PETROLEUM SOURCE ROCK POTENTIAL AND STRUCTURAL SETTING OF NEOPROTEROZOIC BASINS, UTAH AND ARIZONA

Two Neoproterozoic units, the Red Pine Shale (RPS, Uinta Mountain Group, Utah) and the Walcott Member (Chuar Group, Arizona) are recognized for their potential as petroleum source rocks. Both units comprise km’s thick organic-rich shale successions with high total organic carbon (TOC) values. We characterize the subsurface geographic extent of these two units, and investigate their source rock potential. Thirty-one samples of RPS and eighteen samples of Walcott Member were collected and analyzed via Rock-Eval pyrolysis, X-ray diffraction, and hydrous pyrolysis. Well log and geophysical data are currently being integrated and analyzed.

Rock-Eval analyses indicate two distinct sample populations in the Walcott Member, which coincide with stratigraphic position below and above the upper dolomite marker units. Nine samples from the lower strata contain TOC values ranging from 6.33 - 9.06 wt % (avg. 7.91 wt %). The average calculated hydrogen index (HI) for these samples is 86.89, the average calculated oxygen index (OI) is 35.02, and the average kerogen yield (S2) is 6.72 mg HC/g rock. Nine samples from the upper strata contain TOC values ranging from 2.80 – 4.24 wt % (avg. 3.60 wt %). The average calculated HI for these samples is 48.12, the average calculated OI is 57.49, and the average kerogen yield (S2) is 1.78 mg HC/g rock.

Rock-Eval results indicate that 70% of RPS samples have TOC values of < 1.45 wt % (testing cutoff); however, nine samples contain TOC values averaging 2.04 wt % (max. of 3.02 wt %). The average calculated HI for these samples is 63.65, the average calculated OI is 50.75, and the average kerogen yield (S2) is 1.32 mg HC/g rock.

The calculated %Ro-Tmax values suggest immature to peak Walcott maturity (0.53 – 0.81) and peak to post-maturity for RPS (0.69 – 3.73). Kerogen type in both units resembles a mix of types II, III & IV.

Ongoing analysis of seismic reflection data investigates the extent of subsurface Neoproterozoic stratigraphy. Lithologic logs and available drilling samples will aid in identification of Chuar Group at depth. The regional presence of subsurface Chuar Group, Uinta Mountain Group, or equivalent would support evidence for a mid-Neoproterozoic interior seaway, and further aid in the characterization of uncommon Proterozoic petroleum systems.