Evaluation Of Tight Sandstone Gas Reservoir Characteristics In Shan2 Member In Yu81 Block,zizhou Gas Field

Zizhou gas field has been exploited for many years. The geological conditions are complex in upper Paleozoic coal tight sandstone gas pool. In general, this kind of gas poor characterized with low single well yield, quick production decline and short exploration time. In order to expand the product scope and guarantee stable production, it is necessary to deeply understand the geological properties. Yu81 well area is in northwest of Zizhou gas field which locates at the border of Zizhou gas field and Yulin gas field. Although many wells already have been drilled, there are few wells in production. According to the understanding of the problems of the features of distribution of sand body, types of reservoir, effective thickness of gas formation and distribution of beneficial area, I acquired the following results through the research.According to the core observation, well-logging data and previous research results, these materials indicates that the depositional environment is delta plain and delta front. Also, this area can be divided into four types of underwater distributary channel, inter-distributary bay, swamp and estuarine bar. The scale of sandstone of Shan22 formation is the biggest and its drilling ratio is the highest. The average thickness of sandstone in Shan23 formation are 9.26 m, but the average thickness of Shan21 formation and Shan22 formation are 7.92 m and 5.65 m. Superimposition of sandstone includes scouring-cutting type, scouring-contact type and continuous transitional contact type. Distribution of sand body are controlled by depositional environment. Sandstones mainly locate in the underwater distributary channel, and the the thick part of sand body is in middle channel.Sandstone in researched region was divided into quartz sandstone, lithic quartz sandstone and lithic sandstone. In Shan2 member, the content of quartz gradually decreases and the content of lithic gradually increases from bottom to top. Finally, according to the stone-well logging feature, we can establish the standard of well-logging parameter to distinguish sandstone, mudstone and coal.According to the physical properties analysis, mercury injection experiment data and section observation, the average porosity of sandstone in Shan 2 member is 4.15% and the average permeability is 0.41×10~3?m~2, which is a typical low-ultra, low porosity and low-ultra low permeability reservoir. Porosity and permeability of Shan23 formation are superior to the rest formation.We can use fractal dimension method, minimum flow pore throat radius method, and K-algorithm arithmetic method to divide the reservoirs into four types. The main reservoir are class?and class ?, which distribute in Shan23 formation. These areas locate in the northwest, mid and southeast of the researched region. Furthermore, we can confirm the effective thickness by using the fluid identification standard and the lower limits of physical properties. The result shows that the effective thickness is thicker than the main channel and similar to the distribution areas of class ?and class ? reservoirs. The effective thickness of Shan22 formation and Shan21 formation are discrete and thin, which scatters in researched region.It is believed that with thicker sandstone and better physical propertyis, it's easier to get high yield, so they are the key to high benefit. Based on previous research conclusions and the drilling and production situation, it shows that the best production regions of Shan23 formation are located in north-west around Yu44-14 and Yu45-18 and Yu45-20, and in north-east around Tai8, also in south-west around Yu47 and south-east around Zhou16-20 area.