Mapping of thin-bedded crevasse-splay deposits in a low-N/G floodplain environment - Huesca fluvial fan, Ebro Basin, Spain

A good understanding of the reservoir architecture of tough gas reservoirs is necessary to define their potential for the hydrocarbon industry. These tough gas reservoirs have been overlooked until now due to the high economic risks of development. These risks are mainly related to the large uncertainties that exist in the reservoir architecture. Understanding the depositional processes of crevasse-splays will help in minimizing the uncertainties in interpreting the internal depositional setting and deposition processes.This research focuses on the mapping of thin bedded crevasse splays in a low-net-over-gross floodplain environment in the distal areas of the Huesca fluvial fan (Miocene) of the Ebro basin in Spain. This outcrop is a good analogue to the Permian Rötliegend and Triassic Bundsandstein intervals in the West Netherlands Basin. A field study of the Huesca fluvial fan is used to analyse the reservoir architecture (size, shape, spatial distribution and connectivity) of the reservoirs of fossil crevasse-splay deposits. At two outcrop locations an interval of crevasse-splay deposits is mapped, using detailed lithostratigraphical logs, a type section log, photo-panels and lateral characterizations of sand beds. With these data a correlation panel is constructed to determine the connectivity of the mapped layers, and cross-sections are made to determine the lateral continuity, vertical stacking and grain-size variations.On a single layer level, results show both a decrease in bed thickness and in grain size from proximal to distal, related to the energy of deposition. Grain size is also related to distributary-channel proximity, decreasing away from the distributary crevasse channels. In multi-storey stacked sheets, incision of distributary channels into underlying crevasse deposits near the crevasse apex creates sand-on-sand contact. The approximation of a crevasse splay as a simple accumulation of homogeneous sheet like layers should be refined in order to lower the uncertainties in the tough gas reservoir model. A quantitative dataset for the size, shape and stacking patterns of crevasse splay sandstone is composed, which can be used as an input in static reservoir models