ROBE RIVER — AN ONSHORE SHALLOW OIL ACCUMULATION

Abstract

Aeromagnetic depth-to-basement estimates made in 1966 led to the concept of the "Robe River Embayment", a structural depression related to a block of Palaeozoic sediments which has been douwnfaulted into the Precambrian in the northeastern corner of the onshore Carnarvon Basin. During a subsequent seismic survey (1966-67), a deep shothole blew out at 77 m. About 20 litres of heavy brown crude oil were recovered, followed by a strong artesian water flow. The search for a shallow oil accumulation followed with a programme of nine coreholes in 1967-68. Shows of oil and gas were encountered in seven of the wells, but producibility was not established. Further drilling in 1969 (one well), 1972 (five wells) and 1974 (two wells) has better defined the area of hydrocarbon occurrence, but no significant tests have resulted despite promising shows in many of the wells.

Robe River oil is of low gravity (14.5 to 20° API), highly aromatic, and biodegraded. It is found at depths ranging from 65 to 165 m mainly within the low-permeability Mardie Greensand. The greensand is underlain by the highly permeable Yarraloola Conglomerate which is an artesian aquifer and has probably acted as the main conduit for oil migration out of the Barrow Sub-basin. There is no evidence of major structural control on the Robe River oil shows which occur sporadically over a large area. Whilst permeability barriers within the Mardie Greensand probably influence the present distribution of hydrocarbons, there is also evidence that the hydraulics of the Yarraloola Conglomerate have been important in the localization of this accumulation. Water salinity studies suggest that the influx of meteoric water from the Yarraloola Conglomerate outcrop has resulted in a hydrodynamic trap for oil as it migrated updip from the Barrow Sub-basin. During the late Tertiary a much larger accumulation may have existed within the Yarraloola Conglomerate and the Mardie Greensand. The northwesterly flow of water has now ceased and the oil has dispersed, except where it is trapped within the relatively impermeable Mardie Greensand.