Quantitative characterization of shales and their effects on permeability: Examples from tidally-influenced Ferron Sandstone, Utah

Abstract:

This study evaluates the proportion, length, and effective properties of thin shale beds and drapes in tidally-influenced channels within a compound valley fill in the Cretaceous Ferron sandstone Member of the Mancos Shale Formation. The study outcrop comprises of three valley systems. Light detection and ranging derived digital outcrop models have been used to characterize shale length, width, thickness and frequency of each valley systems. Shale geometry and frequency calculated for the valley fills correlate with the degree of tidal influence and channel sinuosity. The long and infrequent shales in Valley-1 (mean length 7.8 m; mean frequency 1.5 m^-1) were deposited in a low tidal influence and braided setting. On the contrary, the short and frequent shales in Valley-2 and Valley-3 (mean length, 4.3 m and 4.1 m; mean frequency 2.7 m^-1 and 3.5 m^-1, respectively) were deposited by more tidally-influenced single-thread meandering rivers. To estimate the effect of shales, a sand/shale model was utilized. The unique character of each architectural unit within the incised valley fills resulted in different k_v/k_h distributions. The valley fill deposits, V1, V2, and V3 had an average k_v/k_h ratio of 0.074, 0.016 and 0.011, respectively. It was demonstrated that the extent of tidal influence determined the shale character of different compartments of the compound fluvial valley fill, which, in turn, uniquely described the potential for fluid flow through these compartments. Therefore, it is important to consider the shale character and related permeability distributions in fluvial valley fill deposits when modeling analogous reservoirs.