Exploring the roles of sediment provenance and igneous activity on the development of synrift lacustrine source rocks, Pearl River Mouth Basin, northern South China Sea

Abstract

Lacustrine mudstones developed in the synrift Wenchang and Enping formations are the major source rocks of the Pearl River Mouth Basin (PRMB) and vary in both richness and quality of hydrocarbons generated. While organic productivity is known to have played a role in the development of these rocks, it has been mostly neglected in prior research. In this study, new zircon geochronological data from igneous intrusive and volcanic rocks in boreholes of the PRMB are used alongside compiled detrital zircon U-Pb ages from Paleogene sandstones to decipher the roles of sediment provenance and volcanic activity in the development of source rocks. The result suggests that during deposition of the Wenchang Formation (49-39 Ma), sediments were mainly derived from intrabasinal structural highs dominated by Yanshanian igneous rocks (180-90 Ma). Similarities in young ages (42–45 Ma) in volcanic and clastic units of the Wenchang Formation suggest volcanic activity concurrent with its deposition. Chemical weathering of the proximal source area together with volcanic materials provided abundant nutrients for the basin and enhanced the primary organic productivity, resulting in the development of high-quality lacustrine source rocks, derived mainly from plankton. In contrast, the detrital zircon provenance of the Enping Formation (39-33 Ma) suggests sediments were derived from the South China Block (SCB), far to the north, with complicated lithologies and more older zircon grains. The long-distance transport of SCB sediments diluted the productive nutrition conditions of the larger and shallower lakes, and introduced organic matter derived from higher terrestrial plants. This study provides a new approach to evaluating the lacustrine source rocks especially for the deep-water areas with limited mudstone samples.