Print this pageComplex Coalbed Architecture in the Fruitland Formation: Northern San Juan Basin, Colorado and New Mexico
NANCE, H. S.
Coal and sandstone deposition along the western margin of the Western Interior Seaway combined to produce more complex coalbed architecture in the Fruitland Formation (Upper Cretaceous) of the San Juan Basin (SJB) than previously documented. Over 1,200 well logs from northwestern SJB were correlated to generate thickness and structure maps of six stratigraphically sequential, operational-scale (net coal approximately 35 ft) coal units. Younger coal units were deposited more paleo-basinward (northeast) in correspondence with the northeast migration of upper Pictured Cliffs shorelines. Net coal maps of operational-scale units show that depositional-dip-aligned (northeast) coal thickness trends dominate, rather than do the strike-aligned (northwest) trends that are suggested in previous studies by maps based on total coal thicknesses. Dip-aligned thins in net-coal maps record thinning of coal beds over underlying channelbelt thicks or pinchout of coal beds against channelbelt or shoreface sandstone bodies. Similarly trending thins in maximum-coal maps additionally reflect coalbed splitting around stratigraphically intervening fluvial sandstones.
Dip-elongate coal trends help explain hydraulic recharge of SJB producing coal units in the high-productivity fairway, which are down dip of recharge areas at the northern margin of SJB. Differential compaction around sandbodies produced unique structural configurations for each coal bed and probably localized enhancement of cleats, thus potentially producing unique patterns of permeability enhancement and conventional trapping for each coal unit.