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Lateral Heterogeneity of Distal Submarine Lobe Deposits, Point Loma Formation, California: Implications for Lateral Facies Prediction in Horizontal Wells

Abstract

Submarine fan deposits are volumetrically the largest sediment accumulations on Earth and host significant hydrocarbon reserves. Turbidity currents—a gravity-flow driven process—carry sediment from the shelf to intra-slope and basin-floor environments where event beds compensationally stack to form radially dispersive submarine lobes. Extensive research has documented the bed-scale architecture of sandy, high net-to-gross proximal and axial lobe environments, which can have event beds several meters thick. Less well-understood are the lateral and vertical relationships of the thinner-bedded, lower net-to-gross event-bed deposits that are deposited in the lateral and distal lobe fringe environments. Understanding the lateral and vertical changes of thinner-bedded event-bed deposits is important for reservoir modeling and lateral facies predictions when drilling horizontal wells.

Sea-cliff outcrop exposures of the Upper Cretaceous Point Loma Formation in San Diego, California, exhibit a wide range of bed thicknesses and stratigraphic architecture, which have been used to interpret their relative position (e.g., axis-to-fringe, proximal-to-distal) in a submarine lobe. In this study, we characterize the low net-to-gross, thin-bedded lobe deposits that are interpreted to be distal lobe-fringe deposits. Outcrops of the Point Loma Formation offer >1.2 km of laterally continuous exposure, for which beds can be correlated and quantitatively measured for lateral and vertical changes in facies and bed thickness.

The results of this study indicate that event beds have rapid lateral facies changes (e.g., from turbidite to hybrid-event-bed) and pinch and swell more frequently than conceptual models for distal lobe deposits would predict. This suggests that a single thinning and/or thickening rate may not be representative of the true architecture of distal beds, even over relatively short (< 1 km) distances. This work provides insight into the degree of lateral heterogeneity that can be expected from similarly positioned deposits in the subsurface, such as those from the Wilcox (Gulf of Mexico) and the Wolfcamp (Permian Basin). The statistical analysis of the thinning rates and lateral facies variability of event-beds in this study provides inputs for reservoir models and horizontal well facies predictions for reservoirs hosted in distal submarine lobe deposits.