Deep-Water Density Current Deposits of Delaware Mountain Group (Permian), Delaware Basin, Texas and New Mexico

Abstract

The Guadalupian Delaware Mountain Group is a 1,000-1,600-m (3,281-5,250-ft) thick section of siltstone and sandstone deposited in a deep-water density-stratified basin surrounded by carbonate banks or reefs and broad shallow evaporite-clastic shelves. The most prevalent style of basinal deposition was suspension settling of silt. Laminated siltstone beds are laterally extensive and cover basin-floor topographic irregularities and flat-floored channels as much as 30 m (99 ft) deep and 1 km or more wide. Channels can be observed in outcrop at the basin margin and can be inferred from closely spaced wells in the basin. The channels are straight to slightly sinuous, trend at high angles to the basin margin, and extend at least 70 km (43 mi) into the basin. Sandstone beds, con ined to channels, form numerous stratigraphic traps. Hydrocarbon sealing beds are provided by laminated organic siltstone, which laterally can form the erosional margin where channels are cut into siltstone beds. Thick beds of very fine-grained sandstones fill the channels. These sandstones contain abundant large and small-scale traction-current-produced stratification. These sandy channel deposits generally lack texturally graded sedimentation units and show no regular vertical sequence of stratification types or bed thickness. Outcrop and subsurface evidence indicates Delaware Mountain Group sediments were deposited by saline density currents. Dense saline water originated on evaporitic shelves and spilled across the carbonate rim, down steep marginal slopes, and into the basin. Basinal waters were density stratified. Denser flows moved along the basin floor cutting channels or depositing sand in existing channels; less-dense flows moved along density interfaces in the water column and carried silt-size material far into the basin where it settled to the floor as thin alternating layers of detrital silt and organic debris. Little proximal to distal change occurred in the size or nature of the channels. Exploration predictions based on submarine fan models formed by turbidity currents would anticipate very d fferent proximal-distal changes in sandstone geometry and facies.