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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

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

Saifullah K. Tanoli1; Mohammad Dawwas Al-Ajmi1; Sayed M. Behbehani1; Raghav Prasad1; Sudhakar R. Mushnuri1

(1) Exploration, Kuwait Oil Company, Ahmadi, Kuwait.

The Ratawi Shale Formation is first Cretaceous dominantly clastic unit which overlies a thick carbonate succession in Kuwait. The formation is divisible into three informal zones; the lower one consisting of mainly shale with limestone interbeds. This part was deposited under transgressive conditions in shallow marine to offshore environments. An MFS is recognized near the top of this zone which follows a variable thickness, usually thin, highstand interval due to incision during the overlying lowstand. The middle zone of the formation consists of better sandstone facies. It was deposited in fluvial to estuarine environments representing sedimentation in incised valleys in western part and in nearshore to shallow marine environments eastward. This zone makes the lowstand deposition and completes one 3rd order cycle of deposition. The overlying upper zone consists of shale with local thin sandstone, siltstone and limestone interbeds and is interpreted as deposited under transgressive conditions with or without a recognizable highstand before its termination by the overlying Pre-Zubair late Valanginian unconformity.

High frequency sequences and important related surfaces such as regressive surface of marine erosion (RSME), subaerial unconformity (SU) or subaerial exposure surface (SES), subaerial erosion surface (SERS), shoreline ravinement (SR) or transgressive surface (TS), flooding surface (FS) and possible existence of basal surface of forced regression (BSFR) were identified and described from the cores. The RSME is usually sharp and wavy separating the underlying offshore muddier facies from the overlying shoreface sandier facies. The sequence boundary is marked by exposed surface locally represented by rooted interfluves and in other areas by the weathered horizons and still in others by incised fluvial surface. The transgressive surface or shoreline ravinement in one case lies at the base of tidally influenced channel but usually makes a thin burrowed horizon with the fining and deepening upward trend. The reservoir facies are variably associated with the falling stage systems tract (e.g. shoreface sandstone above the RSME), with the lowstand systems tract (e.g. fluvial to estuarine sandstones) and locally with the transgressive systems tract (e.g. tidal channel above SR). An insight of the sequence stratigraphic set up along with paleoenvironmental regime is crucial for identification and distribution of reservoir facies in this formation.