Elements Of The Sweet Spot And Their Influence On Shale Oil And Gas

China has3types of organic rich shale developing in marine facies, continental marine interaction facies and continental facies, with large potential of hydrocarbon resources. However, the research of shale oil and gas in China is still at the beginning of development, combined with multi-period tectonic movement in geologic history as well as complex geological conditions, resulting in the great difference compared with North America. It is particularly important to determine the shale oil and gas "sweet spot" for getting the most favorable area and the enrichment zone, and then realizing large-scale economic development.The paper based on published domestic and international academic achievement related to shale oil and gas. First, we summarized the concept of the shale oil "sweet spot" and the difference between shale oil and gas. Although the shale oil and gas are mainly occurred in the mud shale or the interlayer, shale gas is mainly occurred in an adsorbing state, which emphasized the importance of the absorption, while the shale oil is still mainly exist in the free state.Based on the drilling data of shale gas from the lower Paleozoic Silurian Longmaxi formation and Cambrian Qiongzhusi formation in southern of Sichuan Basin, and combining with the geological features of North American shale oil and gas and Chinese continental shale oil, using statistic method as the main means to discuss the elements of "sweet spot". The burial depth of Qiongzhusi formation in Sichuan Basin is3000～5000m, thickness is250～550m, black shale’s thickness is60～300m. The burial depth of Longmaxi formation is2000～3000m, thickness is150～500m, black shale’s thickness is30～120m. Through a synthetic study on shale gas and oil reservoir’s buried depth and its thickness characteristics, in summary, for shale gas and oil the effective thickness is30～120m and the ideal burial depth should be in3500m. the black shale in Longmaxi and Qiongzhusi formation is mainly I kerogen, additional II kerogen. The average TOC values of them are1.81%and2.04%respectively. Organic maturity Ro value is range from2.0%to3.5%.The porosity and the permeability of Longmaxi shale is better than Qiongzhusi formation, and the gas-bearing porosity of Longmaxi shale is more than1.0%, while permeability is less than50nD, worse permeability than shale Barnett. Southern of Sichuan Basin gas test results of the lower Paleozoic shale are gas layer and gas-bearing, referring to the North American shale reservoir water saturation range and shale gas development under water saturation lower limit, considered that shale gas water saturation should be less than45%, gas saturation is above50%. Shale isothermal adsorption experiments showed that Sichuan Basin and Guizhong Depression Paleozoic shale and the adsorption capacity of the North American shale have small difference. Under the same temperature conditions, the content of the shale gas adsorption increased with pressure, a positive correlation between the adsorption of gas shale and different organic carbon content in the same layer shows that with high organic carbon content of the shale can absorb much more gas. By having statistics the total gas value in Longmaxi and Qiongzhusi shale are more than1.5m3/t.According to the analysis of shale mineral composition of Sichuan Basin and Guizhong Paleozoic shale, the mineral composition and content are similar to the shale which produces gas in North America, brittle mineral content of which is more than40%. Compared with the Barnett Shale, Paleozoic marine shale in the south of China have higher maturity, characterized by high maturity of clay minerals, mainly are illite and chlorite, smectite is substantially free, with higher brittleness. The Poisson’s ratio and Young’s modulus of Qiongzhusi and Longmaxi formation of the lower Paleozoic in the Southern of Sichuan Basin is individually0.1～0.27and15.8～40.8GPa, the features of them are high elastic modulus and low Poisson’s ratio. As Qiongzhusi group has higher brittle shale mineral content, high thermal evolution and large depth, which shows the higher rock brittleness compared with Longmaxi formation.Chinese mainland mainly consists of three plates. The single plate area is small and the sedimentary basin formed on the plate is smaller scale and poorer stability than North America platform. Since the multi-cycle and complexity of plate tectonic movements caused the diversity of basin types and sedimentary structure, the type of Chinese shale oil-gas is multiplex and preservation conditions are poorer than North America. Sedimentary environment’s influence on shale oil-gas "sweet point" is mainly in the thickness of source rocks, organic content, mineral composition, reservoir properties and so on. Tectonism can control the distribution of source rocks in the depositional process of source rocks. And the late tectonic action influence mainly on preservation conditions and geostress field of shale oil-gas. The development of sedimentary basins a sediment-suite of China can be divided into5stages from Sinian Period to Tertiary Period. Shale lithological combination, the development scale and distribution of each stage are different. Statistics show that petroleum accounts for a larger proportion in the Cenozoic, and natural gas in the Paleozoic accounts for a significantly higher proportion than the Cenozoic.There is a good correlation between TOC and hydrocarbon potential, the total of gas content, gas-bearing porosity and the capacity of shale adsorption. Through the study of the test data of favorable section in Longmaxi and Qiongzhusi formation and the correlation analysis of the "sweet spot" elements, we found among TOC and hydrocarbon potential, the total of gas, gas-bearing porosity and the capacity of Shale adsorption have good correlation. When the TOC is1.5%, shale has a good hydrocarbon potential, the total gas content, gas porosity and gas saturation have been reached the North American commercial shale gas evaluation lower limits. Ro with shale burial depth, illite content, the organic carbon content are positively correlated. Ro with the total porosity shows the quadratic function correlation, which was first decreased and then increased, had no relationship with the brittle mineral content, indicating brittle minerals are not affected by thermal evolution degree. According to correlation between Ro and organic carbon content, for high-post maturity shale, when we determine the shale gas TOC lower limit value, we may don’t need to recovery the original organic matter abundance.Combined with the "sweet spot" essential elements characteristics and correlations that we have obtained from the elements, this paper summarized and established a set of standard shale gas "sweet spot" evaluation parameters, and applied it to Longmaxi and Qiongzhusi formation in the southern of Sichuan Basin. Finally we obtained the graphs about shale gas evaluation parameters characteristic distribution of Longmaxi and Qiongzhusi formation. The results showed that thermal maturity Ro played great effect on the shale gas "sweet spot" forming.