Abstract: Simultaneous, Multi-Parameter Stratigraphic Inversion of the Mesaverde Clastic Wedge, San Juan Basin
CROSS, TIMOTHY A., and MARGARET A. LESSENGER, Colorado School of Mines
Simultaneous, multi-parameter stratigraphic inversion is a quantitative method of extracting values of process parameters (e.g., sea level, tectonic movement, lithosphere rigidity, sediment supply rate, topography) directly from stratigraphic data (e.g., facies, thicknesses, inferred bathymetry, grain size, ages, stratigraphic cycles). Tests with synthetic data demonstrated that the correct values of process parameters were recovered by stratigraphic inversion of limited data, and the solutions were uniquely bounded with very small uncertainties. Encouraged by these results, we performed similar experiments with well log, core and outcrop data from the Mesaverde clastic wedge of the San Juan basin, SW Colorado.
A basin-scale, dip-oriented stratigraphic cross section containing 62 closely-spaced wells along the northwest side of the San Juan Basin was the data base for the stratigraphic inverse modeling. Fifteen progradational/aggradational stratigraphic units are recognized, nine in the seaward-stepping lower Mesa Verde and six in the landward-stepping upper Mesa Verde. Each unit contains continental, shoreface and marine shelf strata. Data from 5 of the 62 wells were inverted, and the inversion results were tested against measurements from the other wells not used in the inversion.
The cross section shows that the shoreface sandstones in the lower half of the Mesa Verde have a stepped geometry of stratigraphic rise during progradation. The model shows the same style of progradation. The depositional limit of the shoreface, or the turnaround position from seaward- to landward-stepping divisions of the clastic wedge is predicted exactly by the model. Shorefaces of the upper half of the Mesa Verde form 8 to 15 km broad benches which step progressively landward. The model matches the number, position and dimensions of these benches. One of the most surprising results was that the model recreated the 55 km shift of the shoreface facies tract from the turnaround position to the position of the lowest landward-stepping bench.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah