RESERVOIR AND SOURCE ROCK POTENTIAL OF THE LARAPINTA GROUP, AMADEUS BASIN, CENTRAL AUSTRALIA

Abstract

The quality of reservoir and source beds in the Cambro-Ordovician Larapinta Group of the Amadeus Basin ranges from poor to good.A general northerly decrease in porosity from about 12% at the BMR AP3 locality, to 4% in the Palm Valley area about 50 km to the northwest, is evident in the two known reservoir formations (Pacoota Sandstone and Stairway Sandstone). This regional variation in porosity is related to the depth of burial of the reservoir sandstones and their proximity to the thrust zones on the northern margin of the basin in the MacDonnell Ranges. Growth of authigenic silica has reduced primary intergranular porosity. However fracturing - developed locally in response to erosional unloading of the eastern part of the basin during the Rodingan Movement, and subsequently extended and enhanced by the Alice Springs Orogeny provides a secondary fracture porosity. The total storage capacity (average porosity x net porous thickness) of the reservoir sands is greatest in the centre of the basin.The Horn Valley Siltsone averages 1% total organic carbon (TOC) and contains up to 225 milligrams of extractable organic matter (EOM) per gram of TOC, confirming its suspected source potential. The unusual C15+ normal alkane profile of Mereenie oil is found only in the EOM of fossiliferous shales from the Horn Valley Siltstone and the Pacoota Sandstone.A northeasterly increase in the level of organic maturation of Larapinta Group sediments across the central northern sector of the basin accounts for the occurrence of light oil in the Mereenie field, but only gas (with minor condensate) in the Palm Valley field. The reflectivity of coalified graptolite fragments in the Horn Valley Siltstone and Pacoota Sandstone from the Mereenie area indicates a rank equivalent to medium-volatile bituminous coal and places the formation in the 4oil phase-out zone'. At Palm Valley similar particulate organic matter has reached semi-anthracite grade. Accordingly, the proportion of wet gas (C2+) components in reservoir gas (C1 to C4) decreases from 21-38% in the Mereenie field, through 13% at Palm Valley, to 9% in West Waterhouse No. 1.Hence, the most favourable juxtaposition of prospective source and reservoir rocks within the Larapinta Group would appear to exist in the Mereenie-Gardiner Range area.