The Delaware Basin in western Texas and southeast New Mexico is one of North America's most prolific oil-producing regions. Recent activity in southeastern New Mexico has begun to examine the uppermost Wolfcamp Formation (Permian). This interval has been examined in a Devon proprietary core (Lea County, New Mexico) where it is composed of two prominent facies associations. The first is dominated by fine-grained carbonates and calcareous siltstones/mudstones, which are interbedded with organic-rich, non-calcareous mudstones. The second facies association is dominated by dolomitic siliciclastic siltstone/mudstone facies. Both facies associations have comparable porosities, but the carbonate facies have significantly less permeability. The facies are largely organized into fine-grained turbidites derived from different source areas. Each facies association is separated into its own basin floor fan complex with some interfingering between the two associations. The carbonate turbidites were derived from the east off the Central Basin Platform whereas the siliciclastic-dominated turbidites were derived from a source to the west-northwest. The latter strongly resembles the siliciclastic turbidite facies of the overlying Bone Spring Formation. The carbonate turbidites exhibit TOC values that can range from 0.6% to 3.5%, whereas the siliciclastic turbidites generally contain less than 1%. The non-calcareous mudstones that interfinger with the carbonate turbidites preserve as much as 8% TOC. A significant proportion of the TOC within the carbonate turbidites and the interfingering non-calcareous mudstones was derived from terrestrial organics as evidenced by well-preserved fern-like plants with fully articulated leaves (peltasperms). Their presence within the carbonate turbidites indicates that the Central Basin Platform was subaerially exposed during their deposition and indicates carbonate production during a falling to low-stand systems tract. Since these plants are found in both the upper parts of the carbonate turbidites and the non-calcareous mudstones that cap the turbidites, it suggests that the carbonate tubidites rheologically stratified as they flowed, evolving into a carbonate detritus-dominated head and body with a non-calcareous mud wake. Molecular composition, biomarker, and carbon isotope chemistry of oils produced from Wolfcamp reservoirs also indicate that these oils were derived from mostly marine type II with a contribution from type III kerogens.
The association of well-preserved terrestrial leaves (peltasperms) with carbonate turbidite and debris flow facies within the upper Wolfcamp section of the Permian basins of West Texas and New Mexico creates a bit of a dichotomy. Why are well-preserve leaves that are fully articulated to stems being transported into a deep marine system and out on to a basin floor within sediment gravity flows that were clearly sourced from marine deposits? This is particularly vexing when one considers that siliciclastic-dominated, fluvially sourced sediment gravity flows also exist within the Wolfcamp section, but the organics associated with these deposits are composed of finely ground plant debris.
