Abstract

Four cored intervals of the Lower Pennsylvanian Morrow formation of the Empire South Field, Eddy County, New Mexico were studied with conventional methods to determine their diagenetic history. In addition, neutron activation techniques (induced fission track imaging and delayed neutron counting) were used to evaluate the occurrence of uranium, in these rocks, as a diagenetic indicator.

The Lower and Middle Morrow sandstones in the Empire South field area were deposited as fluvio-deltaic sediments in a marginal marine environment during the Pennsylvanian. Compositionally, these sandstones are fine- to coarse-grained subarkoses to quartz arenites. However, diagenesis has modified the original composition through alteration of detrital grains, cementation, dissolution, and precipitation of clay minerals.

The diagenetic sequence of events is: 1) slight physical compaction and the limited formation of quartz overgrowths, 2) complete cementation by carbonate, 3) dissolution of carbonate cement and formation of pore filling clays—mostly kaolinite, 4) development of Fe-rich carbonate cement and precipitation of pore filling chlorite, 5) second episode of quartz overgrowths with dissolution of Fe-rich carbonate cement. The later events (3-5 above) were interrelated with hydrocarbon maturation and emplacement. The alteration and replacement of detrital grains occurred throughout the diagenetic history of the sandstones.

During the latter stages of diagenesis uranium is mobilized from the hydrocarbon source rock and carried by pore fluids into reservoir rocks. In the Morrow this uranium is seen adsorbed onto authigenic clay minerals and organics and occurs with even greater abundance in stylolites. The occurrence of this uranium may be useful in interpreting the diagenesis of the Morrow sandstone and in tracing the migration pathways of pore fluids during late-stage diagenesis.