Geophysical Investigation of the Breccias Impact on Reservoir Quality in the Madison Formation, The Beartooth Region, Montana, U.S.A
AlKawai, Wisam H.1
(1)Exploration Advanced Research Center, Saudi Aramco, Dhahran, Saudi Arabia.
The Beartooth area in Red Lodge Montana (USA) is a highly structured area due to the Laramide orogeny that caused thrust faulting of the strata including the Madison Formation. Geologic models show the existence of different types of breccias within the Formation that resulted from different brecciation processes such as tectonics and sub-aerial exposure. Studies of the Madison Formation propose that breccias (particularly tectonic breccias) do not improve permeability due to the cementation of these breccias. Borehole geophysical data were collected from a well drilled through the uplifted Madison Formation overlain by alluvial deposits to examine the impact of brecciation on reservoir quality. Based on the data obtained, breccias have not enhanced permeability and reservoir quality because because of considerable amounts of calcite cementation present.
The VSP data shows that part of the Madison Formation has a relatively low P-wave velocity of 2650 m/s and a high Vp/Vs ratio of 2.23. These values indicate the Madison Formation has a lower rigidity than typical carbonate rocks. Hence it is likely that brecciation has cause increased attenuation/scattering of seismic energy whilst simultaneously decreasing the rock strength or rigidity. In addition to the VSP data, the gamma ray logs in the Madison Formation show relatively high values suggesting there are considerable amounts of radioactive minerals associated with the calcite cementation. Full waveform sonic logging is used to map the breccias through seismic wave attenuation. The full waveform sonic log data shows variations in in the types of the waves attenuated within the breccias suggesting a low connectivity of the breccias.
AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.