Historically, well logging and interpretation workflows have been developed mainly for use in porous and permeable reservoir formations and are not commonly used to evaluate source rocks or unconventional reservoirs. Instead, the evaluation of source rocks, such as the organic-rich deposits of the Green River Formation, has relied primarily on expensive and inefficient core analyses such as the Fischer Assay and simple log interpretation. With the potential oil equivalent in place exceeding a trillion barrels, there is a need for detailed characterization of these oil shale deposits using high resolution well logs.
We have logged two Green River wells using combinations of standard and advanced logging techniques. This program was supported by extensive core analysis, including Fischer assay and thorough mineralogical and chemical analyses. Methods of determining kerogen content from log responses were developed along with multiple methods of estimating a continuous log of formation water salinity. We developed methods for quantitatively evaluating these Green River oil shales by integrating standard logs with more advanced logging measurements including nuclear magnetic resonance, elemental capture and inelastic spectroscopy, and dielectric dispersion. The results and the petrophysically derived multi-mineral model are validated by the core measurements and then applied to a near-by well.
Oil shale is one of the industry's highest risk but highest reward opportunities, potentially yielding trillions of barrels of high quality oil. Although the potential resource has been known for centuries, formation evaluation is limited mainly to the costly Fischer assay method requiring laboratory pyrolysis of core samples, which is not particularly relevant for in-situ pyrolysis. Little is known of modern well log responses to this unconventional reservoir rock. Wells recently drilled, cored, and logged in the Green River Formation of the Piceance Basin, Colorado, provided an opportunity to study state-of-the-art well logging tools in an oil shale formation.