Contourite sheet sands: new deepwater exploration target

Arguably the largest known, present-day, deepwater sandy deposit is a contourite sand sheet located in the eastern Gulf of Cadiz on the Iberian continental slope. It extends from the Gibraltar oceanic gateway towards the northwest for at least 100 km along a mid-slope terrace at a mean water depth of 500-700 m. It covers an estimated area of around 4000 km2, with a known thickness for a single unit being in excess of 10 m in a proximal location. High-resolution seismic data indicate that this unit is more probably 40 m thick. Deeper penetration (multichannel) seismic data further suggest there may be at least 800 m of sand-rich contourite section in the immediate subsurface. This is inferred to have a high net to gross ratio (>50% sand). Towards its distal extremity the sand sheet passes into three discrete contourite channels, the largest of which (the Cadiz Channel) funnels sand alongslope and downslope to water depths exceeding 1200 m. This channel is floored by a series of large active sand waves and, locally, by gravel lag deposits.Preliminary sediment analyses of selected cores reveal clean, well-sorted sands that would potentially yield excellent reservoir characteristics on burial. This sand sheet is fed mainly from shelf sands that are fed downslope through proximal turbidity current channels and by shelf spillover processes onto the middle slope. The sands are then redistributed alongslope by powerful bottom currents formed from Mediterranean Overflow Water. They form part of a much larger well-established contourite depositional system in the Gulf of Cadiz, which becomes dominantly mud-rich in medial to distal parts as the bottom current velocity and hence transport capacity decreases away from the Gibraltar gateway. Certainly these same processes and deposits will have been equally active in the geological past, along other margins as well as the Iberian margin. We propose, therefore, that such contourite sands should form a serious and specific deepwater exploration target in the subsurface. However, more work is required on modern analogues such as the Cadiz Sand Sheet in order to better characterise and understand these novel reservoir targets.