The Neoproterozoic Gillen Formation, Amadeus Basin, central Australia: an intra-salt petroleum system and viable exploration target?

Abstract

The Gillen Formation, the basal component of the lower Neoproterozoic Bitter Springs Group, occurs throughout the Amadeus Basin of central Australia and comprises a basal sequence of black shales and stromatolitic dolostones overlain by an evaporitic sequence of a thick halite sandwiched between two anhydrite/dolomite units. These Gillen lithologies include potential source, reservoir and seal units for petroleum, suggesting an analogy could be drawn with the prolific evaporitic Ara Group Petroleum System of South Oman. Although well data penetrating the Gillen Formation are very limited, organic-rich shales have been penetrated in both the north and south of the basin, while both well and seismic data indicate the ubiquitous presence of sealing evaporites across the basin. The Ara Group Petroleum System spans the Neoproterozoic–Cambrian boundary and is blessed with carbonate reservoirs composed of metazoan ‘reefs’ comprising Cloudina and Namacalathus, which provide an open framework porosity approaching 50%. By contrast, the Gillen Formation was deposited during the Tonian Period of the Neoproterozoic, some 300–350 million years older than the Ara Group, and has reservoir facies limited to stromatolitic carbonates with fenestral to vuggy porosities that are typically less than 15%. Nevertheless, seismic data reveal numerous intra-evaporite opportunities within the Gillen Formation that, seismically, are virtually indistinguishable from those being exploited in South Oman, suggesting that a Gillen Intra-Salt Petroleum System is a viable exploration target throughout a majority of the Amadeus Basin.