Spatial Dynamics of Tertiary Igneous Intrusion in Raton Basin, Southern Colorado

Between 33Ma and 20Ma, over 500 dikes intruded the Raton Basin in Southern Colorado. Dikes extend radially from the Spanish Peaks double volcano located on the northwest margin of the basin. In addition to dikes, sills permeate the basin, often intruding bituminous rank coal beds used by oil and gas companies to extract Coal Bed Methane (CBM). Previous research focusing on the geothermal and intrusive history of the basin suggests that the pervasive sill complexes are directly related to ??úfeeder dikes.??Ñ This report combines 89 CBM well logs and an aeromagnetic survey to test this feeder dike hypothesis. Sills are identified in well logs by dual induction resistivity (DIR) log spikes exceeding 200 ohm(m). Dikes are identified by rotated to poll (RTP) vertical magnetic gradients exceeding 65 nT/m and dikes are mapped by tracing anomalous gradient ridges on aeromagnetic maps. Sill abundance is quantified for five stratigraphic categories in each well. Abundance is compared to (a) the proximity of the nearest dike to a given well and (b) the number of dikes within ten proximity ranges (500 meter intervals from 500 meters to 5 kilometers). Results show that sill abundance is significantly controlled by the number of dikes within 500 meters from a well, but that dikes outside of the 500 meter range have an insignificant effect on sill abundance in a well. Furthermore, the proximity of the single closest dike has a negligible effect on sill abundance. Results are presented in a series of scatter and bar graphs with accompanying regression analysis. Isopach mapping of sills supports statistical results by showing sill ??úhotspots??Ñ coinciding with areas of dike convergence. Given the quantitative and qualitative evidence that dike frequency within close proximity to a well affects sill abundance, the feeder dike hypothesis can be confirmed. These results may be useful to geothermal energy companies and CMB companies. Previous research suggests that sills control the movement of hydrothermal fluids, so drilling in areas where dikes converge on the surface will increase the probability of intercepting sills and the connected hydrothermal resources. Furthermore, CBM producers may avoid areas where sills have intruded and destroyed coals by drilling in areas removed from dikes.