Diagenetic and Pore Fluid Effects on a Tight Clastic Reservoir (A Case Study)

M. Ahmed¹,O.H. Afif¹, and M. Gupta²

Abstract

Our target formation contains several layers of sand and shale where physical properties of these rocks have been affected by diagenesis. The effect of diagenesis is more prominent on shaley layers where clay has lost its porosity, especially in the deep interval we have investigated. Shaley layers show high compressional stiffness but lower values of shear stiffness. This is related to the bound water inside the clays. In our study area, the cleaner rocks (pure sands or sands with very low clay fraction) show a strong trend with porosity as observed in elastic attribute domain, whereas the clay rich zones exhibit decreasing sonic velocities. Furthermore, shear velocity here is significantly lower than compression velocity. In this study, we have used differential effective medium rock physics models to capture the variation of elastic properties with reservoir properties. Fluid substitution with Gassmann equations (1951) was applied as a first step in order to remove the fluid effect. Next, the elastic moduli of grains were calculated with the mixing method using respective mineral volume fractions (Ruess, 1929). Finally, Kuster-Toksez modeling (KT, 1974) was performed to model the elastic properties of the reservoir intervals. The KT model method was appropriate in this case because porosity, as well as isolated individual inclusion shapes, are used to estimate the effective elastic properties of the rock. Rock physics modeling of elastic properties of the interest reservoir was then applied in two sections: in the clean rock and in the zones affected by increase in clay volume. Based on the modeling results, it was evident that the elastic properties of the target formation have been affected by diagenesis. Clean sands are modeled using suitable values (alpha=0.08) of pore aspect ratio and the effect of clay is modeled using a coated clay model. Fluid properties have a significant effect on pore compressibility where the fluid effect is more prominent at higher porosities. Therefore, changes in fluid properties e.g. brine to gas, have a significant effect on the elastic properties of rocks we have studied. This rock physics analysis provides both a framework model for seismic inversion interpretation as well as a physical basis for petro-elastic characterization of the target formation rocks.

AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain