Reservoir Characteristics and Integrated Method to Illustrate Mishrif Stratigraphic Prospect in Western UAE

Abstract

Abstract As a unique stratigraphic prospect of UAE, the carbonate Mishrif Formation in NN Field is composed of 15-25 m thick rudist grainstone that formed in a shoal environment. The effective reservoir is bounded by inter-shoal packstone-wackestone. Combination of porous reservoir and non-porous baffles indicate high heterogeneity caused by rapid changes in deposition. Current exploration and drilling proposal are precluded due to the ambiguous understanding on reservoir anisotropy and dim-identification from seismic due to the thin reservoir thickness. To mitigate the challenge from reservoir identification, Mishrif whole core was collected and the following analysis performed: thin section description, porosity and permeability (RCA), X-Ray diffraction (XRD), and mercury injection (MICP). The subsurface analysis of the Mishrif reservoir was augmented with litho-facies identification, sedimentary facies recognition, and diagenetic history. Paleogeography was integrated with sequence stratigraphy to predict possible reservoir distribution. Sequence stratigraphy focused on identifying the 4th order sequence interfaces such as first flooding surface (FFS), maximum flooding surface (MFS), and sequence boundaries (SQ). Subsequently, the paleogeomorphology of oil-bearing zone was conducted, and a method using two crucial sequence surfaces was optimized after comparing impression and residual thickness methods. Meanwhile, to quantitatively characterize this set of oil-bearing units, AVO and Pre-stack inversion was implemented to predict reservoir distribution and fluid habitat. The integrated study revealed that the Mishrif reservoir quality is controlled by original depositional facies and diagenetic processes. The rudist grainstone was shoal-related with deposition on a paleo-geographic high and originally high porosity and excellent pore-connectivity. The subsequent fresh water leaching and dissolution contributed to improvement of pore structure. In contrast, the inter-shoal limestone contains higher micrite deposited in slightly deeper water, due to lower porosity it resisted the weathering procedure. To overcome the challenge of thin reservoir thickness, selection of key surfaces which are used to construct the paleo-geographic configuration would be quite crucial. And the identification of such surfaces only from seismic would be of high uncertainty. Finally, the dual-interface method was adopted to delineate the paleogeomorphology of oil-bearing zone. This illustration of paleogeography displayed high similarity to the reservoir quantitatively derived from AVO and Pre-stack inversion, which improved reservoir prediction. This integrated method from core-based reservoir recognition, sequence-driven paleogeography, as well as quantitative AVO and Pre-stack inversion provides new insight to study heterogeneous carbonates and reduce uncertainty for thin reservoir prediction.