Searching for Modern Ferron Analogs and Application to Subsurface Interpretation

Abstract A quantitative approach to selecting modern-depositional settings analogous to those of the Cretaceous Ferron Sandstone is presented as well as an approach to using these analogs to improve subsurface interpretations. Paleotectonic, paleogeographic, and climatic setting of the U.S. Western Interior are integrated to estimate the size of the Ferron drainage network to be 50,000 km 2(19,000 mi 2). Estimates of flow depths, flow velocities, and channel cross-sectional areas suggest maximum trunk river paleo-discharge was on the order of 50 × 10 9m 3/year (250 × 10 9ft 3/year). Analogous modern examples include moderate-sized rivers that drain active mountain belts like the Po (Italy), Rhône, (France), and Ebro (Spain). Continental-scale systems, such as the Mississippi (U.S.A.), Niger (West Africa), Amazon (South America), and Nile (North Africa) deltas, are not appropriate analogs. Incised-valley systems within the Ferron are comparable in depth (about 30 m [100 ft]) to distributary channels in more continental-scale systems. A key difference is that Ferron valleys are filled with multiple channel deposits, with individual channel fills less than about 9 m (30 ft) deep. The relatively uniform size of distributary channels suggests that Ferron rivers experienced only a few orders of bifurcation as they flowed across the delta plain. Shoreline and delta-front deposits are wave-influenced. Locally, the basal Ferron deltas were fluvial dominated, although these fluvial-dominated lobes may lie on the downdrift side of asymmetric wave-influenced deltas, similar in plan to the Brazos, Ebro, and Rhône deltas, and to the southern St. George lobe of the Danube delta. Sizes and geometries of various depositional bodies in modern deltas better constrain estimates of inter-well heterogeneity in subsurface correlations. Correlations of Ferron core and wireline-log datasets are compared with the more complete stratigraphy documented in outcrop. Although the broad clinoform geometry of strata in these deposits can be recreated from subsurface correlations, specific parasequences could not be reliably correlated using only the subsurface data. The subsurface-to-outcrop comparison demonstrates a risk of over-correlating reservoir compartments (i.e. non-connected bodies) separated by minor flooding shales within complex, offlapping fluvial-deltaic reservoir deposits. The Ferron outcrop data provides a measure of uncertainties in correlation of subsurface analogs.