Sedimentologic and Stratigraphic Characterization of Ancient Sandstone Shelf Ridges: An Example From the Campanian Almond Formation, Wyoming, USA

Abstract

Shelf sand ridges are sedimentary bodies on continental shelves formed by transgressive reworking, by storms or tidal currents, of lowstand (coarse-grained) deposits. Common in modern shelves they are highly under-represented in the geological record, partially due to a lack of clear recognition criteria and facies model. These deposits are very clean and well sorted, due to the intense reworking by storm or tidal currents, and are encased in marine muds. This makes ancient sandstone shelf ridges excellent hydrocarbon reservoirs. The correct identification of these bodies and the use of a defined facies model is crucial for an adequate exploration and production plan. For example the Almond Formation in the Hanna and Washakie basins is an active target for gas, condensates and oil exploration. An important part of the production is drawn from the upper-most sandbodies, until now thought to be regressive shorefaces. This work details three of the uppermost Almond sandstone bodies, Hanna Basin Wyoming. The sandstone bodies have distinct characteristics unlike those of a shoreface, therefore we advocate for their reinterpretation as transgressive shelf ridges. Sedimentological sections describing lithofacies, grain size, bed thicknesses, bed contacts, and paleocurrents, with high-resolution photomosaics and LiDAR data, were used to characterize these bodies. Building from this case study, other ancient examples, and modern shelf ridges we propose a shelf ridge facies model, and some diagnostic criteria. Six characteristics that can help the recognition of ancient sandstone shelf ridges are: 1) they are encased in thick marine mud intervals above and below, 2) have a basal unconformity, a transgressive surface that eroded into marine muds or into the remnant of a previous shoreline, 3) the upper boundary is non-erosional and transitions into marine muds, 4) are formed by very clean and well-sorted sandstone, usually cross-stratified where tidal currents were involved 5) contain fully marine ichnofauna, and 6) present compound architectures with large accretion surfaces and lesser order cross-strata inside. We propose a facies and evolutionary model that honors the process regime (tidal vs storm domination), and highlights the complex internal architecture and heterogeneities present in shelf ridges. These facies and evolutionary models have great applicability in reservoir modelling to predict internal flow barriers and reservoir volume and distribution.

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