Distribution and Occurrence of Uranium in Reservoir Rocks

Miles E. Denham, Thomas T. Tieh

Fission tracks were induced by neutron flux from petrographic thin sections of the Lower Pennsylvanian Morrow sandstone from New Mexico and the oolitic limestone of the Upper Jurassic Smackover Formation of east Texas to investigate the occurrence of uranium in these rocks. The results show that uranium concentrates in certain phases during diagenesis and hydrocarbon migration.

The lower Morrow sandstone, a subarkose to quartzarenite, has undergone multiple episodes of precipitation and dissolution of carbonate, silica, and clay minerals. Uranium is associated with late-stage clay minerals and quartz overgrowths. Concentrations of uranium are apparent in cleavage planes of residual carbonate cements, suggesting dissolution of carbonate by fluids relatively enriched in uranium. The uranium-bearing phases show homogenous track densities.

The upper Smackover consists of oolitic limestone sequences with several stages of authigenic carbonate cement. Samples from two cores were subjected to fission track mapping. One core, from a producing gas well, is stained with oil whereas the other, from a dry hole, shows no evidence of hydrocarbon migration. Fission track density is much higher in the samples from the producing well occurring, however, as numerous "point sources" (small areas of high track density) associated with organic material. This suggests the concentration of uranium varied during the influx of the organics. Track density is much lower in the oolitic limestones of the dry hole--tracks occurring largely along ooid-cement boundaries. This uranium was emplaced early in the diagenesis of these rocks, either adso bed on the surfaces of the ooids or released during cement recrystallization. Thus, the fission track mapping technique is potentially useful for investigating chemical changes and migration history of pore fluids in the subsurface.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.