Characteristics Of Tight Sandstone Reservoirs Below Source Rocks And Main Factors Controlling Hydrocarbon Accumulation In Rangzijing Slope Zone In Southern Songliao Basin

As a kind of unconventional petroleum resources, tight sandstone oil and gas is paid more and more attention, and is the important replacement petroleum resource in the future in China. Strengthening the research in geological study of tight sandstone oil and gas has important strategic significance and practical value.According to geological features such as Fuyu oil layer below source rocks and tight reservoirs, and the main problems currently existing in Rangzijing slope zone, based on the studies of sedimentation and diagenesis, this paper makes a study on the characteristics of tight sandstone reservoirs below source rocks, the genesis mechanism of densification, and controlling factors of hydrocarbon accumulation, providing geological basis for oilfield exploration and development in next step.Fuyu oil layer below source rocks developed in early transgression, rapidly transforming from fluvial facies and lake delta plain to fluviolacustrine deposition, and then to fluvial delta front subfacies in the 120 m thickness strata. Sedimentation defines low compositional maturity, fine grain size and poor sorting of sandstone reservoirs below source rocks, which is the main cause of reservoir densification below source rocks. The hydrocarbon filling of Fuyu oil layer in study area did not cause distinct effect on quartz secondary growth. It may inhibit the ferroan dolomitization of ferroan calcite. The diagenetic fluid environment can be divided into three main stages, alkalinityⅠ, acidity and alkalinityⅡ. Based on the principle of “effect superposition and segmented constraint”, we make tight reservoir evolution history by secondary structural zone. It shows that the reservoir tightening periods of Huazijing terrace, Fuxin uplift and Changling depression are 8Ma, 15 Ma and 45 Ma, which are earlier than the hydrocarbon large-scale filling period(about 75 to 65Ma). So the Fuyu oil layer is “after-forming” tight sandstone reservoir and nowadays the oil and gas distribution controlled by structure proves it.The development degree and physical properties of reservoir below source rocks become better as the distance to source rocks increases. Reservoir below source rocks is characterized by low compositional maturity and medium textural maturity. The main type of pores is dissolution intergranular pores, which are small, poorly developed poor in connectivity and heterogeneity. Base level cycle and depositional condition control the relation of source rocks and reservoirs, stacking pattern of reservoirs and original property of reservoirs. Compaction is the main factor leading to the reservoirs tightness, and cementation intensifies the tight degree. The secondary pores produced by organic acids migrating downward into reservoirs improve the reservoirs quality to some extent. Denudation is controlled by acid fluid migrating patterns and the distance to acid source and faults. Tectonic activity plays a positive role in improvement of reservoirs quality in three levels. CO2 filling can significantly improve the permeability of local reservoirs, but it is unfavorable for the formation of effective reservoirs.Source supply condition, palaeotopography and deposition caused the material base conditions of reservoir worse. The basin dynamic environment in which the source rocks developed determined the early compaction of reservoirs below source rocks was sustained and rapid. The fluid movement between source rocks and reservoirs was the key condition leading to the reservoirs tightness. Late carbonate cementation caused by multiple episodic activities of overpressure organic fluid made reservoirs tight eventually. The periods of reservoir densification varied in different secondary structural zones(about 45 to 15 Ma before present). Since the early compaction made a severe effect on fluid filtration channels, they were destructed seriously during the acid fluid migration period. So, the denudation was too weak to effectively improve reservoir properties, and the reservoirs remained tight ultimately.We propose a concept of reservoir “dominant pressure leakage compartment” below source rocks. Under the condition of limited fluid source, only the “dominant pressure leakage compartment” could become the overpressure fluid effective filling site. Its development depends on the reservoir characteristics(size and properties), pressure difference between source rocks and reservoir and channel conditions. Theoretically, with large scale, well physical properties, upper developing location and open channels to fluid source, these reservoirs have the characteristics of “dominant pressure leakage compartment”. Different kind of fluid filling made the denudation, carbonate cementation and hydrocarbon filling of Ⅱ and Ⅲ sand groups intense, which are the hydrocarbon enrichment layers of tight sandstone reservoirs below source rocks.The pressure difference between source rocks and reservoir, oil-migrating fault, and the reservoir are the main controlling factors of hydrocarbon accumulation. The coupling of oil source, overpressure and T2 faults make it possible to migrate downward, and the match of overpressure and conduit system under the source rocks leads to vertical evolution of 4 migration patterns. The oil and gas migrating downward along the fracture under overpressure is the main approach. The assemble pattern of the faults has important control on the migration and accumulation of oil and gas. The closer distance in the plane to the oil migrating fault, lager fault displacement, antithetic normal fault, horst structure, and higher grade of fault are the important controlling factor of accumulation and high production of hydrocarbon. The difference of the genetic type and stacking pattern of reservoir sandstone contribute to the difference in the ability of assembling and migrating hydrocarbon to control the oil and gas enrichment. The coupling of the pressure difference between source rocks and reservoir, oil-migrating fault, and the reservoir lead to the existence of “dominant pressure leakage compartment”. The down-migration distance varies with the change of “dominant pressure leakage compartment” location. Overall, the Ⅱ and Ⅲ sand groups is the preferable layer which is filled with hydrocarbon in Fuyu oil layer.The study results have a strong reference to the next step exploration and development program in study area, and have certain theoretical significance and practical value for the exploration and development deployment of oil fields with similar reservoir characteristics.