Dynamic Reservoir Analytics Engineering Consulting

The Bone Spring formation consists of alternated and interbedded siliciclastic (sand), carbonate, and shale rocks. It is divided into the 1st, 2nd, and 3rd Bone Spring, each containing a package of carbonate followed by a package of sand. Above the 1st Bone Spring carbonate, there are the upper and lower Avalon shales as well as the Avalon carbonate that splits the two shales. The Avalon carbonate is interbedded with dark and carbonaceous shaly siltstones that act as a permeability barrier to vertical flow. The Bone Spring formation was deposited as submarine fan systems. The carbonates are composed of dark and dense carbonaceous wackestones and mudstones that establish an important source rock. The 2nd Bone Spring zone is the most productive, has higher peak rates, and it is thickest across the northern Delaware Basin mainly in the Eddy and Lea Counties. In the Bone Spring formation, interval porosity varies from 8 to 20 percent in the sand layers and from 1 to 4 percent in the mud layers, but all layers have very low permeability, on average less than a few mD. The Bone Spring formation reaches a maximum gross thickness of 3,400 ft. The thickest region of the formation occurs in the eastern portion of the Delaware Basin before it quickly disappears into the Central Basin Platform. Thickness of the individual sands varies from 180–500 ft. In Loving County, the Bone Spring reservoir is found at a depth range of 8,000–11,000 feet. 

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Well performance shows that Wolfcamp formation has a higher probability to produce more oil even though the Bone Spring reservoir has higher TOC content and neutron porosity values but lower normal pressure gradient. Other geological and reservoir descriptions must play role in the reservoir productivity such as thermal maturity, amount of kerogen filling pore space, fossil content type, water saturation, matrix permeability, rock brittleness, presence of "sweet spots" (proximity to faults and presence of natural fractures), completion design, enhanced fracture region extend, and operator's best practices. The Wolfcamp formation is generally a two-lithology based system of mostly shale with interbedded limestone. Minor layers of calcareous sandstone are present. The formation is separated into four units (A, B, C, and D) with slight mineralogy changes between the sub-units. The lithology of the Delaware Wolfcamp is analogous to the Midland Basin, but some differences do occur. Wolfcamp formation in the northwestern section of the Delaware Basin is different from the rest of the basin where carbonates are light colored dolostones. In contrast, the rest of the basin’s Wolfcamp formation are dark colored lime packstones, wackestones, and mudstones. The Delaware Wolfcamp behaves as a reservoir and source rock due to ideal mineralogy and grain size distribution. Depositional and diagenetic processes control this formation's heterogeneity. The carbonate turbidites display TOC values ranging from 0.6 to 6 percent, whereas the siliciclastic turbidites generally exhibit less than 1 percent. The interbedded non-calcareous mudstones contain as much as 8 percent TOC. The argillaceous and siliceous mudstones have effective permeabilities ranging from 10 nD to 30 nD and total porosity between 13.5 and 16.5 percent. In contrast, the effective permeabilities in laminated mudstones and siliceous wackestones and packstones range from 500 nD to 1000 nD and the total porosity is 5.5 percent. These lithofacies are heterogeneously distributed in the stratigraphic section. The argillaceous and siliceous mudstones occupy 70 percent of the thickness and thus house 90 percent of the total pore volume. The laminated mudstones and siliceous wackestones and packstones occupy only 7 percent of the thickness but are 12 to 25 times more permeable. Wolfcamp formation has an average thickness of 2,000 feet but can locally exceed 6,000 feet in the western portion of the basin. The top of the Wolfcamp reservoir in the Delaware Basin lies at an average depth ranging from 10,000 to 12,000 feet.

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A primary controlling factor that affects the high rock brittleness index is a composition of the material. For example, clay or mudstone tend to be more flexible and ductile. Quartz and calcite on the other hand, with a higher density and hardness, are more brittle. In general, the siliciclastic-rich sequences exhibit the highest brittleness index and therefore geomechanical competence.