A Study On Structure Characteristics And Reservior Forming Regularities In Northern Subsag Of Wuliyasitai Sag, Erlian Basin

The northern Subsag of Wuliyasitai Sag is located in the northeast side of Manite depression in Erlian basin, where discovered the industrial oil flow in some Cretaceous strata, it has a good exploration prospect. The lack of unified understanding of layers of the reservoir, the structure of the specific location and the morphological characteristics, reservoir distribution and the main control factors affected the expansion of oil and gas exploration achievements in northern Subsag. This paper is based on a large number of data and predecessors’ research, with modern geophysical exploration, structure geology, petroleum geology, sedimentology such as basic theory as the instruction, then have a deeper study in strata distribution, structure characteristics, sedimentary facies and reservoir characteristics of study area. Based on that, analyzed the accumulation conditions of oil and gas reservoirs and the accumulation process, identified the main controlling factors of hydrocarbon accumulation, established the hydrocarbon distribution sequence and accumulation mode in the northern Subsag of Wuliyasitai Sag, and pointed out the main direction of the next exploration in the study area, it has an important guiding role in the exploration and development of this area.The study area can be divided into two subsags and 5 positive structure belts, developed many faults which can be divided into four groups according to the fault strike on the plane, and show a variety of fracture combination style on the profile. Since the early Cretaceous, it experienced initial stage of rift, the stable stage of rift, the atrophy stage of rift, and the return lifting stage. Aershan Formation and the lower of the first member of Tenggeer Formation is the main oil-generative assemblage and reservoir in the study area. Aershan Formation developed northwest and southeast of proximal high energy deposition of two fan delta. The study area experienced two lake regression and two transgression during A4 period, formed A42, A44, A45 three sets of thick reservoir. The reservoir is mainly composed of glutenite with low compositional maturity and low textural maturity, and the overall reservoir property is poor, is the main places for oil and gas accumulation. The lake deposition is given priority to the lower of the first member of Tenggeer Formation, as the study area’s most important hydrocarbon source layer and regional cap layer, A43, A31 are the more important hydrocarbon source layer and partial cover.It can be summarized three categories, a total of seven types of reservoirs in study area. The lateral contact type of soure reservoir cap assemblages is the main SRCA of study area. The oil generated from hydrocarbon in subsags can be migrated along the oil source fault and unconformity surface, then accumulated in all kinds of traps. It can be summarized as three types of accumulation mode. The distribution and enrichment of oil and gas are controled by tectonic framework, the main oil source subsags, reservoir cap assemblages, fault and unconformity surface. Western nose structure belt, eastern nose structure belt and central fault barrier belt are the favorable tectonic zones in study area. Traps near the oil source faults and T8 unconformity surface in the zone, with favorable facies belt of reservoir development, is the best place for oil and gas to accumulate. Eastern nose structure belt with low degree of prospecting and is the focus of the next important exploration zones in study area. Two cross fault block traps in the east of central fault and one cross fault block traps in the west of H25, with well accumulation conditions for oil and gas, are the three favorable targets in study area.