Evolution From Confined to Relatively Unconfined Strata in a Distal Slope Setting, Magallanes Basin, Chilean Patagonia

Abstract

Ponded sandstones can serve as hydrocarbon reservoirs and are the focus of current exploration and production around the world. Continued production and exploration involving ponded sandstones motivates a better understanding of the stratigraphic evolution of ponding, which aids subsurface prediction and reservoir management. The deposits of unconfined and confined turbidity currents have an expression in stratigraphy that can be quantified with bed statistics. For example, the ratio of mud-cap thickness to genetically linked sandstone bed thickness can be related to the level of the confinement (ponding). We use Cretaceous-age outcrops at Cerro Solitario in the Magallanes Basin (Chile) that span the transition from levee deposits of the Cerro Toro Fm into prograding slope deposits of the Tres Pasos Fm. Stratal lapouts of sandstone beds onto the highly irregular ‘Chingue’ surface, that separates the Cerro Toro from Tres Pasos deposits, correspond with growth faults. This led to an interpretation of ponding in the lowermost Tres Pasos Fm. However, the stratigraphic evolution from this basal ponded interval to an upper interval with variable expression is not well understood. The transition from deposition in a topographically influenced ponded setting at the onset of slope sedimentation to a relatively unconfined setting was documented via facies and architectural analysis from seven measured sections across a 3 km long outcrop transect at Cerro Solitario. Additionally, 1-D bed-thickness statistics were compiled to better constrain the nature of flow confinement history. The analyzed 20-60 m thick succession comprises three turbidite facies associations: (1) siltstone interbedded with fine-grained sandstone (∼2-50 cm), (2) thin- to medium-bedded sandstone (∼10-50 cm), and (3) thick-bedded sandstone (∼85-235 cm). Planar laminations are abundant in all facies. Bed correlations were made across the 3 km transect and indicate that lateral facies changes occur over 100s of meters. Bed thickness measurements, ratio of mud-cap to genetically linked sandstone, and amalgamation ratio of beds are used to identify and quantify changes in degree of confinement. These bed properties can change through time in response to changes in subsidence, presence of partial or fully ponded accommodation, and the dynamic interplay between sediment loading and infilling of accommodation that can result in growth faulting and, potentially, overpressurization of buried strata.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016