Miocene coals in the Hanoi Trough, onshore northern Vietnam: Depositional environment, vegetation, maturity, and source rock quality

Abstract

The Hanoi Trough in northern Vietnam forms the landward extension of the large Song Hong Basin in the South China Sea. Several gas and condensate discoveries have been made in the offshore part while exploration in the Hanoi Trough has been less successful. Encountered hydrocarbons suggest a terrigenous source rock, the most likely candidate being Miocene coals. The composition of these is poorly documented in the public domain, but in the Hanoi Trough with a thinner Cenozoic succession numerous Miocene coal beds have been penetrated by several wells. The current study examines by organic petrography, geochemistry, and sedimentology the depositional environment, paleo-vegetation, thermal maturity and source rock potential of Miocene coal samples from two about 1000 m fully cored wells in the Hanoi Trough. The sedimentological facies, dinoflagellate cyst content and pyrite nodules suggest the coals were formed in coastal mires, in particular the younger section, testifying to an overall relative sea-level rise in the Miocene. This is supported by abundant framboidal pyrite and high TS contents in the coals. Dominance of huminite in all samples suggests water-logged mire conditions while abundant funginite and high concentrations of hopanes indicate high bacterial and fungal activity. Reduced fluorescence, oxidation rims, and elevated reflectance of resinite in a sample with concentrations of liptinite composed of resinite, cutinite, suberinite, sporinite and liptodetrinite might suggest the liptinite-enrichment was the result of degradational removal of less resistant organic matter. Biomarkers indicate the mire vegetation consisted of gymnosperms related to Podocapaceae or Cupressaceae, and perhaps Pinaceae. Presence of angiosperms is indicated by oleanane and aromatic oleananes, lupanes and ursanes. An unknown diterpane derived from Araucaria nemorosa (conifer in the Araucariaceae family) and/or Araucaria cunninghamii (pine) may be the first occurrence in a geological sample. The coals are of lignite to sub-bituminous A rank and are thermally immature with regard to hydrocarbon generation. They are today not at maximum burial depth due to c. 480–1450 m of Miocene uplift before reburial in Plio-Pleistocene time. HImax values around 200 mg HC/g TOC reveal the coals primarily are gas-prone, but exsudatinite in cleats in huminite and cell lumens in funginite is evidence for in situ generation from labile macerals. Oil droplets in cleats/cracks suggest migration of liquid hydrocarbons from deeper buried source rocks and are indicative of a working petroleum system in the Hanoi Trough.