ABSTRACT: Scales of Geological Heterogeneity of a Deep-Water Sand Giant Oil Field

Roger M. Slatt, S. Phillips, J. M. Boak, M. B. Lagoe

To understand the levels of accuracy that can be placed upon different scales of reservoir description, turbidite intervals in part of the giant Wilmington oil field, California, have been numerically described at four scales of heterogeneity. The degree of accuracy of the description, in terms of real geologic variability, is found to diminish with increasing scale.

At the microscale (grains and pores) and mesoscale (near well bore), the following flow units, listed in order of decreasing reservoir quality, were defined by relating various geologic and petrophysical properties: thick-bedded sand, thin-bedded sand, and shale. Mutual relationships among the geologic and petrophysical properties are a result of primary depositional processes.

At the macroscale (interwell), shale beds are laterally continuous over long distances and probably isolate individual sands by acting as vertical permeability barriers. Petrophysical properties, such as permeability, vary between wells within an order of magnitude of measured values. The relationships among petrophysical properties and geologic properties established at the single-well scale are sometimes but not always predictable between wells.

At the megascale (field wide), the turbidites were placed within the context of Vail's integrated sequence stratigraphy model, Walker's progradational submarine fan model, and Mutti's turbidite systems model to illustrate that there is not a unique interpretation when the overall size of a depositional system is larger than that of the data grid. At this scale, petrophysical properties are averaged over a large stratigraphic interval so that there is very little interwell predictability; however, the primary depositional control on gross petrophysical properties is maintained.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990