Recognition and Significance of ‘Scour and Fill’ Structure in Amalgamated Deep-Water Sandstones
A common issue in the subsurface is determining whether amalgamated and apparently structureless sandstones several metres thick fill deep-water channels or form more extensive amalgamated sheets. This is especially the case when there is no guidance from seismic geomorphology. Features in core that help make this distinction are thus important for reservoir characterisation. Here we focus on scour-and-fill structure, subtle features revealed by differential weathering in outcrop, and analogous features that are increasingly imaged by computed tomography (CT) of whole cores in deep-water sandstones.
Cross-stratification filling fields of small, dcm-scale scours are present in the Pennsylvanian Ross Sandstone Formation, western Ireland. Referred to 'scour-and-fill' by Elliott (2000), they superficially resemble trough cross-bedding but were not produced by dune migration. Inclined sandstone laminations downlap onto and drape elliptical scours which are well exposed in plan view and cut one into another. Scour axes and the infilling inclined laminations vary in orientation. Although cut into sand, the scour margins are crisp and the toe-sets are lined with consistently finer grained sand and abundant heavy minerals suggesting flow separation and hence relatively dilute flow. The scour fills are only found within channel-fill sandstones where they occur axially often at multiple levels, and commonly centrally within amalgamated sandstone packages rather than at channel bases. Erosion has been associated with the passage of larger-scale knickpoints that fashioned scours in consolidated sand, triggering secondary flow separation. Knick-points are increasingly recognised in modern channels transmitting sandy sediment gravity currents.
CT scans from the Paleogene Wilcox Formation, offshore Gulf of Mexico, reveal a variety of surfaces and cryptic cross-bedding. Like the Ross examples, the scours are commonly lined by highest density, finest grained sand. Sets of dipping laminae between scour surfaces can steepen-up, flatten-up and also show systematic upward rotation of dip. Paleocurrent trends within 'cosets' can range up to 360°. Combined, these features suggest the cores are intersecting highly 3D inclined surfaces and with both down-stream and up-stream accretion. In both examples, the fine-grained scour linings introduce significant and easily overlooked permeability heterogeneity in otherwise isotropic and potentially good quality reservoir sandstones.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019