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Sedimentological Synthesis and Reservoir Development in Upper Berriasian-Valanginian Ratawi Sequence in Kuwait.

Abstract

An evolution from a widespread low gradient carbonate ramp to a mixed but siliciclastic dominated intra-shelf basin occurred during deposition of the Upper Berriasian to Valanginian Ratawi sequence in Kuwait. The sequence is composed of a basal Ratawi Limestone and an upper Ratawi Shale Formation. The Ratawi Limestone is broadly a regressive carbonate sequence with two prominent shallowing upward cycles. These cycles show negative SP, low gamma and elevated resistivity upwards. Based on lithologic and biotic characteristics five lithofacies were recognized in this formation. These facies were deposited in environments ranging from very shallow inner ramp to outer ramp settings. Ratawi Limestone deposition was most likely affected by the paleo-structural grain and as a result the ramp shallows to SSE towards Burgan high but deepens to NW and NE away from the Kuwait arch. A prominent flooding event is marked at the base of the formation where it corresponds to the high gamma ray values of outer ramp facies. The other less prominent flooding is witnessed at the base of the upper cycle which is overlain by the corresponding highstand shallower facies. The reservoir characteristics are favorable in highstand shallower faices with relatively better showings in paleohigh areas. The Ratawi Shale makes a sharp and disconformable contact with the underlying Ratawi Limestone with a distinct change in sedimentation from carbonates below to dominantly siliciclastics above. It dominantly consists of shale with interbedded argillaceous limestone (mostly wackestone) in the lower half, shale with interbeds of sandstone in the middle, and shale with siltstone and limestone in the upper half. The environments of deposition generally became shallower towards southwest with an increase in the proportion of sandstone. The basal part of the formation was deposited under transgressive conditions in shallow marine to offshore environments with a high gamma zone marking a prominent MFS near its top. Relative sea level drop occurred in the middle of the formation associated with which are widely distributed lowstand related sandbodies. These have good reservoir characteristics and their sedimentological and sequence stratigraphic understanding is critical in finding favorable exploration targets. A major drop in relative sea level terminated the Ratwai Shale sedimentation which is marked by an overlying Lower Valanginian Unconformity (LVU).