ORGANIC GEOCHEMISTRY OF FRESH-WATER AND ALKALINE LACUSTRINE SEDIMENTSIN THE GREEN RIVER FORMATION OF THE WASHAKIE BASIN, WYOMING, USA

Two members of the Green River Formation in the Washakie Basin have be en 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 clos ed, 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 mud stones and coaly lake margin sediments. Potential source rocks in both lake types have the potential for generating high-wax oil at high sub surface temperatures (150-175 degrees C at a heating rate of 5 K/Ma) b ut the alkaline system is clearly more prolific. The most abundant bio logical marker hydrocarbons were the 4-methylsteranes in both lake typ es, though dinosteranes were found only in the Laney Shale. Oleanane w as absent from the Luman Tongue despite an abundance of angiosperm pal ynomorphs. Intraformational heterogeneities were best documented for t he Laney Shale where molecular differences in kerogen type occur at th e 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 t o be linked to kerogen structure, in particular the presence of alicyc lic and oxygen-containing moieties, and these in turn are correlated w ith total organic carbon content and thence productivity/degree of pre servation. Some kerogen components appear to have originated via selec tive preservation whereas others may have been formed by diagenetic co ndensation reactions.