Lake Type Controls on Petroleum Source Facies, Jungger Basin, China
Alan R. Carroll
Junggar Basin Upper Permian rocks may be classified according to lithology, parasequence stacking patterns, organic matter type, and molecular geochemistry as "underfilled", "balanced-fill", or "overfilled" lake facies. These facies, which recur in other nonmarine basins, refer to the balance between the supply of water+sediment to the lake basin and potential accommodation, and provide a useful tool for predicting source rock richness and quality.
Underfilled lake facies occur when the supply of water+sediment is insufficient to fill potential accommodation, resulting in a closed basin with ephemeral lakes. Shoaling-upward parasequences 10-200 cm thick are punctuated by mudcracked intervals indicating subarial exposure. Hydrocarbon extracts contain saline-hypersaline biomarkers, and TOC values are relatively low (0.2-2.0%). Balanced-fill facies reflect an approximate balance between the supply of water+sediment and potential accommodation. Laminated mudstone parasequences 1-4 m thick predominate, but mudcracks or other evidence of subarial exposure are absent. Junggar "oil shales" commonly exceed 20% TOC; biomarker geochemistry indicates saline deposition. Overfilled lake facies occur when the the supply of water+sediment excees potential accommodation space. Shoreline progradation, rather than lake level fluctuation, dominates parasequence development. Interbedded lithologies include fluvial sandstones, conglomerates and freshwater. Biomarker assemblages document increased fluvial influx of terrestrial organic matter.
Source rock-oil correlations indicate that balanced-fill to underfilled lake facies source oils in the Karamay trend, most of the eastern Junggar fields, and the Mosuowan discovery in central Junggar. No oil accumulations have yet been linked to overfilled lake facies.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California