Portability of Outcrop Permeability Data to Subsurface Reservoirs: Diagenetic Complications

Shirley P. Dutton, Brian J. Willis, I. N. Widya Dharmasamadhi, and Michael H. Gardner

Quantification of permeability structure in outcrop reservoir analogs documents the distribution of flow units and barriers important for reservoir management and simulation. However, differences in burial history and diagenesis of the outcrop analog and the subsurface reservoir must be quantified before outcrop permeability data can be used to model subsurface strata. A case study of the Lower Cretaceous Fall River Formation shows that permeability differences between facies are accentuated by diagenesis and permeability variance increases in deeper subsurface fields. 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. Outcropping sandstones were buried to 2 km before being uplifted at the end of the Cretaceous, but the reservoir sandstones remained at depths of 2 to 4 km throughout the Tertiary.

The main controls on permeability in outcrop sandstones are (1) ductile-grain content and (2) grain size, both properties controlled by the energy of the depositional environment, and (3) hematite cement, an uplift-related diagenetic feature not present in the subsurface. Hematite preferentially precipitated along zones of permeability contrast, particularly in finer grained deposits just below sequence boundaries. Permeability in Fall River reservoir sandstones at a depth of 4 km is controlled by ductile-grain content, grain size, and quartz-cement volume. Quartz cementation and compaction due to ductile-grain deformation were both more extensive in the estuarine facies, which lost permeability relative to fluvial sandstones during burial diagenesis. Diagenesis also increases the coefficient of variation of permeability in both fluvial and estuarine facies in the subsurface. This diagenetic overprint must be considered when outcrop permeability data are used to model a subsurface reservoir.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California