oa Biofacies and Palaeoenvironments of the Khuff C Carbonates in Southern Ghawar Field, Saudi Arabia

Semi-quantitative micropalaeontological analysis of closely-spaced core samples from the Upper<br>Permian Khuff C carbonates in the Haradh area of southern Ghawar Field has revealed rich and diverse<br>foraminiferal assemblages and associated microfossils, including bryozoa, calcareous algae,<br>brachiopods, echinoids, ostracods, rare sponge spicules and cyanobacterial sheaths. The foraminifera<br>are typically very small and include agglutinated, microgranular, miliolid and calcareous hyaline forms<br>that display a variety of morphotypes that assist to refine the depositional environment. In addition to<br>the considerable biofacies variations in ascending stratigraphic order within individual wells, lateral<br>variations are present that together reveal regional and temporal palaeoenvironmental changes. These<br>changes can be related to successive transgressive - regressive depositional cycles that compare<br>readily with the distribution of porosity. Rock fabrics range from dense mudstones through<br>wackestones, peloidal packstones to ooid grainstones. Porosity types encountered within the Khuff C<br>reservoir include interparticle, mouldic and intercrystal. Diagenetic alteration of the primary fabrics<br>includes cementation by calcite, dolomite and anhydrite, of which pervasive dolomitization is<br>responsible for creating porosity within otherwise non-porous carbonates. The close association<br>between biofacies and reservoir porosity distribution has led to the recent application of rigsite<br>micropalaeontological analysis to biosteer under-balanced coiled-tube development drilling of the Khuff<br>C reservoirs.