oa Incised valley system and associated hydrocarbon entrapment: An example from the Early Cretaceous Zubair Formation in South Kuwait

The presentation outlines two themes; firstly an example of back-filling of an incised valley, and secondly an associated play development. The top of the Ratawi Shale Formation is an established hiatus of Late Valanginian age during which a drop in base level resulted in subaerial exposure in southern Kuwait. Drainage by northeasterly and easterly streams developed and incised the subaerially exposed areas. The sedimentation in these channels marked the initiation of deposition of the Zubair Formation, which overlies the Ratawi Shale Formation. Pre-existing structures along the NS-trending Kuwait Arch partly controlled the drainage. Incised valleys were developed in the flank areas of the highs. Crestal parts of the structures initially remained exposed as non-depositional surfaces. This understanding was applied for delineating potential hydrocarbon-bearing area for exploration. The facies sequence within one of the incised valleys, in ascending order, consists of stacked fluvial sediments followed by estuarine channel sediments, which in turn are overlain by inter-fluve sediments and finally succeeded by beach sediments. This vertical stacking pattern of facies is a good example of incised valley back-filling during the base-level rise. The laterally constrained channel-fill sandstones are hydrocarbon- bearing along the northern flank of the Burgan High. This entrapment resulted from sealing by the transgressive systems tract (TST) related fine-grained sediments which impinge southward onto the High. No entrapment exists where vertical and lateral seal is lacking. The study incorporated seismic, log and core analysis and can serve as an example for exploration in similar traps elsewhere.