Abstract: Oil Potential of Mesozoic Sediments Beneath Independence Mountain Thrust Fault, North Park, Colorado

Gerald M. Park

Oil is produced from more than 60 wells in North Park basin, Colorado. Triassic, Jurassic, and Cretaceous strata have produced more than 12 million bbl of oil and 500 MMcf of gas. Independence Mountain is a large block at the northern end of the North Park basin that has been thrust southward more than 10 mi (16 km) and has overridden and covers more than 90 sq mi (230 sq km) of these oil- and gas-producing strata. The rocks underlying Independence Mountain have the same characteristics as those which are being produced in North Park today. They also have been subjected to the same structural forces that formed the traps in North Park basin.

Structural relief on the Precambrian surface in North Park ranges from -8,000 ft (2,438 m) along the Spring Creek fault in the basin to over 10,000 ft (3,050 m) in the highlands of the Park and Medicine Bow Mountains. This relief is related to the Laramide orogeny of late Mesozoic and early Tertiary time.

Most of the folding and faulting in North Park took place after the deposition of the Coalmont Formation of Paleocene age. The last stage of the Laramide orogeny was manifested by the Independence Mountain thrust fault which cuts across all earlier structures. The Independence Mountain thrust can be traced for more than 40 mi (64 km) from northwest of Pearl in the Park Range, around the nose of Independence Mountain, and northeast to the crest of the Medicine Bow Range. Positive geophysical evidence, nearby drilling, and geomorphic analysis indicate that several thousand feet of Mesozoic rocks lie beneath the thrust plate.

The northwest-trending subparallel alignment of the oil-producing structures in North Park continues under the upper plate of the Independence fault. Oil-stained sandstones were cut in two drill holes on an open structure 4 mi (6.4 km) south of the thrust fault. These holes contained 30 and 89 ft (9 and 27 m) of oil-cut sand, respectively. Oil is produced in the area from the Pierre, Benton, Dakota, Lakota, Morrison, and Sundance formations.

The flat plane of the thrust indicates potentially productive strata are present beneath the thrust plate at a depth ranging from 1,500 ft (457 m) in Wheeler Creek to 3,500 ft (549 m) along the state line at Sixmile Creek. With modern technology this overthrust hidden trap will soon be producing.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado