Oil Characterisation and Relationship With Source Rocks in the Northern Song Hong Basin, Vietnam by Geochemical Analysis

Abstract

The Northern Song Hong basin is one of the Vietnamese basins which remains largely unexplored since only a few wells have been drilled. The Cenozoic basin fill succession of the basin is believed to contain gas prone terrestrial source rock with kerogen type III. However, oil discovered in some recent wells in the North Song Hong Basin indicates the existence of a petroleum system containing fresh water lacustrine source rock with kerogen type I. Understanding the characteristics of oil and source rocks are useful to unveil the petroleum system as well as exploration strategy in this basin. By geochemical analysis, especially Gas Chromatography‐Tandem mass spectrometry (GC/MS/MS), 5 oil samples and numerous Oligocene source rock samples selected in onshore wells, offshore wells and outcrops samples are used to determine oil/source rock characterization and their correlation. Oil samples taken in offshore wells indicates high maturity level that are up to 1.0% vitrinite reflectance, and one sample is a condensate. However, source rocks in the wells are generally moderately mature and only in the early oil window generation phase. The Enreca‐3 well drilled on the Bach Long Vi island that is located close to the oil discoveries yield 500m of immature deep lacustrine source rocks, with 300m having excellent quality of oil generation with 2‐7% TOC and HI in the range 400‐900mg HC/g. Even though different in maturity level, the characteristics of the oil samples are similar to the rocks in wells and adjacent outcrops. They are generated by non‐marine source rocks, fresh water lacustrine organic matter with contribution from higher plant organic matter. GC/MS/MS analysis results show the dominance of C29 sterane compared to C27, C28, and the existence of Oleanane and Bicadinane in both oil and rocks which indicates terrestrial material input during the source rock’s deposition. Maceral composition of source rocks also shows around 50% of liptinite and 5‐10% vitrinite, proof that the source rock is of type I and type III kerogen. The vitrinite and thermal modeling results also illustrate the high maturity level of oil that come from deeper source rocks that have higher maturity than those in well locations. The results show Oligocene source rocks in the trough center is at around 1.0% vitrinite reflectance, from which oil might have been generated and migrated downward to the Carboniferous‐Permian carbonate basement. After evaluation of source rocks and oil characterization, there are numerous grabens and half grabens containing Oligocene source rocks covering on carbonate basement reservoir, which can be potential plays in the North Song Hong Basin.

AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019