The influence of organic sources and environments on source rock deposition during the periods of Cretaceous-Eocene and Oligocene-Miocene, northern Kalimantan

The sedimentary system of Kalimantan has undergone significant development since the Oligocene. Previous research have largely ignored the capacity of the Cretaceous-Eocene sediments to produce hydrocarbons, focusing instead primarily on the Oligocene-Miocene coal as the principal source rocks. Shales and coals from the outcrops in the northern margin of Kalimantan were analyzed with palynological and geochemical methods to characterize the palaeoenvironmental and palaeoecological differences between the Cretaceous-Eocene and the Oligocene-Miocene samples. The high proportion of Cheirolepidoaceae, Schizaeoisporites and Ephedripites in the pollen assemblage from the Cretaceous—Eocene outcrops reflects an arid tropical/subtropical climate. The relatively low abundances of gymnosperm-derived biomarkers including isopimarane, β -phyllocladane, β -kaurane, suggest the gymnosperm features in flora. High C 27 /C 29 ααα 20R sterane ratios, (C 19 –C 29 ) tricyclic terpanes/C 30 αβ hopane and extremely low oleanane/C 30 αβ hopane, bicadinane T/C 30 αβ hopane, and diterpenoid abundance indicate that there was a dominance of algae relative to higher plants in the organic matter. The gymnosperm-derived biomarkers, including isopimarane, β -phyllocladane, β -kaurane, suggest that palaeovegetation during this period was dominated by gymnosperms. The saline and reducing conditions in the bathyal and abysmal sea, evidenced by rather low Pr/Ph and high Gammarerane index, are beneficial for the preservation of hydrogen-rich organic matter. It is presumed that the Cretaceous—Eocene shales had great hydrocarbon generation potential in the southern South China Sea. During the period of Oligocene to Miocene in the Zengmu Basin and the Baram-Sabah Basin, the climate changed to a dominant humid and warm condition, which is corroborated by abundant pollen of Florschuetzia and Magnastriatites hawardi. Low C 27 /C 29 ααα 20R sterane ratios, (C 19– C 29 ) tricyclic terpanes/C 30 αβ hopane, and high oleanane/C 30 αβ hopane, bicadinane T/C 30 αβ hopane suggest that the palaeovegetation was dominated by angiosperms including the mangrove plants. The extremely abundant higher plants provide ample terrigenous organic matter for the formation of coal-measures in delta facies. The low gammacerane index and high Pr/Ph indicate the fresh and sub-oxic water in delta-neriticabysmal faces, which is not beneficial for the accumulation of hydrogen-rich organic matter. Thus, the Oligocene-Miocene ma