Integrated stratigraphic and petrophysical analysis of the Wolfcamp at Delaware Basin, West Texas, USA

Hydrocarbons stored in low-permeability reservoirs, also known as ‘unconventional reservoirs’, represent important energy resources worldwide. Although current technology allows their production at economic rates, there still are numerous production challenges and unknowns regarding their flow behavior. A better understanding on how fluids stored in these reservoirs are drained by the hydraulic fractures after stimulation may help to optimize completion designs and field development plans. This research is an attempt to describe such drainage behavior in the largest oil producing unconventional formation in the World. I investigated the drainage behavior in Wolfcamp reservoirs at the completion scale by integrating stratigraphic and petrophysical analyses with flow modeling. I interpreted the depositional and diagenetic processes that generated three Wolfcamp cores recovered in the central-eastern Delaware Basin, measured the porosity and permeability of distinct lithofacies, and developed simple models to describe flow in these strata. I found that most fluids (~95% of the pore volume) are stored in low-permeability (e.g., < 60 nD) mudstones that I interpreted as hemipelagics and siliciclastics turbidites. Interbedded with these deposits are the low-porosity (~5% of the pore volume) and low-permeability (e.g., < 50 nD) carbonate lithofacies that I interpreted as gravity flow deposits and diagenetic dolomudstones. The carbonate gravity flow deposits, when dolomitized, are up to 2000 times more permeable than the other deposits and represent preferential flow pathways that drain fluids from the low-permeability strata during production. This drainage behavior increases the reservoir upscaled permeability, and therefore production rates, multiple times higher compared to a reservoir consisting of only low-permeability strata. Hence, the presence of these permeable, dolomitized, gravity flow deposits plays a critical role when producing from Wolfcamp reservoirs as they accelerate drainage. These findings are also applicable to other low-permeability formations exhibiting significant permeability heterogeneity