The Characteristics Of High-resolution Sequence Stratigraphy And The Studying Of Reservoirs Of Upper Triassic Yanchang Formation In Ansai Oilfield

The Ordos Basin is one of the most important energy source bases in China. It has abundance oil resource. Ansai Oilfield is located in the slope of Shanbei of the Ordos Basin. Since the oilfield has been found in 1983, after 20 years prospecting and developing, it has become the important oilfield in the Odors Basin. The main oil-bearing layer is Triassic Yanchang Formation. It belongs to the petrology concealment oil-gas and has the characteristics of low permeability or very low permeability. Because the strong heterogeneity of the reservoir and the complexity of oil distribution, the cost of exploration is very high. In order to reducing the cost of the exploration and development, finding out the rule of the oil and gas distribution, providing the credibility geology gist for the further development, it is necessary to research the fine division and correlation of the strata.Based on the high resolution sequence stratigraphy and the sedimentology theory, by the observing of outcrops, the describing of cores and the analyzing of well log data, the paper establish the Yanchang Formation stratigraphic framework in the first time in Ansai Olifield. And stress to study the Chang 6 and Chang 2 oil-bearing formation in the sequence framework, discuss the characteristics of the sandbodies distribution and sedimentary microfacies. Using porosity and permeability data, the heterogeneity of reservoir has been studied and the controlling factors also have been analyzed. On the basis of above synthesis analysis, the author generalize the factors of the oil collecting and its distribution rules, at last the exploration targets have been assessed. By the study, it gains the results as follows:On the basis of analysis sedimentary evolvement history of Upper Triassic Yanchang Formation in the whole basin, by the studying of the paleocurrent direction and the heavy minerals, it make sure that NE~NEE is the main direction of sediment provenance.By the observing of the typical outcrops, the describing of the cores and interpreting lots of the well logs and seismic data, the author discuss the facies of petrology and the facies of log well, and set up the relationship of each other.The type and the characteristics of Sedimentary microfices in Yanchang Formation have been systematic research. The conclusion is that Chang 6 and Chang 8 dominated by meandering fluvial front-delta facies, Chang 2 belong to the braided river deposition, Chang 3 and Chang 10 is meandering fluvial facies, and Chang 7,Chang 9 and Chang 4+5 are lacustrine facies.There are three kinds of the interface. One is the base level changing from ascending to descending- that is the flooding surface. The second is the base level changing from descending to ascending-that is the erosive bottom surface. In addition, the mud and tuff symbol layers thatpossess the clearly characteristic are the good gist in dividing and correlation strata.On the basis of cores and well log data, Yanchang Formation has been divided into one long term base level cycle, four middle term base level cycles, and 9 short term base level cycles. The strata framework of Yanchang Formation in Ansai Oilfield has been established in the first time. And the short base level cycle SSC5 and SSC2 also has been subdivided, and set up the responding super-short term base level cycle framework.The characteristics of the sandbodies and the microfacies of the reservoir in different order base level cycles indicate that the reservoir of SSC2 is the braided sandbodies while the SSC5 is the front delta channel sandbodies. The direction of the sandbodies is consistent with the direction of the sediment provenance and the microfces distribution-that is the NE-NEE.Different base level cycles have different porosity and permeability properties of reservoirs. By the studying of the petrology component and structure, the porosity and permeability, the porosity types and the porosity structure, SSC2 is better than SSC5 in porosity and permeability properties. Its controlling factors are the sedimentary...