Petrophysical rock typing integrated workflow in a Chilean greensand.

Petrophysical rock typing (PRT) is a key integrated workflow in reservoir characterization that utilizes petrophysical properties like permeability and porosity in conjunction with pore size distribution, typically derived from mercury capillary pressure measurements to classify from rich to poor reservoir quality rock units. However, the PRT workflow must encompass additional properties such as mineralogy, surface area, and clay distribution in tight rocks with a high clay mineral content to capture the microstructure and heterogeneity in such formations.

This case study focuses on the Zona Glauconitica (ZG) reservoir in the Magallanes basin, Chile, greensand with permeability ranging from 0.001 to 1 mD and total porosity between 10 and 25%v/v. Its high iron content is due to substantial amounts of chlorite and/or glauconite. The PRT workflow analyses ten petrophysical and mineralogical parameters using principal component analysis and K-means clustering, aiming to identify crucial patterns and correlations between rock properties and their storage potential. Multilinear regression (MLR) analysis was employed to determine the best-fit correlation for predicting pore throat radius at different mercury saturations from capillary pressure curves, using total porosity and gas permeability as input variables.

Four distinct petrofacies were identified as closely associated with clay minerals content, iron levels, permeability, porosity, and pore throat distribution. MLR best correlated the pore throat radius at 25%v/v mercury saturation with Pittman’s (1992) correlation. These findings offer significant promise as they contribute to enhancing and refining the existing petrophysical model. Future work extends this methodological approach to logging data across ten uncored wells, preceded by a validation process involving two cored wells. This iterative process will contribute to developing and validating an effective petrophysical model for the ZG reservoir, facilitating more precise and efficient evaluations of its production potential.

Pittman, E. D. 1992. Relationship of porosity and permeability to various parameters derived from mercury injection-capillary pressure curves for sandstone. AAPG Bulletin, 76, pp.  191-198.