Thermal Alteration Indices vs. "Pure" Geochemistry--Two Tales of a Rock

Eddie Belle, William C. Cornell, William D. Norland

A series of samples of the Mesilla Valley shale (Albian, Cretaceous) was collected along radial traverses around a 47-m.y.-old andesite intrusion, Cerro de Cristo Rey, in Dona Ana County, New Mexico. Sample distance from the intrusion extended to 750 m. Thermal alteration indices values of palynomorphs range from 5 in samples less than 50 m from the intrusion to 1-2 in samples more than 400 m from the intrusion. TAIs suggest that liquid hydrocarbons should have been generated in the shale in a 250-m wide zone beginning approximately 200 m from the shale-andesite contact.

Geochemical studies of the same samples were conducted using the following techniques: gas chromatography of bitumen extracts, electron spin resonance of extracted kerogen, and pyrolysis gas chromatography (pyrochromatography) of extracted kerogen. These studies indicate the kerogen in the Mesilla Valley shale is overmature and cannot generate significant liquid hydrocarbons.

We believe these disparate results are caused because in this study, thermal alteration indices (TAI) analysis concentrates on autochthonous kerogen whereas the geochemical analyses are dominated by allochthonous kerogen from two sources. Kerogen from syndepositional terrestrial plants is present, as is kerogen reworked from older and more mature sedimentary rocks.

AAPG Search and Discovery Article #91034©1988 AAPG Southwest Section, El Paso, Texas, 21-23 February 1988.