Energy Research, Department of Geology and Geophysics
University of Wyoming
Falling Stage Systems Tract in Siliciclastic Systems and Its Control on Reservoir Architecture and Exploration Strategies
Until recently, sequence stratigraphic models have attributed systems tracts to periods of relative sea level rise, highstand, and lowstand. Recent studies, including high-resolution reflection seismic profiles in late Quaternary shelf-margin deltas, as well as outcrop and core studies of both clastic and carbonate systems, have identified a distinctive falling stage systems tract - the FSST. This is the logical counterpart to the transgressive systems tract and in many shelf to shelf-margin settings the FSST accounts for the bulk of the delta-front sandstones in the succession. For example, most of the shoreface deposits of such well-known Cretaceous units from the US Western Interior as the Gallup (New Mexico), Desert, and Castlegate (Utah) Sandstones, represent the falling stage systems tract. The FSST lies above and basinward of the highstand systems tract, yet below the sequence boundary and the overlying lowstand systems tract. Seismically, the FSST is characterized by stratal offlap. The three other systems tracts are all characterized by onlap.
The existence of an FSST is dictated by logic, documented in numerous studies, and critical in providing the kind of predictive capabilities that makes sequence stratigraphy useful. Logic, in the form of maintaining sediment balance, links the FSST intimately to the formation of the overlying sequence boundary. Sequence boundaries develop gradually over finite periods of time during sea level fall. On the landward side of the regressing shoreline there is a gradually widening zone of sediment bypass. The sediment that is bypassed feeds an advancing and descending shoreface, the falling stage systems tract. If the descending shoreface reaches a shelf edge, some sediments will be fed into a deepwater fan, if not, the entire FSST and subsequent LST will remain on the shelf or ramp. The final stage of shoreline regression is typically associated with a resumption of relative sea level rise, and onlap of strata above the sequence boundary. This is the LST. Examination of numerous outer-shelf, high-resolution seismic reflection profiles document that this ‘final’ phase of lowstand progradation may account for 5% to 15% of the volume of regressive deltaic sediments. Moreover, the lowstand delta front sediments are generally separated from the underlying FSST delta front by the basinward, conformable part of the sequence boundary. Casually walking out an outcrop or correlating shoreface sandstones in well logs or core, therefore, generally fails to reveal this most basinward part of the sequence boundary.
Perhaps the main reason why so many stratigraphic cross-sections fail to recognize the sea level fall that may have attended delta front progradation lies in the conventional choice of stratigraphic datum. As datum one often chooses a pronounced lithologic boundary, such as a ravinement surface that separates underlying fluvial and coastal plain deposits from overlying marine sediments (often mudstones). This datum directly truncates the lowstand delta front sandstones at the basinward end of the cross-section (where the transgression begins), but overlies it by the combined thickness of the LST and the nonmarine part of the TST farther landward, due to the subsidence that occurred during transgression. Consequently, of course, the regressive delta front sandstones beneath will appear to climb whereas, in fact, progradation occurred during a preceding sea level fall
What difference does it make? An early, astute study of the Kenilworth Member of the Blackhawk Formation in the Book Cliffs of Utah asked rethorically “where have all the lowstands gone?”. The answer that study correctly provided was that the lowstands were right there, yet they were not recognized because they appeared to climb relative to this non-horizontal, time-transgressive datum. Lowstand sandstones are in high demand as exploration targets, thus knowing their location in a sequence is critical. Knowing where to extend drilling downdip beyond the basinal pinch-out of deltaic sands and where not to do it has great impact on every E&P company’s finding cost and Wall Street’s perception of the soundness of their science.
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