The Genesis Analysis And Well Logging Evaluation Of High Natural Gamma Sandstone Reservoir In Yanchang Formation In Ordos Basin

Some area in Ordos Basin exist high natural gamma reservoir in Yanchang Formation. The distribution of high natural gamma reservoir is mainly in oil production area in Yanchi of Ningxia and Dingbian, Ansai, Wuqi, Jingbian of Shaanxi and Huachi of Gansu. The high natural gamma reservoir is found in the Chang4+5, Chang6, Chang7, Chang2, and Chang3that accumulated rich oil in vertical profile. The geology research and well logging evaluation of high natural gamma reservoir are not only important to the evaluation of storage capability and well logging interpretation in Yanchang Formation,but also to the determination of genesis of high natural gamma in the sandstone.The petrology and geochemistry characteristics of high natural gamma reservoir show that high natural gamma reservoir has more feature compare with conventional reservoir:①High natural gamma reservoir have more K-feldspar, chlorite and iron muddy clay matrix. Some high nature gamma reservoir has higher level of debris, and complex glycosylation are common. The content of quartz is significantly reduced. Feldspar in some high gamma reservoir is seriously Kaolinitilized.②Content of Th, U, K in high natural gamma reservoir is increased compared with conventional reservoir, in which the content of Th are significantly increased that mainly exist in the Monazite, rutile, iron muddy clay, feldspar, some of the zircon, apatite, Biotite and and debris. The radioactive element of U mainly exists in most of zircon, some of apatite, biotite and magnesium calcite. The radioactive element of K is mainly in K-feldspar, biotite, rutile, iron muddy clay, and some of albite and debris.③The total porosity of high gamma reservoir is larger1.3%than conventional reservoir,and relatively developed secondary pores. The residual primary intergranular pores are small, and medium and small pore is relatively developed, and connectivity of the pore is better.The distribution and sedimentary characteristics of high gamma reservoir show that connectivity of high gamma reservoir is poor.The high gamma reservoir usually disappear in the conventional reservoir, or intersect with mudstone on lateral. The sedimentary facies is not control the high gamma sandstone formation. It can form in the delta environment, and also can form in the turbidite fan environment, or in the braided channel. The study of the relation of volcanic tuff, material source and high nature gamma reservoir show it is not inevitable connection for the volcanic tuff and formation of high gamma reservoir.To a certain degree, high gamma reservoir is indigenous to material source from northeast basin. On the whole, the hydrothermal activity maybe is important to the forming of high gamma reservoir.Comparing with conventional reservoir, the high gamma reservoir show that the value of gamma, compensated neutron, pe increase obviously. Resistivity decrease obviously. Acoustic velocity decrease a little. Density increase a little.All this change bring the difficuly for exactly computing the porosity,permeability, clay content,oil saturation,and make the oil water distinguishing more complex. When distinguishing Oil-water layer by well logging data,we suggest establish well logging interpretation diagram on the basis of reservoir classification. By using new diagram can improve the identification accuracy.In spite of the better porosity and permeability in the high gamma reservoir, and it is the favorite reservior for oil accumlation, but it can be water layer, oil layer, oil-water layer, poor oil layer, water with oil layer, even can be dry layer with poor porosity and permeability.In the area of rich oil, the high gamma reservoir usually is oil layer, oil-water layer.In the margin of rich oil area,it is often water layer, water with oil layer. The research of the relation of oil-water and high gamma reservoir show that it is only a special reservoir.The condition of oil bearing is controlled by many factors.On the base of porosity,permeability, acoustic velocity, density of high gamma and adjacent conventional reservoir, this paper establishes the formula of computing porosity, permeability, and also comes up with clay content computing formula by using clay content and nature gamma value of adjacent conventional reservoir, by using spontaneous-potential data, by using acoustic velocity, by using density data; by using nuclear magnetic resonance data. This paper discusses the drilling core oil saturation difference of high gamma and adjacent conventional reservoir. The result of analysis show the difference between high gamma and adjacent conventional reservoir is very small. But influenced by resistivity and porosity change, oil saturation value from the well logging computing is very different. The oil saturation is usually lower from well logging computing. This paper establishes more exact oil saturation formula by comparative analysis.