Heterogeneity in the overpressure zones in West Baram Delta, offshore-Sarawak, Malaysia

Abstract

Geopressure prediction is crucial because all drilling programs require estimation of abnormal pressured zones. The use of rock fabric deformational relics at variable depths to determine heterogeneity in overpressure zones in reservoir has not been explored. Therefore, the aim of this paper is to use petrography to determine intrinsic variations within overpressure zones. Understanding petrographic expression of overpressure on reservoir lithofacies at variable depths will enhance the prediction of overpressure in reservoirs that are not initially logged by subsurface pressure detecting tools. Thin section and scanning electron microscopy (SEM) petrography was used to study rock fabric. The petrophysical properties of the rock samples were measured using mercury porosimetry. Two lithofacies were discretized in a single reservoir i.e., massive friable fine-grained sandstone (MFFS) (Layer 1) and massive fine-grained sandstone (MFS) (Layer 2). The underlying layer (Layer 2) is dominated by sutured quartz grains, lenticular grain orientation, quartz overgrowth and relatively low porosity and permeability. The quartz grains are polycrystalline and characterized by planar and conchoidal fractures, in contrast to the overlying MFFS layer dominated by monocrystalline and platy quartz grains. The petrographic studies show overpressure variation can be characterized intrinsically by quartz deformational features and overall texture differences.