Stratigraphic variability of mineralogy, rock types and porosity of the Wolfcamp Formation using multi-mineral petrophysical analysis and seismically constrained reservoir modeling, Northwest Shelf, Delaware Basin

Abstract

The Wolfcamp Formation on the Northwest Shelf of the Delaware Basin formed in a low-angle (<1̊ ) slope-to-basin depositional environment and is primarily composed of sediment gravity flows. Multi-scale heterogeneities within the Wolfcamp are reflected in its stratigraphic architecture, rock types, petrophysical properties, and their spatial distribution. The Wolfcamp Formation is informally divided into four stratigraphic zones, from top to bottom Zones A, B, C, D, in which reservoir quality was analyzed. Seismic-constrained 3D reservoir modeling was used to map the Wolfcamp mineralogy, rock types, and petrophysical and geomechanical properties to explore their spatial distribution and relate these parameters to hydrocarbon pore volume. The study area is west of Carlsbad, New Mexico and data include a 93 mi2 (240 km2) 3D seismic survey and logs from 45 wells within the survey that penetrate the entire Wolfcamp Formation. Across the study area, the Wolfcamp Formation gradually dips to the east-southeast or basinward, varies in elevation from -4764 to -5600 ft (-1454 to -1706 m), and varies in thickness from 106 to 1206 ft (32 to 367 m). Each stratigraphic zone displays proportional thickening in the depositional dip direction to the southeast. For the Wolfcamp Formation in the study area, shale volume (Vsh) ranges from 0-100% with an average ~30%. Higher values of Vsh are common in the lower and middle Wolfcamp Formation. Total and effective porosity (t and e) for the Wolfcamp Formation both range from 0-16% and average ~10% and 4%, respectively. t and e vary stratigraphically and general are higher in dolomites and lower in limestones. Average water saturation (Sw) is 35%, ranges from 0 to 100%, and increases moving stratigraphically upward. Lower values of Sw generally correspond to dolomite and high values vary between limestones and sandstones. Based on ρmaa-Umaa analysis and a Vsh cut-off, the Wolfcamp Formation has 29% mudstone, 27% limestone, 22% sandstone, and 22% dolomite. The stratigraphic distribution of rock types reflects the variability in mineral composition for the Wolfcamp Formation. The base of the Wolfcamp is primarily mudstone and mudstone percent decreases upward through the section. Limestone and dolomite percentages are greatest in Zones B and C, respectively. Sandstone percent is low in lower and middle Wolfcamp and is dominant in the upper Wolfcamp. Relationships between rock types and specific ranges in p-impedance values were established and used to map the distribution of rock types. 3D rock-type and petrophysical property models illustrate how reservoir quality varies by rock type and reservoir zone. Dolomites exhibit the highest effective porosity and hydrocarbon pore volume and poor reservoir quality is associated with limestones. Zone C has low Sw, relatively high brittleness index, contains a high proportion of dolomite, and would be an optimal interval to land horizontal wells for development.