History of Methane Seepage in the Delaware Basin and its Relationship to Bacterial Diagenesis

Brian E. Lock, Department of Geology, University of Louisiana, Lafayette, LA

The Late Permian of the Delaware Basin records a late, evaporite phase in the basin's history. The Castile Formation consists mainly of laminated calcite/gypsum (anhydrite in the subsurface) couplets that can be widely correlated across the basin, suggesting deep water conditions. The overlying Salado Formation represents relatively shallow water, with extensive sulfates and other evaporites.

A prominent feature of the Gypsum Plains area of Castile Formation outcrops is the presence of secondary carbonate masses that form irregular hills ("castiles"). These coincide with, in some cases, active methane vents and it has been proposed that conversion of sulfate to carbonate is a product of shallow subsurface bacterial activity. There are similarities to the process of carbonate formation in Gulf Coast salt dome cap rock.

Although the castiles occur over the greater part of Culberson County, the most instructive example straddles TX652 near its crossing of the Delaware River. A complex of several hills includes secondary limestones typical of the uppermost Castile overlain by rocks texturally more characteristic of the basal Salado. The latter include pseudomorphs of bottom-nucleated gypsum crystals, but textures are also seen which are interpreted as primary botryoidal submarine cements, normally associated with highly agitated water conditions inconsistent with the local setting.

Methane seepage and calcitization of sulfate is generally regarded as having started in the Tertiary, when regional uplift and tilting occurred, permitting fluid migration along faults. However, it might be noted that the well-studied "Tepee Buttes" south of Denver consist of marine limestones, with accompanying specialized faunas and botryoidal cements, around sea-floor hydrocarbon seeps within the Cretaceous Western Interior Seaway. It is possible that the Culberson County methane vents were also active in the Cretaceous. The Highway 652 outcrops raise the possibility that seepage and associated bacterial activity may have started in the Permian.