Geochemical composition of oils in the Dunga Field, western Kazakhstan: Evidence for a lacustrine source and a complex filling history

The onshore Mangyshlak Basin, located in western Kazakhstan along the eastern coastline of the Caspian Sea, is a prolific hydrocarbon province and home to numerous oil and gas fields. The primary reservoirs are Jurassic clastic sandstones, but the Dunga Field contains hydrocarbons in Lower Cretaceous (Valanginian, Barremian, Aptian, Albian) clastic and carbonate reservoirs. The general perception is that these petroleum accumulations were charged from marine source rocks of likely Middle Triassic age. A detailed organic geochemical investigation of oils from the Cretaceous reservoirs of Dunga Field suggests that all were generated from the same lacustrine source rock facies (organofacies C) in mature kitchens in the offshore Mangyshlak Basin, followed by a complex filling history. The oils are characterised by relatively high wax content, low sulfur content (mainly < 0.2 wt%), typical lacustrine tricyclic terpane ratios (high T26/T25, high T24/T23, low T22/T21) and low H31R/H30 hopane ratios. Maturity of the source rock that generated the oils would tentatively correspond to a VR of ∼0.9-1.1 %Ro. The oils are non-biodegraded, but some minor water washing might have occurred. Diamondoid data suggest that the Barremian reservoir contains a mixed oil pool composed of the non-cracked lacustrine oil and another charge of oil that is cracked from 79% to 83% and likely also to be slightly fractionated. This requires a complex filling history of the Dunga Field involving two hydrocarbon charges, likely from two different fetch areas. One charge of non-cracked oil migrated into all four reservoirs whereas another charge was 'hoteled' and cracked before remigration to the Barremian reservoir only and mixed with the non-cracked oil.