A Multiproxy Geochemical Approach to Unraveling Hydrocarbon Charge History of the Triassic Shublik Formation of Arctic Alaska

Abstract

The Triassic marine carbonate Shublik Formation is a key source rock for hydrocarbons in the North Slope of Alaska. Although lithological heterogeneity and thickness variability of the Shublik Formation is widely recognized, most of the literature refers to it as one source rock unit. In addition, previous studies of oil types in the vicinity of the Northstar Field suggested presence of “shaly” organofacies of the Shublik Formation, however there remains a gap between biomarker analysis of predicted “calcareous” and “shaly” oil types and source rock geochemistry.

This study of the Shublik Formation investigates how lithologic heterogeneity relates to the distribution of source rock properties through detailed core-based analysis of the Tenneco Phoenix-1 well (OCS-Y-0338), drilled in offshore Arctic Alaska in 1986. Samples from different lithologies were collected for total organic carbon, Rock-Eval pyrolysis, carbonate content, elemental analysis, and analysis of biomarkers and diamondoids. The core was also scanned at 1-ft intervals, using a hand-held x-ray fluorescence (XRF) device. The Shublik Formation in the Phoenix-1 well was subdivided into two nonsource and four source-rock intervals based on TOC and Rock-Eval pyrolysis results, and distinctive geochemical, lithologic and XRF chemostratigraphic features. In four identified source rock intervals, biomarker analysis indicates the presence of two organic facies deposited under anoxic clay-poor and suboxic clay-rich environments. Both organofacies are characterized by oil-prone type I kerogens and are immature to marginally mature. Additionally, analysis of biomarkers and diamondoids combined with Rock-Eval pyrolysis results yields evidence of mature migrated hydrocarbons that may have affected previous interpretations of organic matter type and maturity of this core.

Furthermore, current work fills the gap between biomarker analysis of two genetically-distinct organofacies and predicted oil families. Chemometric evaluation of multivariate biomarker data reveals mixtures with variable degrees of mixing between end members. Analysis of diamondoids confirms mixed oil types and establishes diamondoid signatures of source rock end-members. This allows for correlation of biomarker-poor, overmature Shublik source rock samples to oils, and extends these interpretations over large areas of the North Slope.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018