Origin of gaseous hydrocarbons from Upper Cretaceous and Tertiary strata in the Piceance basin, western Colorado

Abstract

Natural gas samples were collected for geochemical analyses from Upper Cretaceous and Tertiary strata of the Piceance basin in western Colorado to: 1) determine the origin of gases (i.e., microbial versus thermogenic), 2) determine the thermogenic source rock(s) for the gas-rich Williams Fork Formation, and 3) assess the nature of gas migration. Mud logging gases were sampled approximately every 60 m between 350 and 2800 m and analyzed for "C compositions and CI/C,-3 ratios. Samples collected from low gas content intervals above 1950 m define two parallel trends of increasing "Cc, content with depth. Data from the first trend are based on eighteen analyses and range from-69.9 to-38.3%o (R 2 = 0.92). These data suggest a microbial and mixed microbial/thermogenic origin for methane. Only one sample from above 1950 m contained sufficient amounts of C2for isotopic analysis (813 CC2 =-27.0%o at 1718 m). Data from the second trend are based on seven analyses and are offset by approximately +20%o compared with the primary trend at comparable depths. These data range from-65.0 to-38.5%0 (R' = 0.84). 813c ci and C,/CI-3data from both trends are similar when viewed on a crossplot, thus suggesting that large-scale, vertical gas migration has occurred. Migration was probably aided by fractures that formed during maximum burial and peak gas generation. Except for one sample collected at 1718 m, "CC2compositions above 1950 m were not determined due to insufficient sample sizes. Below 1950 m, gas contents abruptly increase and approach 10-4' gas units. These gases have "C compositions indicative of thermogenic origin. Gases between 1950 and 2450 m have relatively uniform geochemistries (8"Cc, =-39.9 0︢.3%ol 613C C2 =-27.4 I︢.i%ol CI/Cl-3 = 0-91 0︢.03), and are chemically distinct and therefore Renetically different from gases between 2450 and 2791 M (513C ci =-37.9 +-O.2%og 813C C2 =-26.4 0︢.5%09 CI/Cl-3 = 0.88 0︢.01). Gases of the latter group were probably derived from coalbeds that comprise the Cameo Group, as abundant coals are found between 2450 and 2630 m. Only three thin coalbeds occur within the Coal Ridge Group between 1950 and 2450 m, so gases from this interval were probably derived from interbedded shales. Core and cuttings samples were also collected and sealed in cans from several intervals for geochemical analyses. Canned methanes at or above 858 m are "C-enriched by 13 to 33%o compared with logging methanes at equivalent intervals. Below 1934 m, however, 813C ci values for core and cuttings are comparable to logging gas values. This observation suggests that 813 Cc, discrepancies above 858 m are related to low gas contents in the core and cutting samples. Therefore, geochemical data from core and cuttings were not used to assess migration or to interpret gas origin.