TO CLINOFORM OR NOT TO CLINOFORM? THAT IS A KEY QUESTION FOR CORRELATING IN SHOREFACE-TO-SHELF SYSTEMS

One of the best places to examine and test shoreface-to-shelf correlation scenarios is the Campanian Book Cliffs of eastern Utah. This region is distinguished by world-class shallow marine outcrops coupled with an extensive subsurface data base, which allows for high-resolution sequence stratigraphic correlation on a variety of scales. Two different correlation methods have been proposed for the Book Cliffs shoreface-to-shelf systems: (i) gradual basinward thinning with no clinoforms, or (ii) abrupt thinning with clinoforms, downlap and pinch-out. The latter method has driven many shoreface-to-shelf correlation over the years.

A variety of stratigraphic units in the Star Point Formation, Blackhawk Formation and Lower Castlegate Sandstone have been correlated over a wide area of eastern Utah both in outcrop and in the subsurface. Most member- and parasequence-scale rock packages show a gradual basinward thinning not the abrupt clinoform-style pinch-outs popularized in previous studies. No dramatic thickness changes are observed, as demonstrated by the average 30 % reduction in thickness over 40+ kilometers. A simple set of correlation rules and guidelines, based on sequence stratigraphic principles, have been distilled for similar shoreface-to-shelf systems: (i) gradual basinward thinning of parasequences, (ii) abrupt basinward decrease in sandstone with a concomitant abrupt increase in mudstone, (iii) basinward compartmentalization of parasequences into bedsets, and (iv) no steeply dipping clinoforms. The gradual basinward thinning with no clinoforms correlation scenario is most appropriate in strandplains, wave-dominated shoreface-to-shelf environments, and the distal portions of wave-dominated deltas. In contrast, the abrupt basinward thinning with clinoforms correlation scenario is better suited to river-dominated deltaic environments and the proximal portions of wave-dominated deltas.

The subsurface implications of applying the correct correlation scenario include the following: (i) avoiding cycle or parasequence skips, (ii) development of a tight correlation framework and robust static geological model, (iii) increased likelihood of history matching the dynamic reservoir model, (iv) improved prediction of fluid flow, and (v) optimization of development activities.