Quantifying the lateral heterogeneity of distal submarine lobe deposits, Point Loma Formation, California: Implications for subsurface lateral facies prediction

Abstract

Submarine fan deposits are volumetrically the largest sediment accumulations on Earth and host significant hydrocarbon reserves. Extensive research has documented the bed-scale architecture of high sand-to-mud ratio, proximal and axial environments, which can have bed thicknesses of several metres; however, less well-understood are thin-bedded turbidites, which are typically lower N:G and deposited in more distal environments. Conceptual models assume that lobe-fringe-to-basin plain environments consist of tabular, sheet-like beds that extend out continuously and predictably over long distances—up to several kilometres. Extensive lateral continuity, however, is not necessarily reflected in ancient outcrop analogues. This study seeks to apply a quantitative approach to the characterisation of thin-bedded turbidites to assess the impact of multi-scale heterogeneity on reservoir predictability. The 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 an off-axis-to-fringe depositional environment. The study area spans 700 m of laterally continuous outcrop, across which 10 correlated stratigraphic sections are used to quantify changes in metrics such as bed thickness, N:G, lithofacies proportions, etc. Results of this study demonstrate that thin sand beds experience both lateral facies changes and rapid thickness changes more frequently than conceptual models would predict. A single measure of lateral heterogeneity does not reflect the true architecture of sandstone beds, and significant information is lost when beds are correlated over ten to hundreds of metres. Sands are commonly deposited in irregular, ‘finger-like’, planform geometries, which compounds at the lobe element scale and influences lateral lithofacies predictability. This study of the Point Loma Formation offers high-resolution bed-to-element scale data, which may be used as inputs for reservoir models and horizontal facies predictions in both conventional and unconventional hydrocarbon reservoirs.