New Depositional Model (Lake-Shelf Hyperpycnites) for Enigmatic Brushy, Cherry and Bell Canyon Formations, Permian, Delaware Basin, USA: Importance for Local and Global Petroleum Exploration and Development
¹Geoclastica Ltd, Bude Turbidite Research Centre, Cornwall, UK
Many oil companies use a popular deep-sea-fan model for exploration and development in the Brushy, Cherry and Bell Canyon ('BCB') formations, despite the Brushy's previous shallow-water interpretation. Moreover, the Brushy is popular as an outcrop analog for worldwide passive-margin, deep-sea-turbidite reservoirs (fans and leveed sinuous channels; e.g., offshore GoM, Brazil, Africa), where economic losses from using incorrect analogs can potentially reach billions of dollars. A new BCB depositional model is offered here, river-fed turbidites (hyperpycnites) on a lake shelf, based on (1) a literature survey, (2) the author's outcrop observations, and (3) his intimacy with the Brushy-lookalike Bude Formation (UK). Evidence for a low-salinity "Lake Brushy" includes: (A) lack of reported marine fossils other than abraded (reworked) ones; (B) lack of exclusively marine ichnogenera; and (C) distinctive "premature amalgamation", whereby sand sheets are up to 10 m thick yet comprise only thin (< 40 cm) beds, reflecting easy resuspension (weak cohesion) of fresh-water bottom muds. Shallow water is indicated by numerous BCB event beds with evidence for storm waves (HCS; near-symmetrical ripples; mud-draped scours), interpretable as wave-influenced hyperpycnites. An inner-Brushy belt of "deep-sea slope" muds with "slope channels" is reinterpreted here as a stack of delta-slope clinothems, each < 15 m thick, separated by ravinement sequence boundaries and fluvially incised valleys with low sinuosity and non-estuarine hyperpycnite fill. Delta progradation at highstand alternated with lowstand deposition of hyperpycnites on an outer-Brushy shelf, fed by incised rivers. Shelf emergence was prevented by storm erosion that maintained an equilibrium profile. The shelf was on an inherited passive margin, facing a SE-subducting remnant ocean cut off from the world ocean by the Marathon salient (of Gondwana) colliding early against Euramerica, raising a sill (Diablo Platform), isolating the Brushy 'oceanic lake', freshened by river inflow. The new shelf model is vital for BCB exploration and production, since predicted sand distribution, geometry and architecture differ strongly from those forecast by the popular model of deep-sea fans fed by slope channels. The BCB formations are unsuitable global outcrop analogs for passive-margin deep-sea-turbidite reservoirs.
AAPG Datapages/Search and Discovery Article #90190©AAPG Southwest Section Annual Convention, Midland, Texas, May 11-14, 2014