Lithofacies And Palaeogeography Of The Middle Jurassic Buqu Formation In The Long Euni-Osma Region,Qiangtang Basin

Lithofacies palaeogeography is the study of sedimentation and a series of basin evolution,analysis of the sediment characteristics under different sedimentary environment and the distribution law of oil and gas and solid mineral formation is analyzed on the basis of material basis and conditions,the distribution law of oil and gas and solid mineral resource potential and guide them further exploration work.The title of this paper is from the sub project of "investigation and evaluation of key blocks of Qiangtang" in the project of "Qiangtang Basin Jinxing Lake-Long Euni area oil and gas resources strategic investigation",The study subjects were selected from the Middle Jurassic Buqu Formation of the Long Euni-Osma Region in the subproject,This paper uses modern sedimentology theory,the study area of Buqu Formation in detail,rock structure,sedimentary structure,sedimentary facies and lithofacies palaeogeography study,and on the basis of the Middle Jurassic Buqu Formation,oil reservoir,favorable oil and gas prospecting area were analyzed briefly.The Qiangtang basin is sandwiched between the Hoh Xil-Jinshajiang suture zone and the Bangong-Nujiang suture zone.The central uplift belt divides the Qiangtang basin into the South Qiangtang depression and the North Qiangtang depression.The study area belongs to the part of the South Qiangtang Depression,west of the Shuanghu County in the area of Tibet,and in the Easmine area near the Andu County in the east,the earth coordinates are 88 ° 30'-90°30'east,latitude 32°30'-33°north latitude.According to the author's field profile,observation profile,observation route and drilling data,combined with the comprehensive analysis of previous research results,made the following understanding.1.The formation of the Longanyi-Osma area is always east-westward in the upward direction,and has the characteristics of the north-south zone in the tendency.According to the rock assemblages and fossil characteristics of the strata,the strata from the north to the south can be divided into three strata,namely,the northern Tiandi-Chalanga strata,the northern part of the Bi Luo wrong-Dazoma area stratigraphy,the southern part of the Dormy-Sauce-Zaqu stratigraphy.And the Cenozoic strata of the Upper Triassic Tumen and the Cenozoic strata.The Bertoche-Ondal error-Dazoma Formation is divided into two groups: Granite limestone and a small amount of gypsum salt,brown and purple sandstone,silty mudstone and so on.The Dumas-Sauce-Zaqu stratigraphy is divided into two groups: muddy shale.2.The rock types in the Middle Jurassic Buqu Formation in the study area are dominated by carbonate rocks,and the clastic rocks are rare.Only sandstone is found in the area of the Quba-Gonduma in the east of the study area,Siltstone and the Tuo-zhi-zhi-sheng section of the Duo-Sue-Zaqu stratigraphy,and found the mudstone.The limestone in the carbonate rocks and the dolomite are mainly distributed in the large area.The limestone is mainly composed of internal debris limestone,oolitic limestone,bioclastic limestone,clump limestone and mudstone limestone.Algae layer dolomite,grain dolomite;parts of the area can be seen a small amount of gypsum.3.Based on the comprehensive study of the lithology assemblage,rock structure and sedimentary structure of the multi-section cross section of the Buqu group,combined with the analysis of the inclusions and the carbon and oxygen isotopic data of the dolomite and the roof and limestone,the sedimentary facies Is divided into: tidal flat phase(including the tide under the shoal phase),lagoon phase,shelf phase.4.Based on the analysis of multiple facies sedimentary facies,combined with dozens of trajectory tracking and analysis and the special rocks(oil shale and ointment Salt rock)distribution and paleogeographic inheritance analysis,the lithospheric paleogeographic map of the Middle Jurassic period is established.The first study area can be divided into five facies belts: uplift eroded area,tidal flat facies,lagoon facies,tidal trough facies belt and continental shelf facies.5.Based on the study of lithofacies palaeogeography,combined with the relevant data and the results of the test analysis,the oil and gas reservoirs,favorable oil and gas prospecting areas in the Longanyi-Osma area are analyzed.It is believed that the oil-bearing rocks in the ancient reservoirs are mainly distributed in the lagoon facies.The reservoirs are mainly distributed in the tidal flat and tidal shoal belts around the lagoon facies.The favorable oil and gas exploration prospecting area should be the western part of the study area.