Research On The Sedimentary Facies And Characteristics Of Yan9Oil Layer In Yang19District Of Suijing Oilfield

This paper is aimed at the research of Yan9oil group of Jurassic Yan’an Formation in Suijing district of Ordos basin. The research understandings and thinking have been built through a large amount of reading about the geologic information of the research area. With the application of theories on sedimentology, petroleum geology and so on, combined with coring, logging, log and production data and previous research achievements, a systematic study has been done on the sedimentary facies characteristics and distribution, sand bodies’conformation and distribution pattern, reservoir features and heterogeneity of Yan9oil group, based on which the influence factors of reservoir growth have been analyzed and the superior and inferior of the reservoir have been evaluated, laying profound geologic foundation for further reservoir exploration and development.Suijing oilfield Yang19well field is located in the mid-north part of the primary structural belt, Shangbei Slope. It is a high in the east and low in the west monocline. There is no faults and large-scale basement uplifts. A series of westwards-pitching low amplitude nose-like uplifts was on the slope and they have various sizes and similar forms, and an impervious bed existed in the updip direction, thus, a good combination of source, reservoir and capping beds was formed. In the area Quaternary, Tertiary, Cretaceous, Jarassic and Triassic formations were developed. The exploration target horizon is the Yan9oil group of Yan’an series. While the Yan9oil group was deposited the topography was flat, the water was shallow and the rivers brought a large amount of mud and silt into the lake. The sand bodies of the work area were mainly deposited in the NE-SW trending rivers of a delta plain, forming litho-structural traps together with the nose-like uplifts.On the basis of core description and logging curves, in this paper the marker beds of the research area have been determined, stratigraphic division and correlation of173wells has been done, the isochronous stratigraphic framework has been built and the Yan9oil group has been divided into three sublayers, Yan91, Yan92and Yan93. There is connectivity in the information thickness, which is represented by the tendency of the northeast part of the area tapering off toward the southwest part. In addition, the source lay in the eastnorth.Based on the geologic background of the region, the sedimentary characteristics of the Yan9oil group have been analyzed with lithofacies and logging facies data. The sedimentary environment has been determined to be river, and then it has been subdivided into3microfacies, channel, channel outer edge and flooding plain, whose distribution situation and features have been described in detail, and among which the channel sand bodies are the dominant reservoir body of Yan9reservoir and they are distributed in banding forms in the area. The connectivity of sand bodies of Yan9oil group is good. The sand bodies of Yan91and Yan92sublayer are8-12m thick, and distributed widely in plane while the sand bodies of Yan93sublayer is4-8m thick. The major oil-bearing horizons are in Yan91sublayer with Yan92to a lesser extent and there is generally no oil in Yan93sublayer.Derived from the cast thin slices, scanning electron microscope data of Yan9oil layer in Yang19district, medium-fine feldspar-lithic sandstones is dominant in Yan9reservoir, the clastic particles are subangular, the sorting is medium-good, and the texture maturity is high. Authigenous clay minerals such as kaolinite and hydromica were filled, and calcite, ankerite and other cements were developed with the pore-type cementation dominant. Intergranular pores, intragranular dissolved pores are dominant, with locally microcracks. The pore throats are dominated by fine throats with slight medium throats and distributed unequally.The porosity of Yan9oil layer in the research area is mainly ranged from10.06%to19.88%and the permeability is mainly between10.12－10-3μm2to68.19×10-3μm2, thus, Yan9oil layer is medium-low-porosity and low-permeability reservoir. Taken variation coefficient, rush coefficient and level difference of intraformational permeability as the major evaluation criteria, integrated with rock properties, sedimentary rhythm and interbed distribution, it has been determined that Yan9reservoir has medium-high intraformational heterogeneity. The set bedding coefficient and sandstone density can reflect the vertical stacking conformation and lateral connection of sandstones, synthesized with impervious beds, it has been turned out that the interformational heterogeneity is high. According to the above information, the sandbody lateral distribution and physical property lateral distribution characteristics have been derived, that is, the sand bodies are distributed in longitudinal zonality, and the lateral connectivity of Yan91and Yan92is good while the sand bodies of Yan93are isolated, it can be further known that the sand body connectivity is good in the direction of channels and the heterogeneity is weak while the lateral heterogeneity is strong.Assigning the score to the main parameters of the reservoirs by the use of cluster analysis, then dividing the reservoir into three parts, good reservoir, medium reservoir and poor reservoir, according to the result. The most area in Yan91belong to good reservoirs, so Yan91is the major reservoir in exploration and development. A great number of reservoirs in Yan92are medium reservoirs, good reservoirs scattered in the middle of the study area. The Yan93reservoirs developed less and have the poorest quality, which is not good for hydrocarbon accumulation.The sedimentary microfacies types control the heterogeneity of lithology and properties of Yan9oil layer, and epigenetic diagenesis alterations improve the vertical and lateral heterogeneity of the reservoir.