Multiscale Gilbert-type delta lobe architecture and heterogeneities: The case of the Roda Sandstone Member

摘要

The Roda Sandstone Member (Lower Eocene, Tremp-Graus Basin, Spanish Pyrenees) is interpreted as a well-exposed multiscale Gilbert-type delta. It is formed by multiple prograding lobate-shaped sandstone bodies up to 5 km(3 mi) long and 2-3 km (1-2 mi) wide, each constructed by a number of smaller scale (hunderds of meters) lobes that stack in a compensational mode responding to relative base level changes. In general, the proximal part of each lobe is dominated by trough cross stratification, the medial part by impressive clinoforms, and the distal part by more tabular cross stratification. Well-developed tidal bundles occur abundantly in delta lobe-front attached and detached tidal bars; the latter are up to 5 km (3 mi) long and 2 km (1 mi) wide. Cemented layers cover both the individual lobes as well as the lobate-shaped sandstone bodies. The three-dimensional (3-D) architecture of small-scale (decimeter to meter) heterogeneities at lithofacies scale formed by millimeter-to centimeter-thickmud layers in the tidal bars have been studied to better understand hydrocarbon flow through heterolithic tide-dominated successions. The Roda Sandstone has been used as an analog to many oil and gas fields with a particular focus on 3-D lobe geometry, compart-mentalization, stacking patterns, and associated correlation concepts.