Reservoir bitumen has been discovered in multiple Paleogene series of strata in the Baiyun Sag

Pearl River Mouth Basin.The abundance of reservoir bitumen gradually increases from the shallower Zhuhai and Zhujiang formations to deeply buried Enping and Wenchang Formations.The reservoir bitumen only sporadically occurs in coarse-grained sandstones with better permeability and porosity

while occurs in siltstones and mudstones in the form of veins.Almost all reservoir bitumen samples do not show apparent anisotropy with bitumen reflectance lower than 2.0%

suggesting that the reservoir bitumen in the Baiyun Sag doesn't mainly result from high-temperature cracking.All bitumen extracts lack 25-norhopane series according to the analysis of molecular geochemical composition.No gas chromatogram base-line "hump" was discovered in all saturated hydrocarbon of reservoir bitumen.The reconstructed burial history of representative individual wells shows that the reservoir strata have not experienced large-scale uplifting and denudation.Therefore

the reservoir bitumen in the Baiyun Sag is not of biodegradation origin.Based on the analyses of the geological setting and characteristics of oil and gas reservoirs

the origin of reservoir bitumen could be attributed to later "gas washing" for deeply buried hydrocarbon-accumulation generated by source rocks of Enping and Wenchang Formations during the early thermal maturity stage.The time differences of thermal evolution for source rocks and reservoirs of various stratigraphic horizons leads to the adjustment and altering of oil and gas reservoirs in the Baiyun Sag

subsequently resulting in the formation of light and volatile oil reservoirs.