Publications

The Sokang sub-basin comprises an underexplored Neogene depocentre in the East Natuna Basin. The Sokang-1 well was drilled on a prominent structural feature within this depocentre in the early 1970’s and tested significant amounts of carbon dioxide-rich gas from Late Miocene sandstones. A comprehensive fluid inclusion study from sample material remaining from this well has been carried out to try and better understand the hydrocarbon generation and migration history within this depocentre. This included Fluid Inclusion Stratigraphy (FIS) screening analyses followed by petrographic analyses, fluid inclusion composition, microthermometry and biomarker analysis of the fluid inclusions. These analyses have highlighted the presence of multiple populations of liquid petroleum inclusions within both the Late Miocene and deeper sandstones, proving the presence of a liquid petroleum system and indicating a complex hydrocarbon charge history for this sub-basin. Contributions from at least three different source rocks are noted based on the biomarker compositions of these fluid inclusions and the range of API gravities detected indicate continued charge and accumulation within these reservoirs. Previous petroleum system analysis and basin modeling work has been used to determine most likely candidates for these source rock units. The presence of liquid petroleum inclusions within the Late Miocene sandstones is in contrast to the tested gas which comprises methane with varying amounts of carbon dioxide (up to 87 mol %). These inclusions provide evidence of an earlier oil accumulation which may have been displaced by late migration of carbon-dioxide rich gas into these sandstones. A deeper zone, with oil shows, also shows evidence of liquid hydrocarbon accumulation and may represent an untested oil reservoir. The work confirms the potential for liquid petroleum accumulations within the Sokang sub-basin and should assist in developing the exploration model to identify prospects associated with this play type.