The Sedimentary Characteristic And Hydrocarbon Geologic Condition Of Permian In Buerjin-Zaysan Area,China-Kazakhstan Border

The oil and gas field(including the Permian,Jurassic and Paleogene reservoirs)has been found in the same basin and in the same structural position in the Zaysan depression on the Kazakhstan side of the Buerjin-Zaysan basin.There has been no discovery of oil and gas in the Buerjin depression on China side.In order to promote China's oil and gas exploration work in the Buerjin area and provide certain geological basis for the companies which engaged in oil and gas exploration in Zaysan depression,it is very important and necessary to carry out the study of the Permian sedimentary filling and oil and gas geological conditions in Buerjin-Zaysan area along the China-Kazakhstan border.This paper is based on the Paleozoic tectonic evolution in the western section of the Paleoasian Ocean tectonic domain,the progress of Permian oil and gas exploration,the oil and gas geological exploration of Buerjin-Zaysan Basin and the geological situation of Buerjin-Zaysan region,and also based on the outcrop geological profile,drilling,the 2D seismic section,sample test results,and previous achievements,etc.We have divided the tectonic units of Buerjin-Zaysan area and analyzed the regional tectonic evolution process.According to the results of field geologic profiles and drilling and seismic profiles,the characteristics of the Permian strata in the study area were analyzed in this paper,and the division of sedimentary facies in the study area was completed,on the basis of former study,the hydrocarbon evolution process,hydrocarbon expulsion dynamics,and secondary migration of hydrocarbons are also analyzed.Combining with the characteristics of source,reservoir and cover,through the analysis of fluid inclusion test results and the results of research on reservoir formation dynamics and methods,this paper produced a petroleum system event table,comprehensively analyzed the hydrocarbon accumulation process,and summarized the hydrocarbon accumulation model.Buerjin-Zaysan Basin is a intermontane superimposed basin which located between Southern Altai and Tarbagatai mountains.The tectonic units are divided into three secondary structural units from south to north: Zaysan depression,Buerjin depression and Central uplift.Controlled by the post-closure structure of the Paleoasian Ocean,the sedimentary facies of Permian in Buerjin-Zaysan area was a lacustrine facies of the foreland basin.The Early Permian was the initial development stage of the lake basin.The types of sediments in the Kalagang Formation in Buerjin depression,the ??????????? Formation and the overlying Kendirek Formation in Zaysan area were debris construction of some coal-bearing;In Middle-Late Permian,the sedimentary center of Buerjin-Zaysan basin moved to the Zaysan side.The Upper-Middle Permian in the Zaysan area were Karaulgur formation,Taranche formation,Maichat formation,and Akkolkan formation,which were mainly detrital sediments under the lake-delta system.There were two main source rocks of Permian developed in Buerjin-Zaysan area: one unit was upper Permian semi-deep lacustrine and deep lacustrine mudstone in Maichat formation and Akkolkan formation,the other unit was lower Permian shallow lacustrine mudstone in Kalagang formation and ??????????? formation.The Early Permian source rocks developed in a slightly oxygen-deficient environment in freshwater lakes,and the Late Permian source rocks developed in lake sedimentary environment with reduced,high salinity algae developed.Comparing with the organic geochemistry parameters between the upper Permian source rocks in Zaysan depression and the lower Permian source rocks in Buerjin depression,we found that the upper Permian source rocks in Zaysan depression overmatch the lower Permian source rocks in the Zaysan depression.The Permian reservoirs of Buerjin-Zaysan Basin are mainly sandstone and conglomerate,and the composition maturity and structural maturity of the reservoir are low.The main types of reservoir porosity is the remaining intergranular pores,followed by the intragranular solution pores.The regional cap rock of Buerjin-Zaysan Basin is a deep lake and semi-deep lake mudstone which developed in the Cenozoic and has good distribution on the plane;The Maichat group and Akkolkan group of Permian and the Tologoy group of Jurassic were local cap rocks.They are well matched.Buerjin-Zaysan area has favorable conditions for hydrocarbon accumulation during the Permian.There were two main hydrocarbon source rocks developed in Permian.The area where the mountain front thrust belt developed has good matching among the source bed,reservoir and cap rocks.The boiling inclusions in the Permian sandstone reservoirs indicates that the production and migration of hydrocarbon had occurred in the early Permian.The burial history of the basin shows that the main source rock underwent multiple heating processes.It reached the peak of oil generation in early Jurassic,while it reached the peak of gas generation in Paleogene.The abnormal excess pressure is widely developed in the process of hydrocarbon generation of thick mudstone layer in the Permian,the initial migration of oil and gas is along the direction which the pressure gradient decreases.The oil and gas can form fault reservoirs through vertical migration of fracture in secondary migration,it can also formed various Permian oil and gas reservoirs.The Paleogene gas reservoir and Jurassic thick reservoir are to a large extent the result of the secondary accumulation after the Permian reservoir failure.The regions of Jurassic heavy oil and Paleogene gas reservoirs should be paied more attention on,and it has important exploration significance for the search of Permian oil reservoir.