Study On Litho-Electric Property Relation Of Chang6~1Reservoirs In Zhidan-Zibei Oildom, The Ordos Basin

With the rapid development of oil industry, the low-porosity and low-permeability reservoirs have become one of the focus issues in hydrocarbon exploration and exploitation. However, because of the particular petrologic characters, poroperm characteristics and pore structure which are different from conventional reservoirs, the low-porosity and low-permeability reservoirs have complex and variable litho-electric property relation, and there are several problems about logging saturation evaluation of this kind of reservoirs based on classical Archie formula. The Ordos Basin is a typical representative of low-porosity and low-permeability lithological hydrocarbon reservoirs, Chang6formation of the Triassic is the major oil-bearing strata. The deposition and diagenesis of Chang61sub-formation in the research area are the primary causes of its complex and variable petrologic characters, poroperm characteristics, pore structure and litho-electric property relation, which leads to the difficulty in quantitative evaluation of oil-bearing probability in each oildom of the research area. Based on the research of sedimentary microfacies and reservoir geological characteristics, the litho-electric property relation of Chang61reservoir has been investigated and analyzed to determine the main influencing factors. Furthermore, the research area has been divided into several districts to obtain the appropriate litho-electric parameters which can be applied in quantitative evaluation of saturation.The main conclusions are as follows:1. The Chang61reservoirs are mainly composed of feldspathic sandstones, which have the characteristics of low composition maturity and high texture maturity entirely. The dominating pore types are remnant intergranular pores, intergranular dissolved pores and intragranular dissolved pores. The reservoir types are primarily III, followed by II and IV.2. Pore structure is the main influencing factor of litho-electric property relation of Chang61reservoirs. Litho-electric parameters m, n are both related to pore tortuous degree. The more complex pore structure is, the more pore tortuous degree is,which leads to bigger values of m, n parameters. According to conductive mechanism of porous rocks, additional conductivity of clay and salinity has little effect on the values of litho-electric parameters which is negligible as a result of the high salinity of formation water. Distinct types of reservoirs have different and complex pore structure, which leads to the diversity and complexity of litho-electric property relation in research area. There is wide range of a, b, m and n parameters. Moreover, regression analysis shows that there is also piecewise linear formation factor(F)-porosity(cp) and resistivity index(I)-saturation of water(Sw) relation in bi-logarithm coordinate.3. In order to ensure the accuracy of saturation evaluation, the main oil-bearing zone in research area has been divided into several districts to determine values of litho-electric parameters according to geological constraints, including sedimentary microfacies, reservoir quality index(RQI), porosity and permeability, pore structure and litho-electric experiment results as well as the planar variation of the above, of which sedimentary microfacies is the foundation and the RQI is used to distinguish different petrophysical facies.4. After the research area divided in accordance to reservoir pore structure and petrophysical characteristics, there is a good bi-logarithmic correlation observed between formation factor(F) and porosity(cp) as well as resistivity index(I) and saturation of water(Sw) in most of the districts. However, m, n parameters shows diverse features in different districts because of pore structure variation influenced by variable sedimentary environment and diagenesis. In some other districts, the results indicate that there is relatively poor correlation between F and (p rather than simple linear relation in bi-logarithm coordinate resulting from vertical heterogeneity. Hence, a, m parameters should be selected appropriately in saturation evaluation in these districts. In the research area, b, n parameters determination of reservoirs which have piecewise linear relation between I and Sw in bi-logarithm coordinate should be combined with practical exploration and production experience to obtain the approximate saturation range. Then, the values of b, n parameters corresponding with Sw can be known. The corrected sealed coring saturation analysis data has been used to verify the saturation quantitative interpretation model in two of these districts, and we have achieved good results, which indicates that the litho-electric parameters determined by this means can be applied in practice.