No abstract

The Qaidam Basin is a big intermountain Mesozoic-Cenozoic petroliferous basin in western China. The huge thickness Cenozoic strata and tectonic deformation formed a good combination of hydrocarbon sources, reservoirs and caps, leading to huge hydrocarbon potential of the basin. The Western Yingxiongling Area locates in the western part of the Qaidam basin, during the Cenozoic era a special tectonic dynamics background was formed by the joint control of the sinistral strike-slip fault of the East Kunlun and the sinistral strike-slip fault of the Altun (Fu et al., 2016). The tectonic deformation in this area is very strong, which makes the distribution of hydrocarbon pools extremely complicated. The breakthrough of wells Shi 20 and Shi 24 in the Western Yingxiongling Area in the 1980s confirmed that there are high oil and gas enrichment zones in this region. However, the distribution characteristics of oil and gas reservoirs in this area are not ascertained due to the lack of technical means and theoretical understandings. In recent years, based on the results of three-dimensional seismic exploration, studies on sedimentology and reservoir, and hydrocarbon geochemical evaluation, the geological characteristics of deep reservoirs in this area were systematically recognized. The structure of this area is characterized by "double layer", with large trap area and diverse types. At the same time, the region is in the favorable source depression area of the basin with sufficient hydrocarbon sources (Jin et al., 2001). The latest reservoir research discovered the commonly developed dolomite intergranular pores and the dissolution pores, which changed the former understanding of single fractured reservoir (Li and Wang, 2001). Since 2015, the exploration of the Western Yingxiongling Area has made remarkable progress on the basis of new theoretical understandings and technical application, and has successfully drilled a batch of high-yield wells such as Shi 38, Shi 205 and Shi 1-3 et al. In order to gain better understandings on the hydrocarbon pool in the Western Yingxiongling area, performing a systemic analysis on hydrocarbon pool stage division is necessary.

The igneous reservoir of Shahejie formation in eastern sag of Liaohe depression is characterized by complex geological environment, variable lithology and high heterogeneity. Reservoir evaluation is difficult only based on conventional logs due to complex lithology and pore structures. Effective igneous reservoirs were identified and reservoir controlling factors were analyzed based on effective porosity calculation, pore structure analysis, lithology identification, lithofacies analysis, fracture evaluation and heterogeneity analysis by combing nuclear magnetic resonance data, micro-resistivity image data, conventional logs as well as mud logging data. Based on our study, the igneous reservoirs in the study area are more related with effective porosity and pore connectivity, and less related with fractures. Good reservoirs are mainly distributed on the top part of explosive facies and effusive facies, where lithologies are mainly Trachyte, volcanic breccia and breccia-bearing tuff. The weathering leaching process is quite important for igneous reservoirs, but the reservoir qulity would not be good if the weathering process is too strong as it will lead to low effective porosity. The accuracy of igneous reservoir evaluation gets improved a lot by this integrated approach and the conclusion from this study will help to optimize igneous reservoire exploration plan.