oa Sequence Stratigraphic Analysis of the Ratawi Shale Formation: Implications for Reservoir Distribution and Exploration Potential in Kuwait

The Ratawi Shale Formation is first Cretaceous dominantly clastic unit which overlies a thick carbonate<br>succession in Kuwait. The formation is divisible into three informal zones; the lower one consisting of<br>mainly shale with limestone interbeds. This part was deposited under transgressive conditions in<br>shallow marine to offshore environments. An MFS is recognized near the top of this zone which follows<br>a variable thickness, usually thin, highstand interval due to incision during the overlying lowstand. The<br>middle zone of the formation consists of better sandstone facies. It was deposited in fluvial to estuarine<br>environments representing sedimentation in incised valleys in western part and in nearshore to shallow<br>marine environments eastward. This zone makes the lowstand deposition and completes one 3rd order<br>cycle of deposition. The overlying upper zone consists of shale with local thin sandstone, siltstone and<br>limestone interbeds and is interpreted as deposited under transgressive conditions with or without a<br>recognizable highstand before its termination by the overlying Pre-Zubair late Valanginian unconformity.<br>High frequency sequences and important related surfaces such as regressive surface of marine erosion<br>(RSME), subaerial unconformity (SU) or subaerial exposure surface (SES), subaerial erosion surface<br>(SERS), shoreline ravinement (SR) or transgressive surface (TS), flooding surface (FS) and possible<br>existence of basal surface of forced regression (BSFR) were identified and described from the cores.<br>The RSME is usually sharp and wavy separating the underlying offshore muddier facies from the<br>overlying shoreface sandier facies. The sequence boundary is marked by exposed surface locally<br>represented by rooted interfluves and in other areas by the weathered horizons and still in others by<br>incised fluvial surface. The transgressive surface or shoreline ravinement in one case lies at the base of<br>tidally influenced channel but usually makes a thin burrowed horizon with the fining and deepening<br>upward trend. The reservoir facies are variably associated with the falling stage systems tract (e.g.<br>shoreface sandstone above the RSME), with the lowstand systems tract (e.g. fluvial to estuarine<br>sandstones) and locally with the transgressive systems tract (e.g. tidal channel above SR). An insight<br>of the sequence stratigraphic set up along with paleoenvironmental regime is crucial for identification<br>and distribution of reservoir facies in this formation.