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AAPG Bulletin, V.
Tide-influenced sedimentation in a rift basin—Cretaceous Qishn Formation, Masila Block, Yemen: A billion barrel oil field
1Nexen Inc., 801 7th Ave. SW, Calgary, Alberta T2P 3P7, Canada; email: [email protected]
2Nexen Inc., 801 7th Ave. SW, Calgary, Alberta T2P 3P7, Canada
Dale Leckie is chief geologist at Nexen. He is a specialist in sedimentology, sequence stratigraphy, and basin analysis. He has worked on Australia, Colombia, Yemen, and New Zealand basins. He has received numerous awards from AAPG, SEPM, and the Canadian Society of Petroleum Geologists. Dale coedited (AAPG Memoir 55), Foreland Basins and Fold Belts and (Canadian Society of Petroleum Geologists Memoir 15), Sequence Stratigraphy: Surface, Subsurface and Sedimentology.
Thomas (Tom) Rumpel is staff geological consultant with Nexen, working in the Yemen Operations Team. In a 27-year career as a professional petroleum geoscientist, Tom has worked in a variety of capacities for several intermediate and junior exploration and production companies, including field geologist, exploration/exploitation geologist, consulting geologist, vice president of Exploration, and chief of Computer Geoscience.
This study has benefited from work and discussion with numerous Nexen staff, including David Dearborn, Michael Kerman-Milne, Allan King, Brian Mills, Brett Norris, Kent Wilkinson, and Fred Watchorn. Scott Gardiner and Brian Mills reviewed the manuscript for Nexen. Adam Yakabuskie did a superb job on the figures. Geoffrey Norris provided palynological data and interpretations. Petrological data were provided by GR Petrology Consultants, Core Laboratories Canada, Continental Rocktell Services, GR Petrology Consultants, and Petrel Robertson Consulting. Nexen is thanked for permission to publish this study, as are Nexen partners—Occidental Petroleum and Consolidated Contractors. The manuscript benefited from reviews by S. Yoshida and J. Robinson.
The Lower Cretaceous Qishn Clastics Member in Masila Block 14, Hadhramaut region, Republic of Yemen, has estimated reserves of 1.1 billion bbl recoverable oil and has produced 600 million bbl of oil. Sedimentation took place in an elongate paleogulf of the Say'un-al Masila basin, with open-marine carbonates to the east. The Qishn Clastics Member unconformably overlies mixed carbonates and clastics of the Sa'af Member. Lower Qishn onlap resulted in deposition of brackish and tidal (likely macrotidal) estuarine to open-bay or gulf deposits. The middle portion of the lower Qishn Clastics Member shows evidence of arid nonmarine sedimentation, including debris-flow deposits, red beds, and shale-clast conglomerates, in turn, overlain by interfingering coastal and nonmarine deposits. The lower Qishn Clastics Member is truncated by a sequence boundary overlain by an extensive sandstone deposited in a low-accommodation braid plain close to the shoreline. A flooding surface is present at the top of the sandstone, over which progradational, tide-influenced deltaic strata were deposited. Delta progradation culminated in shallow-water clastic dolomitic deposits on the coastal plain. With subsequent transgression, a rising water table caused a nonmarine flooding surface with lakes and lacustrine deltas, overlain by tidal-flat deposits. Ongoing transgression resulted in wave ravinement overlain by shallow-shelf clastics and then deposition of slightly deeper shelf carbonates. The uppermost unit comprises bioturbated, clastic-shelf deposits related to a drop in relative sea level. Throughout much of the Qishn Clastics interval, accommodation was moderate, except for low accommodation associated with regional sheet sandstone at the base of the upper Qishn Clastics Member. Virtually all marine and brackish deposits show evidence of tidal sedimentation. Climate was arid and seasonally wet.
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