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Geologic controls on transgressive-regressive cycles in the upper Pictured Cliffs Sandstone and coal geometry in the lower Fruitland Formation, northern San Juan Basin, New Mexico and Colorado
1Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713-8924; [email protected]
2Department of Petroleum Engineering, Texas AM University, 3116 Tamu, College Station, Texas 77843; [email protected]
Three upper Pictured Cliffs Sandstone tongues in the northern part of the San Juan Basin record high-frequency transgressive episodes during the Late Cretaceous and are inferred to have been caused by eustatic sea level rise coincident with differential subsidence. Outcrop and subsurface studies show that each tongue is an amalgamated barrier strand-plain unit up to 100 ft (30 m) thick. Successive upper Pictured Cliffs tongues display an imbricate relation and are offset basinward, reflecting net shoreline progradation northeastward. Upper Pictured Cliffs barrier strand-plain sandstones underlie and bound thickest Fruitland coal seams on the seaward side. Controls on Fruitland coal-seam thickness and continuity are a function of local facies distribution in a coastal-plain setting, shoreline positions related to transgressive-regressive cycles, and basin subsidence. During periods of relative sea level rise, the Pictured Cliffs shoreline was temporarily stabilized, allowing thick, coastal-plain peats to accumulate. Although some coal seams in the lower Fruitland tongue override abandoned Pictured Cliffs shoreline deposits, many pinch out against them. Differences in the degree of continuity of these coal seams relative to coeval shoreline sandstones are attributed to either differential subsidence in the northern part of the basin, multiple episodes of sea level rise, local variations in accommodation and progradation, stabilization of the shoreline by aggrading peat deposits, or a combination of these factors. Fruitland coalbed methane resources and productivity are partly controlled by coal-seam thickness; other important factors include thermal maturity, fracturing, and overpressuring. The dominant production trend occurs in the northern part of the basin and is oriented northwestward, coinciding with the greatest Fruitland net coal thickness. Similar relationships between trends of thick coal seams of coeval origin with stacked shoreface sandstones exist in other Western Interior basins in the United States and serve as models for coalbed methane exploration in other basins of the world.
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