--> Abstract: Well-Log Expression of Lake Strata: Controls of Lake-Basin Type and Provenance, Contrasts with Marine Strata, by K. M. Bohacs and K. Mikell-Gerhardt; #90937 (1998).

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Abstract: Well-Log Expression of Lake Strata: Controls of Lake-Basin Type and Provenance, Contrasts with Marine Strata

BOHACS, KEVIN M., and KIMBERLY MISKELL-GERHARDT, Exxon Production Research Company

The well-log expression of lacustrine strata varies widely, controlled by lake-basin type and sedimentary provenance. These variations affect the evaluation of source, reservoir, and seal potential, net/gross calculations, and even the fundamental choice of the best logs for stratigraphic correlation. No single log or log suite is appropriate for all lake-basin types.

The members of the Green River Formation, Washakie Basin, Wyoming, demonstrate the variations due to lake-basin type. In the Luman Tongue (humid, overfilled lake basin), the combination of sonic and resistivity are the best tools for identifying depositional environments and stacking patterns, and for stratigraphic correlation. (Facies identification from sonic is limited to intervals > 2 m thick.) In the lower Laney member (semiarid, balanced-fill lake basin), resistivity is the best geological tool and matches well with organic content. Its resistivity and gamma-ray responses to lithology are opposite that of the Luman Tongue due to Volcaniclastic sandstones and low U in organics in the Laney member. In the Wilkins Peak member (arid, underfilled lake basin), sonic and density logs best differentiate among siliciclastics, evaporites, and organic-rich shales, which have widely varying, non-unique gamma-ray and resistivity responses.

Variations due to provenance show in the contrast of the lower Laney member (with sedimentary-rock provenance) with balanced-fill lake units from the Hartford, Newark, Songliao, and Liaodong Bay basins. In these latter basins, crystalline provenance or intermittent marine connections result on gamma-ray and resistivity signatures similar to marine clastic strata.

Lacustrine well-log expression can differ greatly from marine strata, especially for gamma-ray and sonic logs. In an alkaline lake with volcaniclastic sands and low U water concentrations, a flooding surface can be marked by a sharp decrease in total gamma-ray activity that increases upward as the radioactive sands prograde. Although general well-log character can be predicted using lake-basin type and provenance, accurate detailed evaluation of depositional environments absolutely requires local rock calibration.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah