RESERVOIR CHARACTERIZATION AND DEPOSITIONAL ENVIRONMENT ANALYSIS OF UPPER WOLFCAMP FORMATION GRAVITY FLOW LITHOFACIES IN MITCHELL COUNTY, MIDLAND BASIN, TEXAS

Mineralogical and petrophysical data derived from a single core of the upper Wolfcamp Formation sourced from Mitchell County, Texas were compared and correlated with core description in order to assess the reservoir quality of sediment gravity flow lithofacies. Deposition records basin-shelf environments near the Eastern Shelf of the Midland Basin including massive thick-bedded, hyperpycnally-sourced calciturbidites with interbedded debris flows (lobe and proximal fan channel complexes) and siliciclastic proximal shallow marine deposits of muds, silts, and muddy sandstones (shelf-edge delta). Siliceous intervals typically show higher reservoir quality, exhibiting an average porosity of 7.13% and permeability of 781.71 nD when compared to calcareous intervals (averaging 4.38% porosity and 528.20 nD permeability). Relatively intact Permian peltasperm fossils (Germaropteris martinsii) are concentrated within fine-grained siliciclastic intervals implying significant organic matter contribution from terrigenous sources via hemipelagic suspension fall out. Rare thin (~3 cm) hybrid event beds appear towards the top of the lower Leonardian succession. These flows record initial fluid flow rheology (basal limestone) followed by suppressed turbulent (cohesive) flow behavior by the incorporation of muddy substrates (calcareous argillaceous siltstone and mudstone). It is likely that these transitional flows and hyperpycnal plumes served as mechanisms for depositing plant material within deeper marine settings. Thick-bedded dolomitic limestones are found towards the top of the Wolfcamp and exhibit higher average porosity (5.73%) than lower undolomitized limestone successions. Additionally, these dolomitic beds are overlain by thinner (~2 cm) porous limestone reservoirs. Porous and highly altered limestones along with hydrocarbon residues in calcite-healed fracture-networks imply that the upper Wolfcamp behaved as an open system through which undersaturated fluids preserved the porosity of limestone beds, allowing hydrocarbons to migrate vertically and laterally before diagenetic cementation. Locating channelized carbonate deposits near sources of hydrothermal fluids such as faults can allow for the discovery and prediction of hybrid limestone-shale plays in slope settings.