Criteria for Differentiating Channel and Sheet Reservoirs: A Deep-Water Field Example

Stelting, Charles E.¹, Jennifer Beall², Lisa Goggin¹, William H. Crane³, William J. Schweller⁴, Janet Murphy⁵, Michael Richey⁶ (1) Chevron Energy Technology Company, Houston, TX (2) Chevron, Houston, TX (3) Chevron Energy Technology Company, San Ramon, CA (4) Chevron Energy Technology Company, San Ramon, (5) Chevron International Exploration & Production, Houston, TX (6) Chevron Interntional Exploration & Production, Houston, TX

Channelized and unconfined (sheet) reservoirs are characterized by thick, clean, sand-rich deposits. These elements compromise the bulk of reservoir bodies in deepwater depositional systems and may be distinguished from one another by their geometries, facies associations and hierarchical stratigraphy. Seismic data reveal that the bulk of the reservoir sands in this field example are confined to a broad, low-relief depositional fairway which trends roughly normal to a structural monocline. Because low seismic resolution precludes delineation of geometric planforms within the fairway, reservoir characterization and prediction are accomplished by integrating borehole data (well logs, cores, OBMI) with the broader seismic framework.

Individual sand bodies are dominated by blocky gamma ray log signatures. Calibration to core demonstrates this log signature is associated with massive, amalgamated sands. Conversely, irregular or serrated gamma ray log signatures are indicative of lithologic variability consisting of intermixed turbidite sands and muddy-sandy mass transport deposits; interbedded sand and shale less common. Stratigraphic correlation and degrees of heterogeneity are the key to classification of the reservoir sands into channel or sheet elements within this ‘mixed channel/sheet depositional system.' Sheets consist of laterally continuous, massive sands with rare internal heterogeneities and exhibit minor facies gradation along the fairway edges. The channel bodies are also dominated by massive sands but are more heterogeneous reflecting facies trends from an axis to margin setting and common slump or debrite deposits. In addition to the integrated field correlations, outcrop analogs proved invaluable in establishing the depositional interpretation.