In the Masila region of Yemen, several hundred million barrels of recoverable oil occur within three Lower Cretaceous siliciclastic lithostratigraphic units of the upper Qishn Formation. The basal unit, S3, locally exceeds over 40 m in thickness. This unit overlies an erosive surface and is formed of blanket deposits of cross-bedded, fining-upward, pebbly sandstones with siliceous and dolomitic paleosols. The S3 unit reflects deposition from coarse-grained fluvial channels and related environments. The middle unit, S2, is composed of complexly interbedded sandstones and mudstones up to over 25 m thick. These deposits grade upward from S3 sediments and display cross-bedded pebbly sandstones, cross-bedded sandstones with mud drapes, and inclined heterolithic units formed of interbedded sandstones and mudstones. The S2 unit is capped by widespread churned and rooted mudstones 3-5 m thick. S2 deposition resulted from the rapid infilling of an estuary, or estuaries, created by the transgression of the fluvial systems responsible for S3 sedimentation. The uppermost unit, S1, at its base is formed of an eastern shelfal package with Helminthopsis- and Chondrites-bearing mudrocks, and western beds formed of sediments generally similar in appearance to the middle S2 unit. Above the lowest S1 deposits are marine muddy coquinas that blanket the study area. In the west, these sediments are abruptly to gradationally overlain by Ophiomorpha-bearing, fine-grained, cross-bedded shelly sandstones up to about 10 m thick. These sandstones form a north-south linear trend and are interpreted to represent a progradational pulse of subtidal shoal sediments that interrupted the general deepening trend exhibited by the upper Qishn interval. Final upper Qishn deposition was younger shelfal muddy coquinas that overlie the shoal deposits. These and younger carbonate sediments act as the uppermost seal to reservoirs in the Masila region. The upper Qishn sediments are compared to modern and Pleistocene estuarine deposits. We propose stratigraphic, depositional, and paleogeographic models for this formation and, from this analysis, we derive several implications for future petroleum operations in the region.

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