Petroleum inclusions atop Unayzah gas condensate reservoir: Signpost for an undocumented chapter of the Arabian Basin filling history?

Abstract

The Carboniferous-Permian Unayzah gas condensate reservoir of the Ghazal Field in eastern Saudi Arabia contains abundant oil inclusions indicative of a palaeo-oil column. The molecular composition of a fluid inclusion rich zone at the top of the reservoir revealed a mid-mature biomarker distribution (0.80% vitrinite reflectance equivalent, VRE) that is considerably lower than that of the co-occurring reservoired fluid (1.41% VRE). Side chain cracking of the triaromatic steroids evident in the produced condensate is significantly less advanced in the inclusion oil, as is also reflected in their alkylnaphthalene distributions. The lower maturity signal in the inclusion oil is combined with calcareous related biomarkers that include relatively abundant 30-nor-17α-hopane, C 24 tetracyclic terpane and alkyldibenzothiophenes, in contrast to the reservoir hydrocarbon fluid derived from the Lower Silurian Qusaiba shale. Inclusion gas is enriched in the higher hydrocarbons, which might be attributed to a high proportion of oil (or wet gas) inclusions. Fluid inclusion ethane and propane δ 13C are very close to the mean of the produced gases, but carbon dioxide δ 13C is about 5‰, and methane δ 13C 2‰ less negative than the average of the produced Ghazal gases, which may signify more than one hydrocarbon generation/population within the fluid inclusions. An earlier contribution into the Unayzah reservoir from a non-Qusaiba source may therefore be inferred to have had preceded the volumetrically more significant and more mature Qusaiba charges. Further research is needed to elucidate the source of this oil charge and its extent, which could include deep (Cambrian or Neoproterozoic) sources that might be present in the region.