Abstract
AbstractSubmarine 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.
Highlights
Submarine fan deposits are volumetrically the largest sediment accumulations on Earth and host significant hydrocarbon reserves
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
We find that a single measure of lateral heterogeneity does not reflect the true architecture of sandstone beds, and that significant information is lost when beds are correlated over tens-to-hundreds of meters
Summary
Submarine lobe deposits host important archives of paleo-environmental change (Hessler and Fildani, 2019) and are major targets for the exploration and production of hydrocarbons (Weimer and Pettingill, 2007). Shortdistance changes in bed character are often overlooked due to the averaging of sparse data over spatially large areas, and yet these relatively small-scale variations seen in the outcrop are necessary to calibrate more realistic depositional models, which in turn can be used to model reservoir flow properties and connectivity. This quantification of bed architecture in submarine lobe deposits provides the data to (1) better understand the depositional mechanics of sediment gravity flows in distal lobe settings, and (2) accurately model reservoir-scale heterogeneity
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