Print this pageLacustrine Source Quality and Distribution: Lake Type Controls on Hydrocarbon Generation
CARROLL, ALAN R., and KEVIN M. BOHACS
Three lake types, expressed by the lithologies and parasequence stacking patterns of lacustrine deposits and by associated facies, commonly recur in otherwise dissimilar nonmarine settings, such as Upper Permian, siliciclastic mudstones of the Junggar basin and Eocene, carbonate-rich facies of the Washakie basin. The terms "underfilled", "balanced-fill", and "overfilled" refer to the inferred balance between potential accommodation and the amount of water+sediment available to fill that space. Organic geochemical characteristics distinctive of each type aid in predicting the type and quality of hydrocarbons generated. Conversely, geochemical analyses of generated hydrocarbons may help predict the distribution, efficacy, and generation timing of lacustrine source facies.
Thin (<2 m), aggradational desiccation cycles are common underfilled facies, reflecting ephemeral-lake and playa deposition. Source rocks are homogeneous, regionally extensive, and often associated with eollanites and evaporites. Source rocks generate mostly oil, which may be sulfur-rich, and in some cases generate at relatively low thermal maturities. Biomarker distributions record saline to hypersaline depositional conditions.
A combination of vertically aggraded desiccation cycles and progradational clastic parasequences 1-5 m thick typifies balanced-fill facies. Carbonate lithologies are common, and relatively homogenous source facies may cover broad areas. Due to an optimal combination of primary productivity and organic matter preservation, TOC content may be very high, and oil-prone Type I kerogen predominates. Biomarker distributions reflect specific populations of algae and other aquatic organisms, and in some cases elevated salinities.
Shoreline progradation dominates overfilled facies, resulting in the stacking of indistinct parasequences up to 10 m thick. The deposits may be laterally discontinuous and contain strong facies contrasts; fluvial sandstone and coal are common. Transported terrestrial organic matter contributes to Type I-III kerogens that generate both waxy oil and gas across a wide range of thermal maturities. Biomarker distributions record the mixed aquatic and terrestrial organic matter.