Extend The Oil Region Of Upper Paleozoic Sedimentary Facies And Reservoir Microscopic Characteristics Of The Ordos Basin Study

Ordos basin is one of the most potential gas reservoir regions in China. For the low understanding about the gas reservoir of the upper Paleozoic in Ordos basin, this paper has integrated geological methods and the observation, description of outcrop and drilling core, through the analysis of single factor with synthesize factor, guided by the theory of petroleum geology, sedimentology, reservoir geology, development geology and reservoir engineering, several methods were united to deeply research the sedimentary facies, the microcharacteristics of sandstone reservoirs and the rule of the hydrocarbon enrichment, and bring forward favorable gas-bearing zone finally.Combined the published data, this paper analyzed the characteristics of lithology and electric property of the drill cores over30test wells comprehensively and paid more attentions to the characters of electric property of the several marked layer in Hutian section in the region of interests, such as the layer of feralite, Jinci sandstone, lower coal group (N8and9layer) and Dongdayao limestone. We consider the layer above the Dongdayao limestone on the top of Taiyuan strata as the new boundary of the Taiyuan and Shanxi formations's.With the analysis of paleocurrent direction and heavy minerals, it shows that the Shanxi and Shihezi formations are located in the junction area affected by north and south source. The provenance is from northeast, northwest and south, the central part is placed in junction area. And through the analyses of the main vertical section to distinguish4sedimentary system types of the Upper Paleozoic,13sedimentary system (facies),22subfacies and some microfacies, and discussed detailly the characteristic of all kinds of facies, subfacies and microfacies and spatiotemporal distribution of them.Based on geochemical index mainly, this study analyzed the sedimentary environment of Shanxi, Shihezi and Shiqianfeng formations. The character of the mudstone samples showed that the mean paleosalinity of the Shanxi formation was4.96‰. A triangular diagram with the contents of mudstone's palaeosalinity, the ratio of B/Ga, Sr/Ba and V/Ni, and B-Rb-Ga as the three end members for classifications of sedimentary environment were applied to the classification of the formation, which is classified as mixed water sedimentary environment. The paleosalinity of Shihezi and Shiqianfeng formation was a little smaller,2.59‰and0.09‰respectively, which indicate the Shihezi and Shiqianfeng formation was deposited in fresh water sedimentary environment. In another word, the marine environment was gradually replaced by continental sedimentary environment from Shanxi to Shihezi formations.It was shown that the sandstones in the research area are medium to granule arkoses, which the distribution of mud matrix is not homogeneous and the type of cements includ calcite, quartz and feldspar overgrowths and clay minerals. The separation of the clastic partieles are mostly middle to fine, sub-angular and sub-rounded. The contact pattern is mainly point-line or lines, and mainly the membrane-Porosity typed cementation. It shows that the diagenesis types included compaction, recrystallization, cemeniation, dissolution and metasomatism. The sandstones of this area have experienced early diagenetic phase to later diagenetic phase on the basis of experimental results. About the pore spaces of reservoirs, it is mainly composed of the iniercrystalline microspores of kaolinite and secondary dissolution pores, always the combination of many types.The development of the pores caused the changes of the permeability. There is a relationship between the size of pore throat and the connecting degree of pores and throats. The main reason of the reduction of property, such as the primary pores, is the mechanical compaction and cementation.Reservoir heterogeneity was studied from inter-beds, among-beds and planar view. The heterogeneity of He-8reservoir is the strongest, in which the coefficient of variation of He-8is0.88,0.83and0.56. Sedimentary environment and diagenesis are the primary factor that results in reservoir heterogeneity.According to the study of geological conditions for oil and gas accumulation, combined so many years'exploration practices, we concluded that sedimentation, diagenesis and tectonism are not only the foundation for accumulation of oil and gas, but controlling its enrichment and distribution.According to the predictive basis for favorable zone and the degree that individual factor impact on reservoir, we selected9predominated factors, then divided the favorable reservoir into three types by fuzzy distinguish value of quantitative evaluation, i.e., the most favorable reservoir, favorable reservoir and relative favorable reservoir. We selected Qian5, He8, Shanl and Shan2as the favorable segment from the Paleozoic reservoirs, which will be the guide for the next exploration.