MURILLA CREEK — AN UNTESTED STRATIGRAPHIC PLAY IN THE SURAT BASIN

Abstract

In 1978 Shell farmed into Western Mining Corporation's (WMC) permit ATP-241P in the central Surat Basin. A major objective of the venture was to explore for Lower Jurassic stratigraphic traps in the Murilla Creek area on the western flank of the Mimosa Syncline.

The Murilla Creek stratigraphic play comprises a series of sand-filled valleys incised into the base Jurassic unconformity surface, forming a dendritic drainage pattern which supplied sediment to the braided stream deposits of the Lower Precipice Sandstone to the east. At their western depositional limit these potential reservoirs are likely to be sealed vertically by a regionally extensive lacustrine shale, and laterally by siltstones and shales within the sub-cropping Middle Triassic Moolayember Formation. Oil charge may either be directly from underlying Triassic source rocks or indirectly from the Permian Blackwater and Back Creek Groups. Exploration of the Murilla Creek area involved the acquisition of 776 km of experimental, reconnaissance and infill seismic and one well (Coalbah 1) which enabled seismic calibration and delineation of the Lower Precipice Sandstone to its seismically resolvable limits.

Seismic interpretation confirmed the potential for several stratigraphic traps in the Murilla Creek area. However trap geometries and seal constraints suggest that any stratigraphically trapped oil accumulation would be small (less than 3 MMBBL). Coupled with the considerable geological risks associated with the stratigraphic play, the low potential rewards rendered the play economically unattractive to Shell and WMC and the acreage was subsequently relinquished. However the Murilla Creek stratigraphic play remains untested and, in view of the potential for numerous small oil accumulations, may prove to be an attractive exploration target under more favourable economic circumstances.