The role of algal blooms in the formation of lacustrine petroleum source rocks — Evidence from Jiyang depression, Bohai Gulf Rift Basin, eastern China

Abstract

Algal blooms are responsible for the laminae in the lacustrine Shahejie Formation in the Jiyang depression, one of the sub-basins of the Bohai Gulf Rift Basin in eastern China. About 65 oil fields have been discovered in this depression during the past 40years and the oil is mainly derived from three types of finely laminated lacustrine rocks of Shahejie Formation (Eocene–Oligocene). These lacustrine petroleum source rocks are oil shales, calcareous shales and calcareous laminated mudstones. Optical, SEM and BSEI analyses of these source rocks reveal that there are algal (coccoliths and dinoflagellates) laminae in the rocks. The individual algal lamina is generally composed almost wholly of one species, such as Reticulofenestra bohaiensis in coccolith laminae, and Fromea as well as Bohaidina in dinoflagellate laminae. This distribution suggests that the algal laminae formed from algal blooms in the ancient lake. Other components of source rocks included organic matter laminae, carbonate laminae and clay-rich laminae. We also discuss the sedimentation of organic matter laminae and carbonate laminae which were related to algal blooms. We conclude that the laminated lacustrine petroleum source sediment of Jiyang depression formed during episodes of algal blooms alternating with chemical and detrital sedimentation, and this represents the primary mechanism responsible for the formation of lacustrine petroleum source rocks.