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Daniel L. Pearson1, Thomas R. Moore1
(1) Phillips Petroleum Company, Bartlesville, OK

Abstract: Coal as a Thermal Insulator and Oil Source Rock

Vitrinite reflectance (%Ro) jumps occur across the coal-bearing Fruitland Formation, San Juan Basin, northwestern New Mexico, and are attributed to coal’s low thermal conductivity. Other variables that can affect %Ro include: 1) hydrogen-rich (perhydrous) vitrinite, 2) vitrinite that has adsorbed liquid hydrocarbon, 3) variables in depositional environment, 4) convective versus conductive heat flow, and 5) systematic identification or standardization errors. Some well sections show marked %Ro increases across the coal-bearing interval. One Fruitland well section increases from 1.04% to 1.14% across 65 feet of core bearing 28 feet of clean coal. Modeling shows this %Ro increase possible with low thermal conductivity.

Thermal maturity is an important quality factor in coalbed gas reservoirs for volumes of methane generated and adsorbed. Published data in coal methane basins are normally given as single %Ro values per well location. If this value is from high within a thick coal-bearing section, gas resources can be markedly underestimated; if from the bottom only, possibly overestimated. Such reserve estimates can vary 5 to 10%.

Coals with abundant liptinite can source waxy liquid crude. Liptinite concentrated in bands should give more efficient liquid expulsion. Some coals studied from San Juan Basin wells have liptinite maceral contents of 13% and produce minor amounts of waxy liquid (pour point 70° F) crude; just enough to be a nuisance. There is evidence that presence of liquid hydrocarbons negatively impacts the coal’s gas sorption capacity and ability to produce coal seam gas. Coals may need 15 to 25% relative abundance of liptinite macerals before significant liquid expulsion occurs.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana