( 5) Through of studying the LA-ICP-MS zircon U-Pb geochrono-logy of the sandstone clastic in Yanshiping group, claiming that the Qiangtang Basin has Pre- Cambrian basement, and the main period of its formation is approximately 1974 Ma ~ 1666 Ma, and the Pre-Cambrian basement also hadsuffered from the post- tectonic reconstruction. Experienced the tectonic thermal events of the Columbia and Rodinia supercontinent, Greenville Jingning tectonic movement of magma, Pan- African movement and a series of tectonic evolution, also supporting that the recognize of the Qiangtang terrane belongs to the Gondwana.( 6) On the basis of the total organic carbon and kerogen microscopic identification and vitrinite reflectance indexes of organic geochemistry, combined with the Qiangtang basin carbonate hydrocarbon source rock evaluation criteria, generally, we believe that the Jurassic of Yanshiping group of Buqu Formation, Suowa formation and Xueshan formation of carbonate source rock hydrocarbon generating ability belongs to medium level, organic matter types for type I, mainly in the oil, which is more favorable hydrocarbon generation area, especially in the Buqu Formation, that is deserving of public attention in future oil and gas survey and evaluation of the target horizon.( 7) Performing geochemical analyze of cold spring gas sample in permafrost area in study area. Based on this analyze, it pronounced that the average content of CH4 is 370 × 10-6. Comparing with the average content of CH4 of cold spring gas in permafrost areas along the Qinghai-Tibet railway( Kunlun Mountain — Anduo), it is nearly 113 times of the latter, and it exhibits a very high geochemical anomaly.The region of the Wenquan basin is favorable for the formation conditions of gas hydrate, such as permafrost thickness and temperature and pressure, hydrocarbon gas and water, fluid migration and reservoir,which is comparing with the formation conditions of domestic and foreign gas hydrates in permafrost regions. Indicating that the area islikely to have the existence of gas hydrates, and it should be the noteworthy of investigation and assessment about gas hydrate zone in the future.