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Facies Description, Depositional Environment and Petroleum Potential of the Lyons Sandstone, Colorado Front Range and Northeastern Denver Basin

Abstract

Abstract

The Permian (Leonardian) age Lyons Formation, composed mostly of fine- to medium-grained well-sorted quartz sandstone, crops out in the Front Range and is present in the subsurface of the Denver Basin in northeastern Colorado. This unit, 0-25 m thick, increases in thickness towards the south, and shows a regional differentiation in cementation patterns: the thicker portions of this unit in the south, but only in outcrop, are generally red, calcite-, clay and quartz cemented with hematite stains whereas in the northern portion of the study are and the subsurface this unit is white to grey with dolomite and anhydrite cement. Some distinct facies changes are also observed in this unit: in the northern portion of the study area, the sandstones are characterized by ripple marks, convolute lamination, water-escape structures and synsedimentary deformations on top of most units showing distinct large-scale cross bedding and preservation of the original dune relief. The southern outcrops, in contrast, show a distinct trend of sub-critically climbing translent strata and planar beds in sand sheet deposits, and pin-stripe lamination in lee side dune deposits in the lower part of the succession with overlying large-scale cross beds with a sharp planar top contact, and small-scale structures are largely absent. This study therefore interprets the Lyons Formation deposition as having taken place in a basin that deepened to the south where subsidence was higher than in the north. Large-scale cross beds in dune structures, well-sorting in individual lamina, sharp contact with underlying fluvial Fountain Formation indicate that Lyons deposition was most likely eolian in origin. Nevertheless, northern outcrops show an initial deposition under partly wet conditions as reflected by the abundant water-escape structures. The Lyons Formation is an active oil-producing unit in Northeastern Denver Basin. Oil traps in the Lyons sandstones are mainly caused by anticlinal folds and are therefore tectonic in origin. In addition, intense anhydrite cementation has formed porosity barriers which helped the entrapment of oil. Porosity of the formation varies from average of 3% to 12%, permeability is generally lower than 0.1 md. Oil production started in 1953 in Keota field, and at present there are five other fields with a total oil production of more than 12 million barrels.