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The marine sands of the Gulf Coast are prolific petroleum reservoirs. Deposition of these sands has taken place during a series of advances and retreats of the sea. A resume of the sedimentation and depositional conditions occurring in a marine advance, or transgression, and in a marine retreat, or regression, is presented. The resulting stratigraphic sequences, the "marine overlap" and "marine offlap" are discussed under ideal conditions, and illustrated by electrical log profiles. Particular reference is made to possibilities for petroleum accumulation as influenced by stratigraphic conditions.
Sands deposited in a transgressive sea, such as the "Cockfield," upper Wilcox (Sabinetown), Marginulina-upper Frio, and lower Miocene sands, are believed to present conditions favorable for the migration, accumulation, and recovery of oil. Although sands deposited in a regressive sea, such as parts of the Rockdale (Wilcox), lower Yegua, Vicksburg-lower Frio, and Catahoula, are not considered theoretically as favorable, local structural or environmental conditions may effect excellent reservoirs.
The compound features representing a marine invasion followed by a retreat, or the reverse, are also considered and electrical log profiles presented. The economic significance in petroleum geology of the resultant sand wedges and shale wedges is discussed.
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