Abstract: Lithofacies Associations Within Complex Slope Channel Reservoirs: Debrites and Turbidites

Sykes, Mark A. - Exxon Exploration Co.; David Mohrig, Christine Rossen and Timothy R. Garfield - Exxon Production Research Co.

There has been considerable debate recently concerning the nature and relative abundance of sandy deep-water debris flow and turbidity current deposits. The debate has focused on both core from petroleum provinces such as the North Sea, Nigeria and the Gulf of Mexico and on outcrop.

The dissimilar fabric of turbidites and debrites has significant implications for reservoir quality. Hence, improved understanding of the nature and relative quantity of these two lithofacies within a deposit is essential for accurate exploration evaluation and prudent reservoir management.

An emerging database of core from slope channels, offshore Angola, is providing new insight into these types of deposits, the relationships between them, and their relative abundance. The Angolan cores show that turbidites and debrites can intimately co-exist within large-scale slope channels, with sandy debrites being volumetrically significant.

Although the turbidites and debrites occur in close association, considerable textural and sedimentologic differences exist between the two lithofacies. The differences are interpreted to be a result of a contrasting mode of transport and deposition.

Turbidites exhibit a relatively consistent and uniform fabric. They are typically composed of fine sand, are well sorted, may be vertically graded and are virtually devoid of detrital clay. Deposits composed of sediment transported as bedload beneath turbidity currents are occasionally observed and can be identified by their planar or cross lamination, sub-horizontal clast orientation and grain imbrication.

In contrast, debrites show substantial textural variability. Sandy debrites are typically less well sorted than turbidites, are composed of medium to very coarse sand and contain up to 10% detrital clay. Permeabilities vary widely depending on grain-size distribution within individual deposits. However, some sandy debrites have rock properties similar to those of turbidites. Fortunately, sandy debrites show additional sedimentologic features which may be diagnostic, such as sub-vertically oriented oversized clasts. Mud-rich debrites are easily identified in core by their fine overall grain-size, lack of porosity and the prominence of floating sand grains.

Recent flume experiments reveal the contrasting flow processes in operation, and the relationships between them. The experiments help explain the genesis of, and the variability within, turbidite and debrite lithofacies. Key controls on flow and resultant lithofacies type are initial grain-size distribution, total sediment concentration and initial flow velocity.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil