oa Carbonate/Evaporite Sequence Stratigraphy of the Subsurface Late Jurassic Arab-C Member, Khursaniyah Field, Eastern Saudi Arabia

This study defines the cycles and sequence stratigraphic framework of the Arab-C Member of the<br>Khursaniyah Field to enhance the understanding of both sedimentological and depositional models of<br>the Arab-C reservoir. The sediments of the Arab Formation in the Arabian Peninsula are typically<br>composed of shallow-water limestones and dolomites interbedded with restricted facies of anhydrites.<br>Each reservoir layer corresponds to retrogradational - progradational cycles. The reservoir-bearing<br>Arab-C carbonate in Khursaniyah Field (150 ft thick) is an overall shallowing-upward composite<br>sequence that can be subdivided into five high-frequency sequences. Each of these high-frequency<br>sequences can be subdivided into multiple fining-upward small-scale cycles.The lower part of the Arab-<br>C Member is made up of cycles that fine-upward from intraclastic/oolitic rudstone into skeletal<br>wackestone and lime mudstone. These are overlain by 9 to 20 ft thick cycles that consist of ooid<br>grainstones capped by anhydrite. The overall evolution is that the basal sequence consists of ooid<br>grainstones and rudstones capped by an anhydrite. The second sequence consists of cyclic ooid<br>grainstones and dolomitized mudstones. The third sequence consists of thick cross-bedded grainstones<br>(which mark the maximum flooding) capped by thrombolite facies. The fourth sequence consists of<br>peloidal grainstones capped by thin evaporites and then there is a thin fifth sequence that has a<br>carbonate stringer in the base that is capped by the regional anhydrite that extends upward to the<br>base of the Arab-C. Grainstones are mostly in the transgressive portions of the sequences and cycles<br>while anhydrite, tidal flat facies and thrombolites mostly occur in the highstand parts of the sequences<br>and cycles. The middle evaporite package thins toward the northeast of the field. Dolomite increases in<br>the southeast of the field and the thromoblites decrease on the crest of the field. This core/log basedwork<br>leads to a better correlation framework for the Arab-C Reservoir where the wire line logs alone<br>are often difficult to correlate. Moreover, the sedimentological work helps to break out facies with<br>differing porosity and permeability relationships that can be imported into geocellular models for<br>matching production history and field optimization planning.