"J" sandstones in northeastern Colorado were deposited primarily in 2 delta systems. The large eastern delta prograded toward the northwest from northwestern Kansas. A smaller western delta prograded eastward between Denver and Colorado Springs.

The eastern deltaic sequence is more than 200 ft thick in places and consists of 3 distinct genetic units, in ascending order, (1) marine delta front of low-energy marine clayey sandstones and clayey siltstones; (2) nonmarine delta plain of alternating sandstone, siltstone, and shale deposited in distributary channels, interdistributary bays, marshes, swamps, natural levees, and crevasse splays; and, (3) transgressive marine of low- to high-energy sandstones and shales.

The western deltaic sequence is up to 190 ft thick and consists predominantly of 1 genetic unit, a nonmarine delta plain, similar to that in the eastern delta. Core and log controls indicate that well-developed lower delta-front sandstones are absent in most areas. Upper marine shoreline sandstones are well developed on the north flank of the western delta.

Eastern and western delta sandstones apparently interfinger in some areas. Sediments of the 2 deltas can be distinguished by mineralogy, because western delta sediments are more mature than eastern delta sediments.

Oil reserves in the southwestern part of the eastern delta total about 250 million bbl. The major part of these reserves is trapped in transgressive marine zones at fields such as Adena, Plum Bush Creek, Little Beaver, and Badger Creek. Some additional reserves are trapped in delta-plain channels. Very little oil has been discovered in delta-front sandstones under the eastern delta plain.

Oil accumulations in the western delta have been found in delta-plain channel sandstones at Peoria, North Peoria, Jamboree, Dull Knife, Hombre, Latigo, Byers, and Bennett fields. Gas has been discovered in the thick upper marine shoreline sandstones at Totem field and south of Totem field.

Further exploration should result in the discovery of additional oil and gas accumulations in delta-plain channel sandstones near the updip edge of the western delta plain. Also, more oil and gas accumulations

End_Page 658------------------------------

should be found in the upper marine shoreline sandstones wherever they are well developed.

End_of_Article - Last_Page 659------------