SOURCE POTENTIAL OF THE HORN VALLEY SILTSTONE, AMADEUS BASIN

Abstract

The quantity of organic matter in the source beds within the Horn Valley Siltstone, as defined by the Total Organic Carbon content, increases westward from low values in the south and east of the Basin to maximum values in the Mt Winter and Mereenie areas. This westerly enrichment trend is paralleled by an improvement in source rock quality, as defined by the Hydrogen Index and Tmax crossplot of samples analysed by Rock-Eval pyrolysis.

Earlier attempts to measure thermal maturation levels of source rocks in the Basin relied on the reflectivity of coalified graptolites but this method was only applicable to unweathered material obtained from the few and scattered bore holes in the Basin. In this study, conodont colour alteration is used to define organic maturation levels. This technique, newly applied in Australia, was used principally on samples collected from the Horn Valley Siltstone and has the practical advantage of being applicable to samples from both outcrop and subsurface localities.

The study indicates that the conodont colour alteration isograds in the Amadeus Basin are primarily related to events of the Alice Springs Orogeny, when the thick mass of molasse sediments (Pertnjara Group) resulting from erosion of the uplifted Arunta Block was deposited. Anomalies in the conodont colour isograds are closely related to timing of structural growth during the orogeny and also possibly to the growth of salt structures.

In addition, the study shows that burial at depths below 1500 m will have led to the catagenetic breakdown of reservoired oil and the production of only gaseous hydrocarbons from source beds.

In combination, these two factors lead to the conclusion that the most prospective area for oil sourced by the Horn Valley Siltstone is north and west of the Mereenie Oil Field in areas of shallow burial.