Oscar E. Weser, 1978. "Oil Trapping Characteristics of Turbidites", Framework, Facies, and Oil-Trapping Characteristics of the Upper Continental Margin, Arnold H. Bouma, George T. Moore, James M. Coleman
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In the cycle of basin filling, gravity-laid reservoir strata (turbidites and associated slide and slump deposits) are deposited during there gressive phase. Such strata usually are separated from normal reservoir rocks (shallow marine and nonmarine strata) by an envelope of pelagic sediments. The latter can serve both as a permeability barrier between normal and gravity sediments and as a source of hydrocarbons. An example of the temporal and spatial relations of these three major facies during one cycle of basin fillinq is provided by upper Cenozoic strata of the Ventura basin.
Gravity strata can be subdivided into proximal and distal facies. This distinction is based largely on electric-log correlatability, sand-body morphology, and predictability of sand percentages.
In the proximal facies, electric-log correlations are uncertain, normal to the direction of sediment transport, but are good parallel with that direction. Sandstone bodies wedge out rapidly, so predicting sand percentages is difficult even with abundant well control. Individual sandstone bodies have a channellike shape and commonly exhibit a complex distribution pattern.
Distal-gravity sediments in the Los Angeles and Ventura basins provide examples of the progressive onlap of this facies against preexisting basement highs where differential compaction can promote updip closure for these sediments. In the distal facies, electric-logs are easily correlated both parallel with and normal to the direction of sand transport. Individual sand bodies are wide spread and sheet like, and sand-percentage values are predictable with a minimum of well control.
Defining the structural history of a prospect area aids in predicting on- and off-structure sand bodies both in the proximal and distal gravity-sediment facies.
Successful oil exploration in gravity sediments also is facilitated by paleogeographic reconstructions which define both their source area and their transport direction.
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Framework, Facies, and Oil-Trapping Characteristics of the Upper Continental Margin
The Gulf of Mexico covers an area of more than 1,500,000sq km, has a maximum depth of about 3,700m, and includes many of the geomorphic features of large oceans.The continental shelf, slope, rise, and abyssal plain comprise the major physiographic provinces of the guldf and contain avariety of subprovinces distinguished by topographic character and geomorphic history.
The gulf shelf is a relatively smooth, gently sloping surface marked locally bylow-relief featuresformed by sea-level fluctuation during the Pleistocene, reef growth, near-surface movement of diapiric salt and mud, and faulting. Shelf width varies from about 280km off the Florida and Yucatan Peninsulas to less than 10km at the Mississippi Delta. The continental slope consists of a considerable variety of physiographic subprovinces and individual features that encircle the deep gulf floor.
The distinctive subprovinces of the gulf slope have evolved in response to reef building and constructional sedimentation on the Florida and Yucatan carbonate platforms; erosion, nondeposition, slumping, and fault ing in the Straits of Florida and Yucatan Channel; salt diapirism and differential sedimentation in the region off Texas and Louisiana; the largeaccumulation of mainly Pleistocene sediment on a former continental slope seaward of the Mississippi Delta; tectonic uplift and diapirism in theGolfo de Campeche; and shale mobilization of feastern Mexico. In contrast to the greatly varied, irregular topography of the continental slope,thedeep seafloor of the gulf (composed of continental rise and abyssal plainprovinces) is an almost featureless plain smoothed by turbidite and pelagic sedimentation and marked locally bylow-relief knolls, sedimentary aprons, and small-leveed channels.