Sedimentology, chemostratigraphy and quantitative pore architecture in microporous carbonates: examples from a giant oil field offshore Abu Dhabi, U.A.E.

Abstract

The Lower Cretaceous Thamama Group host some of the world's most prolific oil and gas reservoirs, yet the key controls on reservoir quality and production are poorly understood. This dissertation focuses on a volumetrically significant, algal-dominated, low-permeability, microporous reservoir within the Thamama 1 interval. The following includes the sedimentology , chemostratigraphy and quantitative pore architecture of the Thamama 1A as it relates to lithofacies, including the stratigraphic framework and depositional environments of this reservoir interval. The Thamama 1A represents an overall deepening upward, transgressive deposit on a low angle carbonate ramp with depositional settings ranging from distal outer ramp to distal inner ramp environments. For correlation purposes [delta]13C and [delta]18O stable isotope analysis resulted in completing a previously incomplete chemostratigraphic section for the paleo-Bab basin. These new and comprehensives curves provide valuable tool for inter- and intra-field correlation. Results from this study also provide a new understanding of the effects of pore architecture on fluid flow. A new methodology for microporosity quantification using QEMSCAN® technology is presented which is capable of imaging microporosity that is below optical resolution. This new method provides a more robust and accurate estimate of porosity, pore size, and pore shape distribution in microporous reservoirs. Results are compared to a variety of other analytical techniques including nitrogen gas adsorption and mercury porosimetry. Total porosity, pore size, and pore shape distribution, permeability and specific surface areas are measured and tortuosities are calculated. Porosity values as well as pore size distributions agreed both quantitatively and qualitatively across all techniques. In addition, pore architecture data showed a clear distinction between lithofacies types, indicating that physical rock properties are likely to control fluid flow and reservoir response. Understanding the pore architecture and its effects on fluid flow will help to optimize reservoir production and ultimately perhaps increase EOR recoveries from this large oil and gas field.