Las Animas Arch Oil Geochemistry

Abstract

Sixty-nine crude oils from wells in the southeastern Denver Basin and Las Animas Arch areas are grouped into three major oil families based on genetic-specific biomarkers and stable carbon isotope compositions using multivariate statistics. These oils are largely distinguished by sterane distributions, biomarkers for different algal populations. High amounts of C28 steranes are very unusual for Paleozoic source rocks (Family 1) while high C27 steranes with upwelling signatures are more common in oils generated regionally from Middle Pennsylvanian sources (Family 3). Higher C29 steranes often indicate a component of terrigenous, coaly sources, consistent with the waxy character of some Family 2 oils. Since Sub-families 3a & 3b are almost exclusively produced from Middle Pennsylvanian reservoirs and 80% of Sub-families 1a & 1b oils are from Mississippian reservoirs, it is fair to propose that source horizons for these families are also Middle Pennsylvanian and Mississippian, respectively. Many of Family 2 waxy oils are reservoired in, and likely sourced, in part, from the Lower Pennsylvanian Morrow Formation. Pennsylvanian source rocks for Family 3 oils were deposited under the influence of strong and persistent upwelling conditions, promoting high phytoplankton productivity. Oils that contain abundant aryl isoprenoids – only Family 3 – suggest shallow water anoxia and enhanced preservation of source organic matter. Family 3a oils in Pennsylvanian reservoirs, likely generated from Desmoinesian source marls, occur only on the northwestern flank of the Las Animas Arch and within the Denver Basin. These oils originated and migrated from Denver Basin depocenters. Pennsylvanian Family 3b oils only occur on the eastern flank of the Arch. These are likely derived from depocenters to the east, perhaps deeper within the Hugoton Embayment. Similarly, Family 1b oils in Mississippian reservoirs, likely sourced from Mississippian shales, occur exclusively on the eastern flank of the Las Animas Arch and are, again, likely derived from depocenters to the east. Morrow Family 2 oils only occur in the Denver Basin. A number of maturity-sensitive biomarker ratios were subjected to principal component analysis and converted into an vitrinite reflectance equivalent (VRE) values. Mississippian shale source rocks to the east of the Arch generated oil in the 0.82–0.90% VRE range. In contrast, the Pennsylvanian-sourced oils were generated at lower levels of maturity, 0.69–0.74% VRE.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015