Petroleum potential of Oligocene lacustrine mudstones and coals at Dong Ho, Vietnam — an outcrop analogue to terrestrial source rocks in the greater Song Hong Basin

Abstract

The outcrop of Oligocene age at Dong Ho, northern Vietnam, may constitute an immature analogue to offshore terrestrial source rocks in the greater Song Hong Basin. The outcrop includes an interval with two source rocks: (1) highly oil-prone carbonaceous mudstones containing kerogen types IIA and IIA/I, and with TOC contents from 6.48 to 16.89 wt%, and HI values from 472 to 690; and (2) oil-prone humic coals (kerogen type III) with HI values from 200 to 242. The mudstones were deposited in oxygen-deficient lakes, which on occasion were subject to marine influence, and the coals accumulated in freshwater peat-forming mires. The coals have broad activation energy ( E a) distributions, while the mudstones have E a distributions characterised by a pronounced principal E a. During artificial maturation about 16–17% of the organic carbon in the coals and 45–50% of the organic carbon in the mudstones participated in petroleum formation. The two source rocks primary generate oil and secondary generate gas, however, the mudstones realised the majority of their potential over a more narrow temperature range than the coals. The excellent generative potential of the terrestrial source rocks at Dong Ho is encouraging for offshore exploration for reservoirs charged by Cenozoic rift-lake successions.