ABSTRACT: Offset Stacking of Elongate Lobes: Primary Control on Reservoir Geometry in Submarine Fans

Victor B. Cherven, Peter J. Fischer, Charles W. Hatten

High-resolution seismic profiles across several modern submarine fans (e.g., Indus Fan and Rhone Fan) and detailed well-log cross sections across ancient fans (e.g., Winter Fan, Sacramento basin, and Stevens Fan, San Joaquin basin) demonstrate that many submarine fans consist of large, wedge-shaped sediment bodies (fan lobes or channel-levee complexes) that are offset rather than vertically stacked. In addition, generalized lithofacies maps and sections across outcrops of ancient fans (e. g., Yorba Linda Fan, Los Angeles basin, and the Great Valley Sequence, Northern Diablo Range) can be used to demonstrate offset stacking of successive channel deposits, successive lobes, and even successive fans. Isopach maps of lobes in both modern fans (e. g., Conception Fan, Santa Bar ara basin, and Mississippi Fan, Gulf of Mexico) and ancient fans (e. g., Lathrop and Stevens Fans, San Joaquin basin, and Winters Fan, Sacramento basin) indicate that most lobes are elongate, regardless of overall shape (i.e., radial, elongate, or broad) or tectonic setting of the fan.

The geometry of fan lobes implies longitudinal sediment dispersal. Most published fan models, however, imply, tacitly or explicitly, that elongate fans result from longitudinal transport and radial fans result from radial transport. Broad fans are often attributed to coalescence of adjacent radial or elongate fans fed through discrete input points (the so-called line source). In our model, fan shape is controlled by a combination of (1) offset stacking of elongate lobes, (2) basin shape and topography, and (3) lateral migration of the feeder channel(s). Channel confinement by erosional channel margins or aggradational levees promotes longitudinal transport and elongate lobe growth. Radial fan growth occurs by channel avulsion near the fan apex, causing lobe abandonment and the growth f a new lobe in the topographic low bordering the abandoned lobe. If the feeder channel is stationary and the fan is unrestricted laterally by basin margins or basin-floor topography, the fan commonly assumes a radial shape. If the fan is more restricted laterally, it will tend to become elongate. Finally, if the feeder channel is ephemeral, as in most cases, or migrates laterally, the fan will broaden.

In many cases, fan lobes in producing fields are encased in and sealed by thick shale blankets, and form elongate reservoirs that are not in fluid communication (e.g., Winters Fan in the Walnut Grove and Union Island fields). Because of offset stacking, adjacent pools or fields produce from different lobes and at different depths, and may have somewhat different reservoir pressures (e. g. Stevens Fan in Ten Section field). Geologists and engineers seldom recognize the presence of offset stacking, and attribute such complexity to faults or facies changes.

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