In the Paradox Basin of southwest Colorado, a major facies change from thick, porous algal dolomite to thin, tight anhydrite occurs within the Desert Creek zone of the Pannsylvanian Paradox Formation. This northwest-trending algal mound belt is subparallel to and separated from the ancestral Uncompaghre mountain front by lagoonal anhydrite and nearshore arkosic fanglomerates. Carbonate rocks attain a maximum gross thickness of 200 ft., which can include 100 ft. of reservoir quality dolomite. These rocks have an updip and abrupt eastward facies change to thin evaporites which form a regional master trap for potentially significant hydrocarbon accumulations. Reservoir parameters and trapping mechanisms are similar to Utah's Aneth Field, where oil production exceeds 340 million barrels.

The subsurface stratigraphic changes have been seismically modeled with a microcomputer to define the extent of each facies. The modeling is based on sonic logs from two wells that exhibit the end-member facies of thick, porous dolomite versus thin, tight evaporite. An interpolation routine creates intermediate logs between these end-members, thereby defining the geometry of the model. The depth model is convolved with a zero-phase wavelet of 10/20=40/60 hz, and the result is an extra seismic cycle where reservoir thickness exceeds 40 ft. Updip loss of the cycle defines the transition from reservoir dolomite to trap anhydrite.

The modeling demonstrates that this major facies change can be detected at frequencies reasonably attainable by modern seismic methods. Regional subsurface stratigraphic analysis and modern seismic data provide an integrated approach to hydrocarbon prospecting in the Pennsylvanian Desert Creek zone.

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