GEOCHEMICAL INTERPRETATION OF SOME OILS AND CONDENSATES FROM THE DAMPIER SUB- BASIN OF WESTERN AUSTRALIA

Abstract

Twenty oils and condensates from the Dampier Sub-basin have been analysed for their gross composition and normal alkane distribution.

The chemical composition (GLC traces) enables two groups to be distinguished:

Oils and condensates low in C22 + n-alkanes. These are essentially found in the Rankin Platform. In this group, the sample from Eaglehawk 1 shows characteristics of a bacterially-altered product.

Oils richer in C22 + n-alkanes. These are found in the Lewis Trough, but also in the Rankin Platform/Kendrew Terrace where subtle differences may suggest different origins.

The geological interpretation puts strong emphasis on time of trapping. The oils and condensates of the first group (except at Angel) are trapped at the contact with the 'Main Unconformity'. Oils formed earlier than the Neocomian have been destroyed, altered (Eaglehawk 1) or trapped in deeper reservoirs, so that only mature to very mature hydrocarbons (low in C22 + n-alkanes) were available for trapping after sealing by Neocomian shales. Maturity considerations indicate that a deep source within the core of the Rankin Platform unit (Middle/Early Triassic Locker Shale and/or older) is the most likely generator of these hydrocarbons.

The oils of the second group, on maturity grounds and to some extent on type correlation, may originate from Jurassic sources in the Lewis Trough. Some oils attributable to the second group are present on the Rankin Platform and/or Kendrew Terrace trapped by intraformational/fault seals and having the full composition of crudes; however, lower land-plant contribution inferred from the n-alkanes, suggests a 'Locker Shale' origin.

The source of the Angel hydrocarbons is more enigmatic, but their strong affinity with the hydrocarbons from the Rankin Platform points to a similarity of origin.