Spreading of salt structures in the Gulf of Mexico

Abstract

Past and present overhangs on diapirs (ductile intrusions) of salt are potential hydrocarbon traps and increasing numbers of larger overhangs are being recognised as seismic acquisition and processing improves. Salt overhangs develop by three process: drag by ductile surroundings sinking around salt diapirs; thinning of diapiric stems; or the topic considered here—gravity spreading. Gravity spreads salt where it is easier for salt to flow sideways than it is to float or sink. The potential level to which individual salt diapirs rise depends on the pressure applied to their source by the overburden load and any lateral forces. The potential level to which salt diapirs rise can be independent of both the top free surface and the level of neutral buoyancy of the salt. Even the most vigorous diapirs cannot rise indefinitely; they gravitationally spread at barriers they cannot penetrate: below, at, or above their level of neutral buoyancy. It has been suggested that salt diapirs spread below their level of neutral buoyancy in weak layers beneath stiff barriers. However, no case of deep subsurface salt spreading appears to have been documented; instead, shales rise with salt along the US Gulf coast. Some salt diapirs in the Gulf of Mexico may simulate ductile ice flows and spread both upwards and downwards to a subsurface level of neutral buoyancy in surroundings of similar strain rate. However, most diapirs in the Gulf are driven above their level of neutral buoyancy so that they spread downwards back towards it. These spread over denser or stifter layers in less dense and weaker barriers of air, water, or unconsolidated sediments. This work focuses on the geological implications of the shapes of the tops of small-scale secondary salt bodies that spread superficially in weaker and less dense barriers under the northern Gulf of Mexico. Analytical, material and natural models are used to show that the shape of the top free surface of spreading salt contains information about both the properties of the materials involved and their kinematics. Many salt diapirs in the Gulf spread downslope from viscous salt fountains in muds over sand. Salt flowing downslope towards a lower level of neutral buoyancy exhibits dynamic extrusion. The temporary, dynamic nature of salt sheets is emphasised by restating standard arguments for overburden strength in terms of low strain rates. Contrary to recent views that salt spreading occurs in weak horizons during fast sedimentation, it is argued that most salt spreads superficially during periods of slow sedimentation. Small salt laccoliths advance downslope at long-term rates between 1 and 5 mm/a. After burial, spreading and gliding of their overburdens can detach such laccoliths from their source and smear them to salt sills that advance downslope at 8 mm/a. As development wells are likely to be complicated by such active salt flow, we should begin now direct measurements of the rates at which salt spreads at superficial levels beneath the northern Gulf of Mexico.