Study On The Jurassic Sedimentary Facies Of Suganhu Basin And Its Periphery

The medium-small basins in North Margin of Qaidam Basin,including Suganhu Basin,are potential strategic replacement fields of oil and gas resources in western China,and the preliminary work has revealed that Suganhu Basin develops Mesozoic source rocks with a certain exploration potential.However,up to now,the little cognition of the sedimentary characters and environments has restricted the further oil-gas exploration work in this area.Through a series of technical means,such as outcrops observation,drilling data and geochemical data analysis,it is the sedimentary systems and environments in Jurassic that have been studied preliminarily.All of those will support the further oil-gas exploration.In the study,fan delta and lake depositions are main facies in Suganhu Basin in the Early and Middle Jurassic,and the dominating facies are braided river depositions in this basin in the Late Jurassic,meanwhile,during Jurassic Period the sedimentary source provenance probably comes from the northern Qilian Mountain;in the Middle Jurassic Period,the delta-marsh-lacustrine depositional system,containing rich coal seams,is well developed in the southern margin of Sertengshan Mountain,and the sedimentary source provenance comes from the north-western and north-eastern areas;in Lenghu area,the primary depositions are deep to semi-deep lacustrine depositions that present the Huxishan Formation in the Early Jurassic,and the sedimentary scource provenance probably comes from the nearby Altun Mountain.Based on outcrops observation,drilling stratigraphic classification and correlation and seismic stratigraphy study,the Jurassic residual strata distribution in the research area is preliminarily studied: the Lower Jurassic strata are mainly found in the Lenghu tectonic belt and the Xiangyang Coal Mine;Well Sucan-1 and Well Sutan-1 in the central depression of Suganhu Basin reveal the Middle-Upper Jurassic series;the Middle Jurassic coal-bearing strata are exposed to the southern margin of Sertengshan Mountain and the Xiangyang Coal Mine;the Upper Jurassic can been observed in the southern margin of Sertengshan Mountain.The distribution of residual strata may be controlled by the Yanshanian structural movement.According to the geochemistry analysis of mudstone samples,rocks in the study area mainly form in reduction environments in the Jurassic,and different samples contain the similar distribution patterns of REE.The analysis of tectonic-sedimentary evolution shows that the western North Margin of Qaidam Basin was on the initial rift stage in the Early Jurassic,and the sedimentary center of the lacustrine basin was located in Lenghu area;in the Middle Jurassic,North Margin of Qaidam Basin was in an extended depression sedimentary environment,while the sedimentary area was expanding and the sedimentary center was moving toward to east;in the latter term,corresponding to Member 7,lake flooding was developing,and a big lacustrine basin was formed in this area.The source rocks of Jurassic are mainly developed during the rising stage of the lake level,namely Member 5 to Member 7 of Dameigou Formation.The overall evaluation of the Jurassic source rocks in Suganhu Basin and and its periphery is medium to good.The types of organic matters are mainly Type Ⅱ and Type Ⅲ,and the organic matters are mature and begin to produce hydrocarbon in big mass.The reservoirs are mainly lithic sandstones,and metamorphic quartzite lithic fragments are the main ones.The reservoirs are compact with porosities less than 10%.The reservoir spaces rely on residual intergranular pores and intergranular dissolved pores,and the reservoir properties are relatively good.