New Mexico Geological Society Annual Spring Meeting — Abstracts

Intrusion of the Muleros Andesite pluton (middle Eocene) into Cretaceous rocks in southern Dona Ana Co., N.M., resulted in an increase in the local geothermal gradient. Shale, siltstone, and limestone samples were collected in the Mesilla Valley Formation at 1.5 meter intervals along radial traverses extending outward from its contact with the Muleros Andesite. Particulate organic matter (kerogen) was freed from these samples by acid maceration and has been examined, via light microscopy, to determine its thermal maturity, or Thermal Alteration Index (TAl). TAl values ranging from 1 - 5 record color changes from yellow to brown to black and indicate immature, mature, and metamorphosed facies of organic matter.

Within 10 meters of the Muleros Andesite/ Mesilla Valley Fm. contact, kerogen is highly volatilized and consists mostly of black, carbon-rich, inertinite particles whose TAl = 5.10 to 32 meters from the contact. dinoflagellate cysts, spores, pollen, and amorphogen are moderately to poorly preserved, are brown to brownish black, and have a TAl of 3 to 4.160 to 200 meters frpm the contact, color of the kerogen, including dinoflagellate cysts, spores, pollen, and amorphogen, ranges from orange to brown, TAl equals 2 to 3, and preservation is markedly improved.

These TAl values indicate that temperature exceeded 200°C up to 10 meters from the pluton, but was as low as 100°C 200 meters from the contact. Hydrocarbons generated by contact metamorphic effects have si nce been lost through erosion. Outside the metamorphosed zone, TAl values of 2 to 3 suggest that hydrocarbon generation may be occurring in part of the Mesilla Valley Fm. As the kerogen is chiefly of marine origin, the formation should be oil-prone.