Organic geochemistry of freshwater and alkaline lacustrine sediments in the Green River Formation of the Washakie Basin, Wyoming, U.S.A.

Two members of the Green River Formation in the Washakie Basin have been analysed by organic geochemical and organic petrographic techniques and the results placed in a sequence stratigraphic framework. One of these members, the Laney Shale, was deposited in a hydrologically closed, alkaline lake under an arid climate and was characterized by high concentrations of alginite-rich organic matter whereas the other, the Luman Tongue, was deposited in a hydrologically open, freshwater lake under a humid climate and consisted of organic-poor profundal lake mudstones and coaly lake margin sediments. Potential source rocks in both lake types have the potential for generating high-wax oil at high subsurface temperatures (150–175°C at a heating rate of 5 K/Ma) but the alkaline system is clearly more prolific. The most abundant biological marker hydrocarbons were the 4-methylsteranes in both lake types, though dinosteranes were found only in the Laney Shale. Oleanane was absent from the Luman Tongue despite an abundance of angiosperm palynomorphs. Intraformational heterogeneities were best documented for the Laney Shale where molecular differences in kerogen type occur at the parasequence level, with early transgression-, maximum transgression- and rejuvenation stages of lake history having its own kerogen type and distinctive high-wax oil fingerprint. Thermal lability was shown to be linked to kerogen structure, in particular the presence of alicyclic and oxygen-containing moieties, and these in turn are correlated with total organic carbon content and thence productivity/degree of preservation. Some kerogen components appear to have originated via selective preservation whereas others may have been formed by diagenetic condensation reactions.