Abstract
The Bone Spring and Wolfcamp formations of the Delaware Basin consist of mixed sediment gravity flow and suspension sedimentation deposits. These deposits exhibit high levels of heterogeneity both at and below core and log scales. A comprehensive approach integrating core and sub-core (nanoscale) data from two key wells and well logs within central Ward County was used to characterize small scale changes in lithology, rock properties, and reservoir quality. With this approach, a total of nine facies were identified; three siliceous mudstones [1, 2, 3], three siltstones [4, 5, 6], and three carbonates [7, 8, 9]. Each is comprised of different grain size distributions, textures, mineralogies, and pore types. Facies are not unique to an individual facies associations and cannot be predicted laterally in this study. Core-based measurements of source and reservoir properties were used along with qualitative observations from thin sections and high-resolution SEM images to identify facies as primary reservoir facies, secondary reservoir facies, and non-reservoir facies. Properties concerning source, reservoir, and mechanical quality were evaluated with respect to each facies and within each stratigraphic unit; 3rd Bone Spring, Wolfcamp A, Wolfcamp B, and Wolfcamp C.
Within the study area, 210 sq. miles in central Ward County along the eastern flank of the Delaware Basin, the Bone Spring and Wolfcamp formations are in the early mature oil window (0.69% – 0.88%Ro) and consist of an intercalation of siliceous mudstones [1, 2, 3], siltstones [4, 5, 6], and carbonates [7, 8, 9]. The four reservoir facies [1, 2, 4, 5] identified are organic rich with average wt.% total organic carbon (TOC) as follows; argillaceous siliceous mudstone [1] (3.1 wt.%, n=21), calcareous siliceous mudstone [2] (3.0 wt.%, n=15), argillaceous siliceous siltstone [4] (2.0 wt.%, n=7), and calcareous siliceous siltstone [5] (2.3 wt.%, n=7). Primary reservoir facies [1, 2] are richer in type II kerogen than the mineralogically comparable but coarser-grained secondary reservoir facies [4, 5], which contain more detrital grains and type III kerogen. Lower organic content in secondary reservoir facies [4, 5] is related to the dilution of organic matter via an extrabasinal influx of detrital grains and possible consumption by benthic fauna in oxygenated conditions. Degree of anoxia, bioturbation, and silica origin all have significant implications to reservoir quality as seen in the mineralogically similar non-reservoir biogenic siliceous mudstone facies [3] and the primary reservoir argillaceous siliceous mudstone facies [1]. The former contains the least amount of detrital silica and organic matter of all facies observed. Early diagenesis of radiolaria and siliceous spicules source the microcrystalline authigenic quartz that was observed to occlude pore space in this non-reservoir facies [3]. Despite the poor source potential and reservoir quality of this facies [3], the high amounts of microcrystalline authigenic quartz are beneficial to reservoir geomechanics. Implications to reservoir quality identified in this work have limited utility outside of the study area away from the flank of the basin, where bioturbation, degree of anoxia, and prevalence of extrabasinal facies differ. GRI saturations, MICP measurements, NMR (T2LM) data, and core-based TOC measurements indicate siliceous calcareous siltstone [6] as a facies potentially making up water-bearing carrier beds. Carbonate-rich facies [6, 7, 8, 9] were sampled least from core and more work must be done to better evaluate reservoir potential of these facies.
Core-based measurements of composition and reservoir quality indicate that porosity and permeability trend positively with clay, pyrite, and TOC, and negatively with carbonate. This relationship with porosity is most evident and statistically significant in the fine-grained facies [1, 2, 3, 4, 5], where silica is always the primary constituent. Relatively high clay content, upwards of 34 wt. %, in this study is not observed to negatively impact mechanical behavior. Porosity and TOC are highest in the Wolfcamp A and lowest in the lower Wolfcamp B subdivision, a trend observed beyond core control within the two key wells and on logs throughout the study area. This is largely a function of facies distribution. Based on stratigraphic architecture, facies distribution, and lack of thick non-reservoir carbonate barriers, the Wolfcamp A and upper Wolfcamp B may be considered one flow unit. This may allow well spacing and number of wells to be strategically optimized per drilling unit. Development strategies with respect to well spacing and well planning, may be better constrained with an understanding of each facies’ source potential, reservoir and mechanical quality, and distribution within each stratigraphic interval. Findings and interpretations from this research contribute to larger scale efforts being made to: 1) understand the role of diagenesis in unconventional reservoir quality; 2) recognize implications of depositional processes in unconventional reservoirs; and 3) image unconventional facies at the nano, micro, and macro scales.
