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The New Windsor field, on the west flank of the Denver basin 50 miles north of Denver, produces oil from two superposed stratigraphic traps. The shallower is an updip sand pinch-out of the Upper Cretaceous Sussex sand at 4,200 feet. The deeper is an updip permeability barrier in the Permian Lyons sandstone at 9,000 feet.
The discovery of the field provides an illustration of the successful integration of geophysical, stratigraphic, and structural geology. The first well, located on a seismic structure, found the Lyons tight, with an oil show, and encountered no Sussex. A second well, located down the northeast plunge of the structure, encountered porous, permeable Lyons and Sussex sands and proved potential stratigraphic traps. A third well, Calco's Brunner No. 1, which was located between the two, indicated commercial production from both the Sussex and Lyons and was completed for 108 barrels of 41° oil from the Lyons.
Seismic and subsurface data show the New Windsor structure to be a northeast-trending anticline approximately 6 miles long and 2 miles wide. The traps are formed by the nearly coincidental transection of the northeast plunge of the anticline by the Sussex pinch-out and the Lyons permeability barrier.
The Sussex, which is called the second or middle Hygiene along the Front Range and is correlative with the Sussex of the Powder River basin, consists of a series of coalescing bar-like sands 6-12 miles wide from east to west and at least 100 miles long north and south. Five wells are currently producing oil from the updip pinch-out of this "bar."
The Permian Lyons sand is fine-grained, cross-bedded and well cemented by silica and anhydrite. The only apparent stratigraphic change across the field is an abrupt loss of porosity and permeability from east to west. A dry hole east of production cored 62 feet of porous sand with 2,436 millidarcys permeability. A dry hole west of production had a total permeability of only 1.8 millidarcys. Three wells are currently producing oil from the Lyons.
Stratigraphic traps in the Lyons and Sussex sandstones are proved by the small but significant
accumulation of 1,250,000 barrels. The true potential of such reservoirs still lies in the minds of the exploration geologists.
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