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

Abstract

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