UNCONVENTIONAL HYDROCARBON RESERVOIRS IN THE NORTH AMERICAN CORDILLERA

Fundamental geological concepts and extraordinary technological advances are breathing new life into hydrocarbon exploration, including the intramontane and foreland basins of the Rockies and the coastal margins of California. Such “resource plays” include organic-rich shales, e.g., the Cretaceous Niobrara and Mowry, and Devonian/Mississippian Bakken, and the Oligocene Monterey of coastal California.

Other such plays include “tight,” gas-bearing Cretaceous/Tertiary sandstones in Colorado and Utah. The shales, source rocks for conventional porous sandstone and limestone reservoirs, are now potential reservoirs themselves, if they contain significant oil/gas accessible by controlled hydrofracturing along precisely controlled near-horizontal boreholes. Keys to success include hydrocarbon-filled paleofractures, high organic content and maturity, stratal brittleness, precise borehole positioning, and experienced-based hydrofracturing in relation to the ambient stress field. In tight, typically gas-filled sands, hydrofracturing is essential and, especially in very thick sand units, may involve multiple vertical, rather than horizontal, boreholes. In the case of both vertical and horizontal wells, the contemporary stress field controls the orientation of induced hydrofractures; however, existing faults and fractures can either augment or interfere with production.

Monitoring of microseisms produced by hydrofracturing guides optimal well-spacing and pathing, as well as hydrofracturing stages and spacing. As the results of such exploration efforts are made public, enhanced basic geological understanding, including stress histories, results. Other parts of North America, from the Gulf Coast and extending into the Northeastern United States, are actively applying equivalent concepts and technology, not only increasing hydrocarbon reserves, but also providing new challenges for environmental protection, including sequestration of carbon dioxide, which, additionally, will require even greater motivation for improved understanding and application of key geologic concepts.