Forward Modeling of Late Pleistocene Shelf-Edge Deltas Offshore Louisiana

Susan E. Nissen, Janet M. Combes, Robert W. Scott

Sand-prone deltas were deposited at the Louisiana shelf edge during Late Pleistocene glacio-eustatic lowstands. High-resolution multichannel seismic record sections, which resolve beds on the order of 3 to 5 feet at 150 milliseconds (370 feet or 112 meters) below the seafloor, document in detail the prograding clinoforms within two deltas. Regional seismic stratigraphic boundaries traced on the seismic data have been tied to important sequence stratigraphic contacts in four coreholes on the outer shelf and upper slope. The ages of these contacts are dated by the graphic correlation integration of fossil tops and oxygen isotope events in the cores. The correlation provides data for the timing of a sea level curve for this region. The integration of the sedimentologic and b ostratigraphic data by modeling is a high-precision technique for defining the magnitude of relative sea level changes.

In order to analyze the detailed interrelations between sea level and stratal geometries, a two-dimensional forward stratigraphic model of the Louisiana shelf-edge from 168 ka to the present was constructed using the University of South Carolina's SEDPAK program. Forward stratigraphic modeling creates a computer simulation of the stratigraphic cross section using a set of user-defined input parameters, including initial basin shape, a sea level curve, subsidence rates, and sediment supply. For the present model, the initial basin depths were determined from paleoecological analysis of the cores; the sea level curve was modified from published curves to incorporate the oxygen isotope data; and subsidence and sediment supply were held constant through time. The resulting model effective y reproduces the two lowstand deltas and the overlying transgressive-highstand systems tracts identified on the seismic sections and within the coreholes. Modeling indicates that most of the stratal geometries in this cross section can be produced solely by sea level changes. Modeling also predicts the distribution and lateral extent of lowstand sands between the corehole control points.

Two-dimensional forward models can predict stratal geometries within reservoirs and potential stratal discontinuities that may define traps and seals. The modeling process constrains the amount of sand and clay within the prospective section. Modeling also provides estimates of rates of subsidence, sedimentation, and burial to be used in hydrocarbon systems modeling.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995