Stratigraphic and Sedimentological Characterization of An Unconventional Carbonate Deposit: An Example from the Late Cretaceous Shilaif Formation, United Arab Emirates

The Late Cretaceous (Cenomanian) Shilaif Formation is an unconventional oil deposit located in the Emirate of Abu Dhabi in western United Arab Emirates (UAE). Presently, it is undergoing early-stage appraisal drilling and development. Due to the reservoir in its natural state having a very low permeability (well below 1mD), traditional primary recovery techniques cannot be used. Instead, extraction of the oil will occur via hydraulic fracking of the reservoir. To better optimize landing zone placement of horizontal wells, which in turn will help lead to improved design of the wells, operations require a thorough understanding of the depositional fabric and stratigraphic architecture of the reservoir. The Shilaif Formation represents a deep-marine carbonate succession deposited within an intrashelf basin. Using Arabian Plate boundary nomenclature present in offsetting wells from various offsetting countries, three primary sequences can be delineated (from oldest to youngest): K120 (Lower), K130 (Middle), and K140 (Upper). Each of these three sequences can be further sub-divided into lowstand (LST), transgressive (TST), highstand (HST), and falling stage (FSST) systems tracts and their corresponding parasequence sets. At the core level, massive bedded and bioturbated organic-rich carbonate facies in each of the three sequences frequently correspond with TST and early stage HST successions. Thin-sections reveal significant planktonic foraminifera and calcispheres. Biostratigraphic analysis of core samples suggests much of the organic material is a by-product of phytoplankton (algal) blooms. Conversely, massive bedded and bioturbated non-organic carbonate core facies in each of the three sequences commonly correspond with FSST and LST successions. Thin-section analysis reveals a higher abundance of fossiliferous fragments thus indicating sediment transport from adjacent platform margins into the basin. It must be noted that different combinations of these organic and non-organic facies, however, can occur within various systems tracts and therefore underpins the need to do detailed core logging and subsequent correlation across the field. Finally, many key stratigraphic surfaces are punctuated by examples of Glossifungities ichnofacies and their recognition in core are of critical importance towards building the basin model and understanding water benches.