Hydrocarbon Accumulation And Enrichment Rules Of Yan-chang Formation In Zibei Region

Chang2, Chang4+5and Chang6members of the Zibei Oil Region in Ordos Basin are taken as the srudy objects in this paer.Under the guidance of theroies of sedimentology、 petroleum geology、geochemistry and well logging gepphysices, on the basisi of abudant data from geology、drilling、logging, etc, the Chang2, Chang4+5and Chang6members of the Zibei Oil Field is deeply studied in strata、structure、sediment、reservoir and characteristic of oil-pool, taking Zibei Region as a whole for the first time. Based on the integrated analysis of electical property, lithology and logging data, according to the regional key bed, on the principle of equal thickness and step control, the division and comparison of reservoir group Chang2, Chang4+5and Chang6of the Zibei Region are deeply studied and the stratigraphic framework of the region is built in this paper; and then the superface structures of Chang2, Chang4+5and Chang6layers are studied; under the east-weat trending monoclinal structure, influenced by the effect of differential compaction, Chang2, Chang4+5and Chang6of reservoir groups developed partly nearly east-weat trending nose-shaped uplift.Through the study of core observation, grain size&rock type analysis, sedimentary structure, sedimentary cycle, logging and testing, the sedimentary micro-facies of Zibei region is analyzed. Reservoir group of Chang2mainly displays delta plain subface deposit, and where braided river delta is developed. Reservoir groups of Chang4+5and Chang6are delta plain, lower part of reservoir group Chang4+5and reservor group Chang6mainly are characterized by delta plain distributary channel deposit. The delta plain distributary channels of Chang2and Chang6distributed generally in NorthEast-SouthWest banding, the distributary channels are separated by interdistributary bay facies.By analyzing the porosity and permeability parameters, the formations of Zibei region are low porosity, extra-low permeability and extra-low porosity, ultra-low permeability type, and these formations are divided into Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ classes. For classes Ⅲ and Ⅳ, they are futher divided into a and b semi-classes. The class Ⅲ and Ⅳ are the main reservoir types in Zibei Region, the class Ⅰ, Ⅱ expressed mainly on Chang2formations; the class Ⅲb, Ⅳa and Ⅳb and mainly presented in Chang4+5and Chang6. Sedimentary micro-facies and lithogenesis are the primary factors controlling and influencing the formation development and spreading of Zibei region.On the study of petroliferous system elements (hydrocarbon source rock, reservoir rock, cap rock, and trap) and their association characteristics, oil and gas mainly originated from the deep laketone and shales. The two serts of reservoir-cap combination of Zibei region are Chang6(reservoir)-Chang4+5(cap rock) and Chang2(reservoir)-Chang1(cap rock), with self-storage, self-cover assemblage locally developed within Chang4+5and Chang1formations. Due to severe erosion, preservation condition is poor and oil distribution area is limited.According to hydrocarbon show evidence and oil-source correlation results in Zibei Region, the system of Zibei Region can be categorized as yan-chang formation of upper-triassic system for the first time, and the semi-systems are Chang6, Chang4+5and Chang2. Through inclusion temperature measurement, basin thermal history, dating of illite and other methods, the reservoir forming period of mesozoic Yanchang formation in the study area is early cretaceous epoch The source-reservoir-cap assemblages of Zibei Region is determined:Chang2reservoir is lower generating-upper reservoir type, Chang4+5and Chang6reservoirs are upper generating-lower reservoir type,which presents the two reservoir-forming patterns of Yanchang formations, that means sandstone lens reservoir forming and updip pinch-out reservoir forming. On that basis, the planar distribution pattern and controlling factors of reservoir groups of Chang2, Chang6and Chang4+5in Zibei Region are systematically studied for the first time, Chang61reservoir are major reservoir, Chang62and Chang4+5reservoirs take second place, Chang63and Chang2reservoirs distributed narrowly. The forming and distribution of Yanchang formations in Zibei Region are controlled by multiple factors such as sedimentary facies, high-quality formations, nose-shaped uplift, regional cap, migration pathway and etc. The favorable sedimentary facies and high-quality formations are the basic conditions for the forming and accumulation of triassic reservoirs; the nose-shaped uplift structure plays akey role in enrichment and high productivity of oil and gas.At last, on the basis of the sedimentary microfacies, formation physical properties, structures, formation distribution patterns, the favourable exploration blocks are evaluated. The block to the north of JianYuCha area and block of Well Li448of NanGouCha area are selected as major region for future exploration.