Reservoir Evaluation Of The Yanchang Formation In Fuxian Area, Ordos Basin

Ordos basin, located at the western part of North China platform, is a large-scale superimposed Craton basin and a significant oil-bearing basin in China. Fuxian oil field is on the northwest trend monocline background, it is located in the south of Ordos basin.Over the past ten-years experience of exploration, Fuxian area has been found servial oil-bearing layers from mesozoic to lower Paleozoic. Yanchang formation is one of the most important exploration target layer.And it's a lithologic reservoir with low-porosity and low-permeability characteristics.Study on the distribution of sedimentary system, reservoir characteristics and effective reservoirs shows that reservoir heterogeneity in Yanchang formation behaves strongly on planar and vertical in Fuxian area.Applied with cores observation, thin section identification, pore texture analysis, SEM analysis and the studied results of former geologists, this thesis deals with the reservoir features, the factors which controlled reservoir characteristics and the reservoir evaluation. Many theories and technological related to sedimentology, reservoir sedimentology, petrology geophysical well-logging have been used. Results indicate that Chang8~2 Members are the main productive layers. Chang-8 Member developed delta front deposition. Delta front and turbidite fan developed in the Chang-7~6 period when the lake basin shrinked. Chang-4+5 Member developed early delta plain deposition and the lake basin was still shrinking. As for Chang-3~2 period, the lake basin was gradually dying out, and delta plain developed.Through the research of petrologic feature, pore types, pore structure and the relationships among lithology, poroperm characteristics, electric properties and oil-bearing properties, it is thought that the lithology of Yanchang formation is mainly fine grained feldspathic sandstone and lithic arkose sandstone, which brings about the feature of low compositional maturity and low textural maturity. Average porosity of the reservoir in Fuxian area is 9.3%, average permeability is 0.46×10-3μm2.It indicate that the reservoirs characteristics are totally of low porosity and permeability, fine pore throat and poor sorting. Remaining primary pore is the dominating pore type in this area, secondly for corrosion pore. By thin section identification and physical property analysis, it's found that dissolution, cataclasis, cementation of chlorite rims curbed quartz overgrow and protected lots of primary pores. But compaction, carbonate cementation, clay minerals cementation such as illite are the destructive diagenesis which resulted in reservoir properties getting worse. Moreover, the grain size of sandstone, the sedimentary thickness of reservoir and the distribution of sedimentary microfacies are also the controlling factors of reservoir development. By sedimentary analysis, distributary channels deposition enjoys better physical properties than mouth-bar and distal-bar.Above the comprehensive Research, it can be concluded that the low limit of porosity is 8% and the low limit of permeability is 0.1×10-3μm2. Reference to the standard of reservoir evaluation in changqing Oilfield, the reservoirs of Yanchang formation in Fuxian area are classified as five types:Ⅰ,Ⅱ,Ⅲ,Ⅳ,Ⅴ(non-reservoir). Type-Ⅲand type-Ⅳreservoir can be seen through the Yanchang formation from Chang-8~2 Member, type-Ⅱonly exists a few in Chang-3~2 Member. Based on the research of effective reservoir, storage factor distribution and combined with sedimentary facies, oil-gas distribution, the result of exploration, the thesis discussed the reservoir comprehensive evaluation and the available exploration block prediction of Chang-8, Chang-6, Chang-3 Member at last.