Study On The Forming Conditions Of The Lower Crataceous Reservoir In Central Structural Belt Of The Chagan Sag

The main research object of this papers is oil reservoir in the Barun Central Structural Zone of western secondary sag of the Chagan Depression, this paper summarizes the physical properties and geochemical features of the extract from source rock, oil and reservoir. Then, on this basis, it discusses oil-source relationship by using geological and chemical examination data. This article also use fluid inclusion analysis technique to analyse hydrocarbon accumulation period and the accumulation time, summarized the reservoir forming mode as well. The main achievements are introduced as follows.(1)Through analyzing the related to the geochemical data about source rocks in this area, There are three sets of source rock in the oil field, K1s1, K1b2 and K1b1, on the whole, the abundance of organic matter in source rocks is relatively high, organic matter type is II1-II2, the mature of source rocks in the central structure region is still basically a low maturity, while well Y11 and well Y1 have reached a mature stage;the effective source rocks of K1b2 reached a maximum of thickness more than700 m in well Y1 area, K1b2 is the main source rocks.(2) By analyzing the data of the testing and analysing of rock flakes, cast thins,SEM, the clastic reservoir rock type is mainly feldspathic litharenite and lithic sandstone, and it has a low compositional maturity,interstitial material is mainly gray shale and dolomitic and mud. the main porosity type is intergranular dissolved pores,micro-cracks, intragranular solution pores, the results of analyzing of poroperm characteristics show that the reservoir in this area are mostly low porosity and ultra-low permeability reservoirs. By analysing macroscopic parameters of mudstone, The results show that the mudstone of the K1b1 and K1s1 are middle regional cap, but Yingen is just a regional cap.(3)The physical properties and geochemical characteristics of crude oil indicate that the physical properties of oil is relatively good, low gravity, slightlyhigh viscosity and high paraffin content, the main body is medium and waxy crude oil.Through the analysis of the components of crude oil and the characteristics of carbon isotope, normal paraffin hydrocarbons, sterane and terpane, indicating that oil ofK1s1 from Y10, Y10-1, Y11, Y15 wells on the central structure belt are mainly from K1b2 of Hule sag; A relatively low degree of maturity of oil is a mixture of oil from local source rock and other from the zone of Y11 well source rock, Y6(K1b2),Y7(K1b1) and Y9(K1b2) wells.(4)Reservoir types in the study area are mainly fault block reservoirs and fault nose reservoir.Research on migration pathways believing that hydrocarbon accumulation occurs mainly vertical migration and lateral migration, the main channel of vertical migration is faults. the sand body connectivity and unconformities are the main migration pathway of lateral migration. through combining the data of inclusion thermometry with the study area the burial history and thermal history showed that the oil accumulationof K1b2 experienced two hydrocarbon accumulation, suhongtu formation sedimentary period and Yingen formation sedimentary period. while K1s1 reservoir experienced one period of accumulation in Yingen formation sedimentaryperiod.