Fractured Shale Reservoirs and Basement Faulting, San Juan Basin,

New Mexico and Colorado

By

A.C. Huffman and D.J. Taylor

U.S. Geological Survey, Denver, CO

The eastern and northwestern margins of the San Juan Basin are interpreted as Late Cretaceous – Eocene Laramide right-lateral wrench systems with strong basinward thrust components. Lateral motion on these faults produced overlying monoclines that included a predominance of fracturing parallel to the monoclinal trend. Mancos Shale of the Puerto Chiquito field along the eastern basin margin is fractured by thrust and backthrust faults within a flower structure overlying a right lateral basement wrench fault. We interpret the fracturing in the Verde field on the Hogback monocline along the northwestern basin margin to have resulted from a similar mechanism but the data available to us are not sufficient to demonstrate this conclusively. The generally east-west regional extensional regime predominant since Rio Grande rifting (Miocene) has kept these arrays of longitudinal fractures open.

Analysis of regional and high-resolution 2-D seismic data along the northern margin of the basin demonstrates the presence of multiple thrust and back-thrust faults distributing the basin margin thrusting within the thick Upper Cretaceous Mancos Shale and other Cretaceous units. Nearly pure Laramide compression with little or no lateral component along the northern basin margin resulted in joint sets at acute angles to the Rio Grande extension that are less likely to be open.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming