S. R. Bereskin1 and G. E. Christiansen2
1Tesseract Corp., Salt Lake City, UT
2Burlington Resources Inc., Farmington, NM
ABSTRACT: Geologic and Production Characteristics of the Lewis Shale (San Juan Basin) as They Apply to Other Cretaceous "Shales" of the Western Interior: A Rocky Mountain Hydrocarbon Future, USA
Numerous behind pipe pay-adds have provided impetus for geologic/engineering studies of the Lewis Shale in the San Juan Basin. Such studies are not only essential to the comprehension of Lewis production, but are also applicable to many so-called offshore "shales" and to coarsening/fining upward sequences of the entire Western Interior region. Recognized Lewis pay is correlative to distal pulses of individually identified sandstones of the La Ventana clastic complex. The three inner shelf facies responsible for production are commonly recognizable on formation imaging logs and include (1) a muddy, fine-grained sandstone characterized by complex bioturbation, (2) an interlaminated facies consisting of dark mudstone and light gray rippled sandstones, and (3) a submerged sand ridge possessing more traditional reservoir characteristics. A fourth rock type of dark gray organic mudstone (up to 2.2% TOC) is likely responsible for internally sourcing the other three reservoir types.
Lithofacies (1) and (2) also produce gas through desorption processes as recognized in other North American "shale" reservoirs. Apart from the desorption phenomenon, the theoretical production mechanism involves dissolution of microscopic features, microporous authigenic illite/smectite, numerous open microfractures, and finally to subvertical, calcite-lined fractures that dominantly trend NNE. Fracture generation appears related to strong contrasts in mechanical stratigraphy where comparatively ductile beds are in stratigraphic contact with more brittle strata.
Although Lewis production parameters are likely analogous to other Cretaceous "shales", each "shale" should be approached with caution because of inherent geological and geochemical variation common to the mudstone realm.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado