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Process controls on the\ud development of stratigraphic trap potential on the margins of confined \ud turbidite systems and aids to reservoir evaluation

By W.D. McCaffrey and B.C. Kneller


Stratigraphic trapping at pinch-out margins is a key feature of many\ud turbidite-hosted hydrocarbon reservoirs. In systems confined by lateral\ud or oblique frontal slopes, outcrop studies show that there is a\ud continuum between two geometries of pinch-out configuration. In\ud type A, turbidites thin onto the confining surface—although the\ud final sandstone pinch-out is commonly abrupt—and individual beds\ud tend not to erode into earlier deposits. In type B, turbidite sandstones\ud commonly thicken toward the confining slope, and beds may\ud incise into earlier deposits. These two types may occur in combination,\ud to give a wide spectrum of pinch-out characteristics. Our\ud analysis suggests the principal control in determining pinch-out\ud character is flow magnitude, with smaller flows producing type A\ud and larger flows producing type B.\ud In areas of poor seismic control it can be difficult to assess either\ud pinch-out character or the proximity of wells to confining slopes.\ud Because estimates of paleoflow magnitude can be made from core\ud or high-quality log image data, however, it is possible to make reasonable\ud estimates of pinch-out character even from wells such as\ud exploration wells, which may be placed conservatively, away from\ud the field margins. Furthermore, systematic paleoflow variations and\ud thickness trends are commonly seen in individual turbidite sandstones\ud as they approach confining slopes. For example, dispersal\ud directions indicate flow deflection parallel with the strike of confining\ud topography; beds thin toward type A onlaps and thicken toward\ud type Bonlaps. These relationships can be exploited via analysis of\ud vertical successions to constrain well position with respect to the\ud slope. Similarly, the presence, location, and frequency of locally\ud derived debrites can provide information on the presence and proximity\ud of confining slopes

Year: 2001
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