NEW FIELD AND GEOCHEMICAL EVIDENCE ON THE NATURE AND EXTENT OF THE LISBURNE PETROLEUM SYSTEM (NORTHERN ALASKA)

The Lisburne Group consists mainly of Carboniferous carbonate rocks formed in a variety of shallow- and deep-water settings. It crops out throughout the Brooks Range and is found in subsurface beneath much of the North Slope in northern Alaska. Sulfur-rich oils found in a number of North Slope wells from the Barrow peninsula 200 miles east to Mikkelsen Bay are interpreted as derived from a Lisburne source. Our reconnaissance field and geochemical data provide information on generation and migration of hydrocarbons within the Lisburne petroleum system. Potential source rocks within the Lisburne in the western and central Brooks Range are deep-water black shales interbedded with radiolarites, carbonate turbidites, and local phosphorites (Kuna Formation and related units). These strata are more than 200 m thick in the Red Dog area (western Brooks Range) and at least 30 m thick at Skimo Creek (central Brooks Range); shales with total organic carbon (TOC) values greater than 2% (locally as high as 20%) make up more than two-thirds of these units. Coarse crystalline dolostones in shallow-water facies of the Lisburne that contain vuggy and intercrystalline pores and fractures filled with solid hydrocarbons are found locally throughout the western and central Brooks Range. Although much of the Lisburne outcrop is overmature for oil generation, some sections in the Howard Pass and Chandler Lake quadrangles (central Brooks Range) have thermal maturities within the oil window. Geochemical parameters in rocks with low thermal maturity indicate a good match between organic-rich shales of the Lisburne, hydrocarbon residues in porous dolostones of the Lisburne, and proposed “Lisburne oils” in North Slope wells. Fluid inclusion observations on diagenetic and fracture-filling cements and organic petrography of solid hydrocarbons in fractures in the Lisburne demonstrate that deeply buried, organic-rich units also generated natural gas in the western Brooks Range (Red Dog area), central Brooks Range (Lisburne #1 well), and central North Slope (Inigok #1 well). Structural and burial history studies suggest that Lisburne hydrocarbons were generated during the Early Cretaceous as a result of thrust sheet emplacement during the Brookian orogeny in the south and rapid burial to the north by sediments shed from the thrust sheets.