Influence of Micropalaeontologically Determined Antecedent Topography on Reservoir Quality in Saudi Arabian Carbonates

G.W. Hughes, R.D. Adams and *F.O. Meyer

Saudi Aramco, Box 5000, Geological R&D, Dhahran 3131, Saudi Arabia, * Carbonate Research Consulting, Inc., Elbert CO 80106, U.S.A. 

The Arabian Platform was a site for the accumulation of extensive sheets of shallow marine carbonates, most of which have important hydrocarbon reservoirs. In many reservoirs, primary porosity is significant; in others, dolomite intercrystalline porosity is significant. Investigations into controls on reservoir development and variations in reservoir quality typically focus on core-based sedimentological aspects. This approach produces limited success because of limited palaeoenvironmental resolution achieved from carbonate sedimentology alone.

Recent studies of Paleozoic and Mesozoic carbonate reservoirs of Saudi Arabia have involved sedimentological and sequence stratigraphic studies that closely integrate semi-quantitative micropalaeontological data with core, geochemical, and wire-line log data. Sedimentological, micropalaeontological, geochemical and log data permit the confident recognition of depositional cycles, parasequences, and high-frequency sequence sets.

In addition, micropalaeontological elements posses subtle palaeoenvironmental significance, especially within the maximum flooding zones, which result in a refined interpretation of vertical and lateral variations of palaeoenvironment and palaeobathymetry. Biofacies also serve to provide, for the first time, variations in the depositional environment across the area, in which localised regions of relatively shallow conditions prevailed atop antecedent topographic highs.

These micropalaeontologically-determined, regional palaeoenvironmental patterns highlight antecedent topography and provide potential guides for an improved understanding of the distribution of primary vs. intercrystalline porosity. Shallower maximum-flooding events are distributed beneath micropalaeontologically barren sections related to adverse shallow marine conditions prone to exposure at the cycle boundaries. It is these exposure episodes that provide appropriate conditions for freshwater leaching and dolomitization within the mixing and marine phreatic zones.

Hughes, G.W., R.D. Adams, and F.O. Meyer 2003, Influence of Micropalaeontologically Determined Antecedent Topography on Reservoir Quality in Saudi Arabian Carbonates: American Association of Petroleum Geologists Annual Meeting, v. X , pX .