New Mexico Geological Society Annual Spring Meeting — Abstracts

The New Mexico Bureau of Mines and Mineral Resources (NMBMMR), with outside participation, has been involved in long-term Coal quality and resource studies for the last 30 years. A continuing 13-year cooperative project with the U.S. Geological Survey to enter data into the National Coal Data System (NCRDS) has led to the development of a coal data base for New Mexico, in particular for the San Juan Basin, that includes information from public and private sources on both coal quality and coal resources. From 1983 through 1988 the NMBMMR was the lead agency for a coal quality drilling project in the San Juan Basin funded by New Mexico Research and Development Institute and several coal companies. Both of these studies examined the major coal-bearing sequences in the San Juan Basin within the Late Cretaceous Fruitland, Menefee, and Crevasse Canyon formations. Economic interest in the Fruitland and Menefee coals has created a data base sufficient for examining the differences and trends in the quality characteristics of these two major coal-bearing sequences.

Fruitland Formation coals are economically viable because they are relatively thick (5-30 ft), have some lateral continuity, and have a comparable or better overall quality than other coals in the basin. These high-ash, low sulfur coals display a decrease in coalification towards the southeast, away from the San Juan volcanic complex. The degree of thermal maturation is particularly evident in the geographic trends of the moisture, calorific value, and vitrinite reflectance (R_o). The sulfur values of the Fruitland coals,. although rarely greater than 1%, is significantly lower in the southern part of the basin; this is apparently a function of the depositional environment and outcrop trend. The northwest trend of the Fruitland Formation is nearly perpendicular to the Cretaceous shorelines. The coals, deposited en echelon behind barrier beach sandstones, represent peat swamps that developed during several minor oscillations in the overall regressive shoreline. The southern Fruitland trend subparallels the Cretaceous shorelines and the coals here represent peat formed behind barrier beach sands and peat that developed on the lower coastal plains as the coastline retreated to the northeast.

Menefee Formation coals in the southeastern San Juan Basin are of economic interest. The upper coal member peat swamps developed behind an overall transgressive shoreline during periods of standstill with minor regressive oscillations creating massive barrier beach sandstones with intertonguing nonmarine coal-bearing units. These low-ash, moderate sulfur coals have a significantly lower ash yield and volatile matter/fIxed carbon ratios than the older Cleary Coal Member coals, also in the Menefee. The regressive Cleary coals were deposited in an environment similar to that of the Fruitland coals, although the Cleary coals have a significantly lower ash yield and greater sulfur values due to differences in rate of shoreline shift. Variations in the degree of thermal maturation are not evident in the surface minable coals of the upper and Cleary Coal members in the southeastern San Juan Basin, although there is a significant increase in the coalification of the Monero field Menefee coals on the northeast edge of the basin. Higher calorific values and R_o and low moisture values of the Monero coals indicate heat advection by groundwater flow from the San Juan Volcanic complex (Clarkson and Reiter, 1988) greatly influenced the degree of thermal maturation of these coals.