Contourite porosity, grain size and reservoir characteristics

Abstract

Contourites are now recognised as having a significant potential as hydrocarbon reservoirs in the subsurface, and several fields have been interpreted as comprising bottom-current reworked turbidite sands. However, very little has been published on the porosity characteristics of contourites. This study documents porosity data from IODP Expedition 339 sites in the Gulf of Cadiz. We use grain size analyses, porosity-depth plots and exponential models to yield a better understanding of grain size characteristics and facies, porosity characteristics, and the reservoir potential of contourites in the subsurface. New grain size data for over 350 samples from the Cadiz contourites is presented, building on earlier work. These data confirm the distinctive trends in textural properties linked to depositional processes under the action of bottom currents. The finest muddy contourites (<20 μm) show normal grain size distributions, poor to very poor sorting becoming better with decreasing grain size, and zero or low skewness. These are contourite-hemipelagite hybrids. Muddy to fine sandy contourites (20–200 μm) trend towards better sorting and initially fine-tail and then coarse-tail skew. These represent typical depositional trends for contourites, affected by current capacity and then increased winnowing at higher current speed. Clean sandy contourites (around 200 μm) are the best sorted. Medium and coarser-grained contourites show a trend towards poorer sorting. They result from the action of dominant bedload transport, extensive winnowing, and mixed sediment supply. Porosity-depth relationships from four Cadiz sites show a moderately high initial porosity for both sand and mud facies (50–60%) and a systematic decrease with depth to around 35–40% near 500 m burial depth. According to the exponential models of porosity with depth, contourite porosity should exceed 10% at 2500 m burial depth. We compare the data from the Gulf of Cadiz Contourite Depositional System, with those of the Eirik Drift, Newfoundland Drift, Gardar Drift and Canterbury Slope Drifts. Similar depth trends are observed, and all show anomalies linked to interbedded sandy and muddy facies, composition (carbonate vs siliciclastic), and the presence of hiatuses in the sediment record. These data provide good insight into the likely reservoir characteristics of contourites, for both conventional and unconventional reservoirs. They are comparable with those of existing contourite fields, most of which are mixed turbidite-contourite systems.