Lateral Fractures Identified as Communication Pathways for the Movement of Gases and Condensates Across the Hogback Monocline

As part of the DOE funded San Juan Basin (SJB) CarbonSAFE project seeking to identify faults and potential communication conduits impacting carbon dioxide injected into Jurassic formations, cuttings from four wells forming an approximately 12-mile transect were analyzed using rock volatiles stratigraphy (RVS). RVS was developed by Advanced Hydrocarbon Stratigraphy to provide direct measurements of the volatile compounds in rock samples and rock properties related to the ease with which the measured volatile compounds are released. Compounds measured include the C1-10 hydrocarbons, carbon dioxide, and several others including sulfides and noble gases. Legacy cuttings analyzed covered the Jurassic section of each well and typically several thousand feet above and below. Transect wells include a basin well (Kirtland-1, drilled 1961), one effectively on the Hogback monocline (HM) (a fault/fold feature which results in 3-7 thousand feet of "upthrown" vertical displacement; Stephenson-1, drilled 1981), and two wells on the western side of the HM (Ute Dome - Gotor-1, drilled 1981; and Barker Dome - Ute-8, drilled 1975).

Comparisons of data acquired at the time of drilling (mudlogs) to RVS data show several matching features providing confidence in RVS analyses of legacy unpreserved cuttings and indicative of the presence of fractures. Compositional signatures paired with structural data for the study area suggest that gases/condensates in both the deep Paleozoic carbonate gas zone and the shallower gases associated with the Mancos Shale and Dakota Sandstone moved laterally across HM through fracture networks. On the "upthrown" western side of the HM, these fractures, charged with gas and condensate, appear to propagate for at least 4 miles but not more than 8 miles. The observations and proposed mechanisms of lateral migration across the HM are consistent with historical field data. The RVS data will be discussed in the context of existing geologic/structural models to evaluate the potential implications of the HM for CO2 sequestration within the SJB and vicinity.