Abstract: Cost-effective Fracture Mapping in Rocky Mountain Basins

HARTHILL, NORMAN, and DAVID R. PHILLIPS

Fracture detection in tight gas sands in Rocky Mountain basins is frequently problematic. However, we show that fractures develop systematically, and that aeromagnetic, gravity and seismic data can be combined to define regional faulting ,and macro-, and microfracturing which control generation and production of natural gas. Basement stresses controlled Laramide tectonics and created paired uplifts and basins separated by major fault systems. The regional fault systems and intrabasement intrusions may be accurately mapped using aeromagnetics; the intrusions are the heat sources for hydrocarbon maturation. The boundary between Mesozoic and Tertiary sediments is a major density discontinuity; faulting in the sedimentary section may be mapped at this level. Using these two sets of fault data together with mapped faults, the macrofaulting pattern of the basin may be identified and characterized. Based on the fault characterization, P-wave seismic data, from company files, or acquired from a broker, may be analyzed for amplitude variation with offset and azimuth ( AVOA) to define the microfracturing patterns at the stratigraphic level of interest. We used this technique at the Bluebell Altamont field in the Uinta basin. Aeromagnetic data revealed regional east-west basement faulting which controlled basin subsidence, and rise of a mafic intrusion. Gravity showed that the basement faulting continued into the sedimentary section, and that the south flank of the Uinta Mountains were overthrust to the south. AVOA analysis of seismic data revealed N 30 degrees W fracturing at the productive top Green River and Mahogany Bench intervals, which is verified by subsurface data.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado