Organic-Rich Triassic-Jurassic Rocks, Thermal Maturity Anomalies, and Oil and Gas Source Rock Potential, Brooks Range Disturbed Belt and Southern Colville Basin, North Slope, Alaska

By

C.G. Mull (Alaska Division of Oil and Gas), E.E. Harris, and P. Peapples (Alaska Division of Geological & Geophysical Surveys)

Geologic mapping and organic geochemical analyses in three widely separated areas along a 350 mile trend in the northern foothills of the central and western Brooks Range reveal a persistent pattern of lower thermal maturity in organic-rich Upper Triassic to Middle Jurassic rocks than in coeval rocks at the mountain front. At the mountain front, allochthonous Triassic-Jurassic rocks with up to 10% total organic carbon (TOC) and vitrinite reflectance >2.0 Ro in Type II kerogen are thermally overmature for oil and gas and are well into the zone for dry gas generation. Similar coeval parautochthonous rocks in the foothills are commonly in either the oil window or in the range of oil and wet gas generation, with 0.8 to 1.9 Ro. Rocks in the foothills belt apparently had a significantly shorter residence time at depth with a lower thermal history than the coeval rocks in the Brooks Range to the south or beneath the Colville Basin to the north. Early uplift of foothills thrust sheets containing Triassic-Jurassic source rocks was probably syndepositional with the Aptian-Albian Fortress Mountain, Torok and possibly Nanushuk Formations and predates Late Cretaceous and Tertiary uplift of the central and western Brooks Range. It sets the stage for complex combinations of trap timing, source rock maturation, and migration into local traps in the foothills belt, and longer distance migration in the Colville Basin. Dead and live oil shows and gassy odors are found in rocks ranging from the Carboniferous Lisburne Group to the Albian Torok and Nanushuk Formations.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.