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-
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
1997. The American Association of Petroleum Geologists. All rights reserved.
received October 9, 1995; revised manuscript received October 21, 1996;
final acceptance February 25, 1997.
Robertson Ltd., 2801 - 18th Street NE, Calgary, Alberta T2E 7K5, Canada.
Petroleum Inc., Bakersfield, California.
Occidental Petroleum Ltd., Calgary, Alberta, Canada.
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.