Organic Facies of Some Mesozoic Source Rocks on Alaskan North Slope: ABSTRACT

Abstract

Geochemical screening (TOC and Rock-Eval pyrolysis) of picked cuttings of Mesozoic age from seven wells located between Prudhoe Bay and the Colville delta distinguished three types of organic facies and one subtype. Identification of the organic facies was based on the organic content of the sediments and their position in a modified van Krevelen diagram. This paper demonstrates relationships between organic facies of the sediments and their inferred depositional environment. The organic matter type and quantity serve to separate the Mesozoic stratigraphy into five intervals: (a) Shublik Formation; (b) basal Kingak Shale; (c) upper Kingak Shale; (d) Pebble Shale/Hot Zone; and (e) Torok/Seabee Formations. The hydrocarbon-generating potential and predicted hydrocarbon products differ considerably and are controlled by the sedimentary environment of each interval. The Shublik Formation in the study area was deposited on a carbonate platform with a deep basin lying to the south. High organic-carbon content and relatively high hydrogen content of the Shublik can be explained by preservation of marine organic matter in anoxic lagoons or local depressions. Deposition of the basal Kingak Shale and of the Pebble Shale/Hot Zone is the result of major transgressions during the Jurassic and the Neocomian, respectively. Both intervals were deposited as bottomsets of prograding sequences (prodelta) and contain high quantities of relatively hydrogen-rich organic matter. Their organic facies are the result of distal sedimentation coupled with high organic productivity and moderate to good preservation. The Fishbone Shale, deposited elsewhere in an environment similar End_Page 463------------------------------ to that of the basal Kingak and the Hot Zone, is not developed in the study area. The upper Kingak Shale and the Torok/Seabee Formations are of a similar organic facies that is characterized by relatively low quantities of hydrogen-poor organic matter. Both intervals were deposited as foresets in a progradational sequence. The former extended toward the south during the Jurassic and the latter toward the north during the Cretaceous. The organic facies of these intervals are the result of clastic dilution, increased input of terrigenous organic matter, and increased bioturbation during early burial. End_of_Article - Last_Page 464------------