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AAPG Bulletin


Estuarine Deposits of the Upper Qishn Formation (Lower Cretaceous), Masila Region, Yemen1

Peter E. Putnam, George Kendall, and David A. Winter4


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

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

1Manuscript received October 9, 1995; revised manuscript received October 21, 1996; final acceptance February 25, 1997.

2Petrel Robertson Ltd., 2801 - 18th Street NE, Calgary, Alberta T2E 7K5, Canada.

3Occidental Petroleum Inc., Bakersfield, California.

4Canadian Occidental Petroleum Ltd., Calgary, Alberta, Canada.

We would like to acknowledge the following organizations for their support of the project, as well as for granting permission to publish: Canadian Occidental Petroleum Ltd., Pecten International, CCC Ltd., Occidental Petroleum Inc., and Petrel Robertson Ltd. The manuscript was improved by the comments of AAPG reviewers R. P. Lockwood, A. S. Al-Sharhan, and 
W. J. Devlin. Devlin's insights were especially helpful; however, any errors of fact or interpretation are our responsibility.

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