Title: Geologic And Production Characteristics of The Lewis Shale (San Juan Basin) As Results Apply to Other Cretaceous “Shales” of the Western Interior: The Next “CBM Play” of the Rocky Mountains, USA and Canada
Speaker: S. Robert Bereskin, Tesseract Corporation, Salt Lake City, UT
Date: November 2, 2001
Publication: The Outcrop, November 2001, p. 4
Numerous discoveries of behind pipe pay-adds have provided incentives for geologic/engineering studies of the Lewis Shale in the San Juan Basin. Such studies are not only germane to the comprehension of Lewis production itself, but are also applicable to many so-called Cretaceous “shales” and to coarsening/fining upward sequences of the entire Western Interior region. Recognized Lewis pay is correlative to coarsening-upward, distal pulses of individually-identified progradational sandstones of the La Ventana deltaic complex. The three shallow marine facies responsible for Lewis production are generally recognizable on formation imaging logs and include (1) a mud-rich, fine-grained sandstone characterized by complex and thorough bioturbation, (2) an interlaminated facies consisting of dark gray mudstone and versions of light gray rippled sandstone, and (3) a submerged sand ridge assemblage possessing more traditional reservoir characteristics. A fourth rock type consisting of dark gray organic mudstone (up to 2.2% TOC) is likely responsible for partially sourcing the other three reservoir types. Bioturbation can, in fact, be present in all four intergradational facies types.
Facies (1) and (2) also produce gas through various amounts of desorption as recognized in other North American “shale” deposits. Apart from desorption, the theoretical production mechanism also involves free gas derived from dissolution of microscopic organic-rich features, microporous authigenic illite/smectite, numerous open microfractures, and finally to subvertical, calcite-lined fractures that dominantly trend NNE with a subordinate set possessing NW alignment. Fracture generation appears related to strong contrasts in mechanical stratigraphy where comparatively ductile, muddy rocks are in stratigraphic contact with more brittle, sand-rich strata.
Although Lewis production parameters are likely analogous to other Cretaceous “shales”, each “shale” should be approached individually because of inherent geological and geochemical variation especially common to mudstones.