Genesis analysis and identification of low-resistivity contrast oil-bearing reservoirs: a case study in the Triassic Chang 3 Member of the Yanchang Formation in Pengyang Region, Southwestern Ordos Basin, China

Comparing with conventional formations, the resistivity difference between the oil-bearing reservoir and water-saturated layer is little in the Triassic Chang 3 Member of the Yanchang Formation in Pengyang Region, southwestern Ordos Basin, China. This makes the oil-bearing potential reservoirs’ identification face a great challenge. To understand the genesis that makes the low-resistivity contrast in oil-bearing formations, several core samples are chosen to apply for routine physical property and X-ray diffraction analysis, nuclear magnetic resonance (NMR), and mercury injection capillary pressure (MICP) experimental measurements. Meanwhile, the formation water is also extracted from 19 wells to measure the total salinity and analyze the chemical components. The results illustrate that the Chang 3 Formation contains good pore structure and relative high porosity and permeability (the average porosity is 13.80%, and the average permeability is 6.81 mD). The effect of clay mineral to formation resistivity can be ignored. The main factors that cause low-resistivity contrast in oil-bearing layers are high formation water salinity and formation porosity. Combining the spontaneous potential (SP) with formation deep induction resistivity (RT), a parameter, named as the corrected resistivity index (CRI), is constructed, and the crossplots of reservoir porosity and CRI and flow zone index (FZI) versus CRI are raised to identify low-resistivity contrast oil-bearing layers in the northeastern and southwestern areas, separately. In addition, after normalizing porosity and CRI, the correlation coefficient between these two parameters is also calculated and used to indicate low-resistivity contrast oil-bearing potential layers. After these methods are extended to field applications, the oil-bearing potential formations in the Chang 3 Member are consecutively identified. The identification results match well with the drill stem test (DST) data, verifying the reliability of the proposed methods. © 2021, Saudi Society for Geosciences.