Research On The Reservoir Characteristics Of The Chang 8₁ Formation In Zhenjing Oilfield, Ordos Basin

Ordos basin is one of the significant oil-bearing basin that located at the western part of North China platform. Zhenjing oilfield in the Zhenyuan-Jingchuan county that located at the north of Tianhuan synclinal in the western of Ordos basin. It has a huge potentiality of oil resource, so it has a better exploration and development prospect. The Chang 81 oil pool with a large area, low oil abundance, and is a typical of low porosity and permeability oil pool. Its heterogeneity is so strong. Therefore, this will have practical significance in the study of the characteristics of the Chang 81 oil pool.Based on the previous study achievement, applied with cores observation, thin section authentication, logging data, test date, this paper systematically studies reservoir characteristics of the Chang 81 formation and the factors which controlled it. Many theories and technological about sedimentology, reservoir sedimentology and geophysical well logging have been used. This thesis have been classified and correlated the formation based on the existing stratigraphic classification and have divided the Chang 81 formation into three small formation. Chang 812 is the main sand layer and subdivided it into three parts. By research of sedimentary facies marks and logging curve shape, the sedimentary facies is determined to be the front-delta deposition of delta deposits, and five microfacies types are recognized including subaqueous distributary channel, subaqueous natural barrier, subsea crevass-splay, mouth-bar, subaqueous interdistributary embayment.Under the research of petrologic feature, pore types, pore structure and the relationships among lithology, porosity and permeability characteristics, electric properties and oil-bearing properties, the average porosity is 9.25%, and the average permeability is 0.224×10-3μm2 in the Chang 81 formation. The lithology are middle～fine grained feldspathic litharenite sandstone, lithic arkose sandstone. The reservoirs have low porosity and low permeability. The type of the pore in the reservoirs are mainly remanent intergranular pores, secondary intergranular dissolved pores and intragranular dissolved pores. We can concluded from the observation of thin section that the diagenesis in the research area are mainly compaction and pressolution, cementation, metasomatic alteration and dissolution. The constructive diagenesis are dissolution and cataclasis, and the destructive diagenesis are compaction and cementation. In which the main factor destroying porosity is carbonate cementation. And the factors controlling the reservoirs are the granularity of reservoir rock, sedimentary facies and diagenesis.By the analysis of logging date and core date, combining well test date, it can confirmed the effective low limit of reservoir parameters of the Chang 81 formation in this research area. The low limit of porosity is 10.5%, the low limit of permeability is 0.24×10-3μm~2, the low limit of GR is 70API and the high limit of GR is 100API, the lower limit of DT is 225μs/m, the lower limit of ILD is 15·m and the high limit of ILD is 33?·m, Oil-bearing grade of the reservoir is oil flecked and above, the lower limit of effective sand thickness is 11m.Based on the research of the porosity and the effective sand thickness of three sand body in Chang 81 formation, discovered that the best region of porosity nearby the well HH105 and HH12. Moreover, the porosity of three sand body in different distribution. The thickest effective sand thickness in the Chang 812-2 formation and it has widely distribution. The difference of the thickness of sand bodies would result in the difference of high-quality reservoir distribution. After the research of the porosity and the plane distribution of effective sand thickness, it can get the plane distribution of reserve-coefficient and predict the beneficial reservoir region of Chang 812 in the research area. Summarized as follows: the most favorable sand body in the Chang 812-2 formation, and the beneficial reservoir region are concentrated in areas of the zone of well HH105 to well HH269 in the southwest, well HH12 to well HH16 in the northeast, and nearby well ZJ24-2 in the research area.