Organic Rich Shale in Permian Fjords — A Potential Resource Play in the Arckaringa Basin, South Australia

Menpes, Sandra A.

In South Australia, Early Permian sediments infill an erosional land surface shaped by the Late Carboniferous-Early Permian Gondwana glaciation. The Arckaringa Basin in South Australia comprises a number of deep erosional troughs recording Australia's time in the southern polar region.

A recent sequence stratigraphic interpretation of the Arckaringa Basin has resulted in a better understanding of the basin fill, in particular the distribution of the organic rich marine shales in the succession. A sequence boundary cuts down into underlying glaciogenic sediments in the landward part of the basin but shows little evidence of erosion in the more seaward part of the basin. Sediments above the sequence boundary, deposited during the marine transgressive phase, onlap palaeo-topographic highs. A maximum flooding surface at the base of the overlying delta succession has been identified and flattening of seismic lines on this horizon demonstrates the downlapping of clinoforms onto the surface. A prominent peak in the gamma-ray curves of wells in the basin coincides with the maximum flooding surface.

Thick organic rich shales with Type II source potential have been intersected in both the southern Arckaringa troughs (TOC's ranging 4.1 - 7.4% over 160m) and the Boorthanna Trough (TOC's ranging 3.9 - 10.4% over 70m). The organic rich shales fall below the gamma ray peak marking the maximum flooding surface, and are transgressive systems tract deposits.

Seismic data shows that the valleys were not completely filled with sediment when the sea transgressed, resulting in long and narrow fjords with a complex palaeo-bathymetry. Abundant algal material recorded in the organic rich shale, including the marine alga Tasmanites, suggests that nutrient rich surface water and anoxic bottom water conditions were present at times in the restricted seaways. These conditions have been recorded in modern fjords associated with density stratification of the water column. Micropalaeontological assemblages indicate lacustrine to brackish to restricted marine environments through much of the Early Permian succession.

The thick organic rich shales of the Arckaringa Basin are now being pursued as a potential shale oil play. The shale is at the threshold of oil generation over parts of the basin, but oil shows have been recorded elsewhere. Exploration efforts are currently directed at determining if a shale oil play fairway is present in the basin.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013