Geochemical Characterization of Source Rock in Lufeng Sag, Pearl Mouth River Basin

Abstract

Lufeng sag in the eastern Peral River Mouth basin comprises several sub-sags, LF13 sub-sag and LF15 sub-sag are the most important hydrocarbon generation kitchens among them. Geochemical analysis of seventy-eight core and cutting samples of Wenchang formation and Enping formation from newly drilled wells that locate in both sub-sags were carried on to determine the basic characteristics and depositional environment of the source rocks. The Eocene Wenchang formation deposited in medium-deep lake with TOC values higher than 1.5% has better hydrocarbon generation potential than the Oligocene Enping formation deposited in shallow lake with TOC values lower than 1.5%. Rock-Eval and biomarker thermal maturity parameters all indicate that the organic matter of both intervals lies within the oil window. Maceral composition analysis demonstrates that vitrinite dominates Enping formation while saproperinite dominates Wenchang formation. Biomarker assemblages demonstrate that the Enping formation is characterized by lower 4-methyl steranes/ΣC29 regular steranes(<0.1), higher gammacerane/C30-αβ-hopane(>0.1), lower oleanane/C30-αβ-hopane(<0.1), whereas the Wenchang formation is characterized by higher 4-methyl steranes/ΣC29 regular steranes(>0.2), low gammacerane/C30-αβ-hopane(<0.1), higher oleanane/C30-αβ-hopane(>0.1). Bicadinane is a typical biomarker of higher plant input and is widely used in Pearl River Mouth basin for distinguishing Enping formation source rocks from Wenchang formation source rocks, however, in study area, the amount of bicadinane show no obvious difference between the two intervals. The difference in biomarker assemblages indicate different organic matter input. Biomarker assemblages and trace elements indicate a fresh-brackish water environment with a tendency of salinity increase and a semi-humid to humid climate condition when Wenchang formation and Enping formation were deposited. LF13 sub-sag has Pr/Ph ratio below 3, 4-methyl sterane/ΣC29 regular sterane below 0.5%, bicadinanes/C30-αβ-hopane below 0.40%. In contrast, LF15 sub-sag has Pr/Ph ratio above 3, 4-methyl sterane/ΣC29 regular sterane below 1.5%, bicadinanes/C30-αβ-hopane below 0.1%. The differences in biomarker assemblage between the two sub-sags indicate more terrigenous organic matter input in LF13 sub-sag and significant input of algae in LF15 sub-sag. Different boundary fault activity and sedimentary process of the two sub-sags are believed to be the main causes.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016