Characterization of Rocky Mountain Paleozoic Oils – Not the Usual Suspects!

Abstract

Most of the oil in the U.S. Rocky Mountain region is considered to have been generated from Cretaceous (e.g., Niobrara, Mowry & Greenhorn), Tertiary (Green River), Ordovician (Red River), Devonian (Bakken), and Permian (Phosphoria) sediments. While the geochemistry of several Paleozoic petroleum systems has recently been well-characterized (e.g., Phosphoria, Central Montana Heath, and Williston Basin Bakken, Madison Group and Red River), this presentation addresses the detection and characterization of Paleozoic petroleum systems that have not been the focus of recent exploration efforts.

The presence of an oil sample is indisputable proof that a petroleum system exists, with the source rock generally deeper than the reservoir location of the oil. The molecular and isotopic composition of produced oil may be used to predict various geological and geochemical aspects of the oil’s corresponding source rock, including organo-facies, lithology, depositional environment, source rock age, thermal maturity, and, at times, migration distance and relative direction. Biomarkers, such as terpanes and steranes, function as molecular fossils. Even though the oil may have migrated far from its source, biomarkers provide evidence as to the nature of the source organic matter.

A detailed analysis of 242 oil samples that have been generated from Rocky Mountain Paleozoic source rocks is the basis for this study. By evaluating this suite of Paleozoic-sourced oils we have a) documented their presence, b) established and mapped the extent of coverage (footprint), c) determined the character (e.g., %S, oil vs. gas, thermal maturity), and d) identified the probable source rock. This identification resulted from multivariate statistical analyses of terpane and sterane biomarker ratios as well as stable carbon isotope values of the C15⁺ hydrocarbon fractions. Triple Quadrupole GC-MS-MS (aka QQQ) was used to determine the alkyl aromatic and diamondoid character and mid-range hydrocarbon number thermal maturity of a subset of these oils, to complement the biomarker work.

Examples will be presented from three (of five) study areas that have been evaluated:

Covenant-like oils in central Utah and northwest Colorado

Pennsylvanian-sourced oils in the southeastern portion of the Powder River Basin

Pennsylvanian and Mississippian-sourced oils in the SE Denver Basin and the Las Animas Arch

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