Controls of grain-size distribution on geomechanical properties of reservoir rock-A case study: Cretaceous Khafji Member, Zuluf Field, offshore Arabian Gulf

The Khafji Member of the Cretaceous Wasia Formation in Zuluf Field, offshore Saudi Arabia is an important siliciclastic hydrocarbon reservoir. The reservoir consists of a lower Main Sand and an upper Stringer Sand. The present study investigated the relationships between the geomechanical attributes (Young's modulus and Poisson's ratio) of the reservoir and its grain size distribution. In addition, an attempt was also made to relate the reservoir quality with the depositional environments deduced from factor analysis of the grain size distribution data. The Young's modulus for the Khafji reservoir range from a low of 8.43×10⁵ psi at 1000 psi confining pressure (1.13×10⁶ psi at 2900 psi confining pressure) to a high of 1.27×10⁶ psi at 1000 psi (1.91×10⁶ psi at 2900 psi). The study shows that there is no significant relationship between grain size parameters (mean, median, skewness, and kurtosis) and geomechanical properties. However, a good correlation was observed between geomechanical parameters and sands deposited in different environments. The clean sand showing moderate to good sorting and interpreted as beach sand showed low Young's modulus (average 1.07×10⁶ psi) and Poisson's ratio (average 0.16), while the parameters were higher (average of 1.57×10⁶ psi Young's modulus and 0.204 Poisson's ratio) for the fine-grained sediments representing possible lagoonal or offshore environment.