Quantitative evaluation model for tight sandstone reservoirs based on statistical methods - A case study of the Triassic Chang 8 tight sandstones, Zhenjing area, Ordos Basin, China

摘要

Petroleum exploration for tight sandstone reservoirs in the Ordos Basin has attracted much attention in recent years. However, the effective evaluation of the quality of tight sandstones is challenging due to their low porosity, low permeability, non-Darcy flow, continuously or quasi-continuously distribution and strong heterogeneity. Using constant velocity mercury injection, cathodoluminescence micrography, energy spectrum analysis, scanning electron microscopy and casting thin sections, a set of parameters, including the clay mineral index, Lorentz coefficient, compositional maturity, and plane porosity are tentatively proposed to evaluate the quality of tight reservoir rocks. Coupled with statistical methods, i.e., Principal Component Analysis, Grey Correlation Analysis, Analytic Hierarchy Process and Hierarchical Cluster Analysis, a comprehensive quantitative reservoir evaluation model is established. Then, the evaluation index (REI) is calculated, and the thresholds of various types of reservoirs are determined. The case study based on the above evaluation model shows that most of the type I reservoirs are petroleum-enriched, which are favourable targets for exploration and development. By contrast with sedimentary microfacies, the sandbodies in favourable targets were deposited in ancient subaqueous distributary channels, and they show a large herringbone-like distribution. Whereas the poor type-III reservoirs and non-prospective layers are mainly located in the southeastern part of the study area. The case evaluation based on the statistical methods is consistent with the results of previous multi-parameter evaluations, which has important implications for further exploration in the study area as well as other tight sandstone reservoirs.