PALEOTOPOGRAPHIC CONTROLS ON CEMENTATION IN THE PERMIAN CEDAR MESA SANDSTONE IN THE CANYONLANDS OF SE UTAH

The Permian Cedar Mesa Sandstone is well exposed in the Canyonlands of southeast Utah. This 300 meter thick unit grades laterally from erg interior sands to erg margin sands and intertongues with fluvial deposits of the time-equivalent Cutler Formation. In the study area the Cedar Mesa exhibits dramatic red and white bands in sands showing consistent grain size. Meter-thick red wind-rippled sandstones are separated by similarly thick white tongues of avalanche strata. A three meter-thick white sandstone is interrupted by 100-200 meter wide red intervals which formed in interdunal lows. As white sands are produced from the migration of hydrocarbon-bearing fluids, this color change illustrates potential heterogeneities in oil reservoirs.

A 20-25 meter thick unconformably bound eolian sandstone was mapped in three dimensions over a 5 km² area in the Needles District of the Canyonlands using GPS. After correcting for a regional tectonic dip of 2° to the northeast, 3-D maps of surfaces within the sandstone were compared to investigate paleotopography of the dunefield. 234 GPS points were used to delineate bounding surfaces which define packages of eolian dunes, interdunes and fluvio-lacustrine units.

Twenty-five rock samples collected characterize different depositional and diagenetic facies. White sandstone bodies in the Cedar Mesa underwent less early cementation and possessed higher porosities at the time of migration. More intense cementation by iron, carbonates and clays reduced sandstone porosity and prevented fluid invasion of the red sandstones, preserving their color. Large-scale and small-scale bounding surfaces, many at high angles to primary bedding serve as effective barriers to fluid flow. Topographically-defined depositional changes also obstruct the flow of hydrocarbon-bearing fluids.