Integration of rock physics and facies-based seismic inversion for reservoir characterization of the Ordovician Red River formation at Cedar Creek Anticline, Williston Basin, USA

Abstract

Cedar Creek Anticline, located on the southwestern flank of the Williston Basin, will undergo carbon dioxide (CO2) enhanced oil recovery operations (EOR) in the near future. The target reservoir is unit 4 of the Red River Formation within Cabin Creek Field at approximately 9,000 feet below the surface. The objective of this reservoir characterization study includes understanding the heterogeneity within the reservoir and its potential influence on the CO2 flood. Available data consist of a 2014 3D compressional wave seismic survey and borehole measurements from 27 wells existing within, or in close proximity to, the study area. An anisotropic rock physics model was developed to predict the reservoir's matrix and fluid effects on the seismic properties. Variations in porosity and pore shape dominate the compressional signal (P-wave) of the Red River reservoirs; while changes in pressure, mineralogy, and fracture density have a secondary effect on the signal. Reservoir heterogeneity is characterized from the inverted elastic volumes using a conventional simultaneous inversion and facies-based pre-stack inversion method applied to the Cabin Creek seismic dataset. Reservoir facies are dolomite rich with low acoustic impedance and velocity ratio (Vp/Vs) compared to non-reservoir facies that are calcite rich characterized with high acoustic impedance and Vp/Vs ratio. Porosity quantified using the facies-based inversion elastic volumes and the rock physics model is similar to the mean interval porosity from logs, while porosity calculated using simultaneous inversion elastic volumes is under-predicted with respect to the expected porosity values. In this study, the facies-based inversion technique is able to generate an improved result compared to the conventional simultaneous inversion.