oa Deposition of Retrogradational Carbonate Grainstone Shoals and Subsequent Basal Barrier (Sole Seal) Development by Deposition and Diagenesis - Lower Cretaceous Ratawi Oolite Reservoir (Minagish Formation) Wafra Field, PNZ-Kuwait

The Wafra field is located in the Partitioned Neutral Zone (PNZ) between Kuwait and Saudi Arabia and<br>has been producing medium oil from the Ratawi Oolite Limestone reservoir since 1956.<br>The Wafra Ratawi is an example of a detached rimmed shelf carbonate shoal depositional system<br>deposited on a paleo-high in the Late Cretaceous. The depositional architecture was analyzed for major<br>depositional controls and to explain the existence of a sole seal (Basal Barrier) that restricted early<br>water encroachment and contributed greatly to the productivity of the reservoir.<br>The carbonate platform is made up of allochem Oolitic limestone shoals (grainstones) during relative<br>sea level stagnation in an overall transgressive retrogradation shoal system with shallowing upward<br>sequences. The reservoir is structurally trapped on three sides and stratigraphically trapped on one<br>side where tighter, off-shoal deeper marine carbonate wackestones and mudstones provide the<br>necessary reservoir seal.<br>As the leeward and windward carbonate grainstone shoals developed, a tighter carbonate mudstone<br>was deposited by reduced circulation of nutrients and aeration, and increased excessive heat, resulting<br>in a low stand system tract (LST) lagoonal facies. The retrogradation of these grainstone shoals with<br>intervening lagoonal facies resulted in a continuous development of a tighter facies across the field,<br>enhancing a substantially facies-driven sole-seal to the reservoir called the Basal Barrier. Diagenetic<br>cementation of the Basal Barrier (possibly due to Neomorphism or re-crystallization of pre-existing<br>carbonate fines) further reduced the sole seal permeability restricting bottom aquifer support, resulting<br>in lower water production for much of the production life.<br>As transgression continued, the grainstone forming carbonate factory was over-whelmed and drowned<br>by a rapid rise in sea level during the transegressive system tract (TST), and tighter, deeper marine<br>carbonates capped the main reservoir grainstone shoals. Complete entrapment came with continued<br>transgression during the highstand system tract (HST) and deposition of tight deeper marine<br>argillaceous carbonates grading to calcareous shales at the top forming the cap rock to the reservoir.