SCOTT, ANDREW R.¹ and H. S. NANCE^2
1Bureau of Economic Geology, The University of Texas at Austin
²Consulting Geologist, Leander, TX

Abstract: Factors Affecting Geochemical and Production Heterogeneity in Coal Beds

A detailed evaluation of coalbed geometry and coal gas and water geochemistry was performed in the northern San Juan Basin to assess hydrogeologic factors that affect production heterogeneity. Production and geochemical heterogeneity reflect the generation, migration, and trapping of coal gases. Water and coal-gas production trends are controlled by a complex interplay among coalbed geometry and continuity, structure, hydrodynamics, and gas generation.

The depositional fabric of five informally defined coal packages strongly affects water and coal-gas production trends. The basinward flow of low-chloride ground waters and distribution of geochemical anomalies are related to coal-bed geometry and continuity, local structure attributes, permeability contrasts, and multiple recharge events. The amount of CO₂ in the coal gases and the d¹³C isotopic values Of CO₂ and bicarbonate are a function of bacterial activity and residence time.

Areas of greatest water production lie near the recharge zone along the north and west margins of the basin and in the high-productivity fairway to the south. The east limit of higher water production (>4 bbl/ft) corresponds to the pinch-out of aquifer coals against fluvial channel sandstones. Higher water production trends in the high-productivity fairway correspond to synclinal axes, whereas higher coal-gas production in the fairway generally occurs south of water-production trends. Higher coal-gas production is also associated with a large structural promontory in the north part of the basin. A coalbed methane producibility model, developed for basinwide evaluation of coalbed methane potential, can also explain local heterogeneity of water and coalbed methane production.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas