Facies Relationships and Statistical Measures for Reservoir Heterogeneities in the Cretaceous Ferron Sandstone, Central Utah

Ann Mattson and Marjorie A. Chan

The Cretaceous Ferron Sandstone of the Mancos Formation is a wave modified fluvial-deltaic system containing seven parasequence sets. The lower two seaward stepping parasequence sets Kf-1 and Kf-2, are the focus of this reservoir heterogeneity study. Statistical summaries of lithologic parameters are used to evaluate the distribution of permeability data for up-scaling in fluid flow simulation. The database consists of measured sections, cored drillholes, permeability, petrography, and scaled photomosaics. The area is studied at two scales: (1) 2.5 × 2.9 km covering the Kf-1 and Kf-2; and (2) 0.8 × 0.8 km centered on prograding dinoforms in a parasequence within the Kf-1.

Four facies are characterized in terms of sedimentary structure, grain size, bedding thickness, % sandstone, and geometry. The proximal delta front facies is >95% fine-grained, horizontal and trough cross-stratified sandstone. The medial delta front facies is 85% fine- to very fine-grained, horizontal-stratified sandstone 15% shale. The distal delta front facies is 60% very fine-grained sandstone 40% shale) and contains the highest degree of burrowing. The mouth bar facies is > 99% medium- to fine-grained sandstone, and is dominantly trough cross-stratified. Petrographic samples from each facies indicate complex diagenesis with up to 5 stages of cementation. Highest permeabilities (attributed to secondary porosity) correlate to the coarsest sandstone lithologies. Permeabilities n fine-grained sandstones are uniformly low (several md) with the exception of some secondary porosity in bedding-parallel, gypsum microfractures.

These statistical measures allow quantification of reservoir heterogeneity at the parasequence level. Lithofacies maps allow visualization of these data. The Kf-1 clinoform surfaces (2-D) are generated deterministically from photomosaics, and Kf-2 unit surfaces (3-D) are generated by kriging data points derived from photomosaics and well data. Relationships between permeability and geologic field data can be used to populate geometric units for reservoir simulation.

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