ABSTRACT

Quantification of permeability structure in outcrop reservoir analogs documents the distribution of flow units and barriers important for reservoir management and simulation. Differences in burial history and diagenesis of the outcrop analog and the subsurface reservoir must be quantified, however, before outcrop permeability data can be used to model subsurface strata appropriately. A case study of the Lower Cretaceous Fall River Formation shows that permeability differences between facies are accentuated by diagenesis, and permeability variation increases with the extent of burial diagenesis. The Fall River Formation, which is exposed in outcrop around the Black Hills uplift in Wyoming and South Dakota, USA, produces oil from fluvial and estuarine valley-fill sandstones in the adjacent Powder River Basin. Outcrop sandstones were buried to 2 km before being uplifted at the end of the Cretaceous, whereas reservoir sandstones remained at depths of 2-4 km throughout the Tertiary.

The permeability of outcropping Fall River sandstones has a log-normal distribution. The main controls on permeability in outcrop sandstones are (1) ductile-grain content, (2) grain size, both properties being controlled by the energy of the depositional environment, and (3) hematite cement, an uplift-related diagenetic feature not present in the subsurface. Hematite was preferentially precipitated along zones of permeability contrast, including sequence boundaries, complicating the determination of changes in primary depositional permeability at important flow-unit boundaries.

In contrast, the permeability of Fall River reservoir sandstones at a depth of 4 km has a bimodal distribution and is controlled by ductile-grain content, grain size, and quartz-cement volume. Quartz cementation and compaction due to ductile-grain deformation were more extensive in the estuarine facies, resulting in a greater permeability reduction during burial diagenesis than was observed in fluvial sandstones. Abundant stylolites developed in sandstones with clay partings or mud clasts, features that are common in estuarine sandstones. The stylolites acted as an internal source of silica. Sandstones near the stylolites typically have low permeability that probably resulted from above-average volumes of quartz cement. Diagenesis increased the coefficient of variation of permeability in all facies in the subsurface. This diagenetic overprint should be considered if outcrop permeability data are used to model a subsurface reservoir having a very different burial history.