CARPENTARIA BASIN: A REGIONAL ANALYSIS WITH REFERENCE TO HYDROCARBON POTENTIAL

Abstract

The Carpentaria Basin is an epeirogenic intracratonic downwarping which formed during the Mesozoic. It is the northernmost of three main Jurassic-Cretaceous downwarps forming the Great Australian Artesian Basin. The basin is filled with clastic sediments, predominantly derived from Precambrian ridges on the flanks of the basin, which thicken gradually from the basin margins towards the depocentre.

The Carpentaria Basin is analogous to the Eromanga Basin immediately to the south, and several rock units were deposited contemporaneously in both basins so that they can be mapped in the subsurface from one basin to the next. There is no change in sedimentary style or lithology.

The character of the Base Mesozoic Unconformity, over which the Carpentaria Basin was deposited, changes abruptly from east to west. East of a point approximately coincident with the Queensland-Northern Territory border the unconformity surface is flat and featureless but to the west it becomes very rugged and irregular. This rugged horizon is believed to reflect either ancient karst topography or extensive reefal build-up equivalent to dolomite intersected in Burketown 1. Based on correlations with the McArthur and Georgina Basins it is probably Cambrian or Proterozoic in age. These rocks are known elsewhere to be porous and permeable.

The unconformity shelves to the west towards Groote Island and the Northern Territory mainland and sediments of the Carpentaria Basin onlap a shallowing ancient landscape. The western boundary of the basin believed to be prospective for hydrocarbons is taken as the limit of deposition of the Cretaceous Gilbert River Formation.

Lithologies and the existence of aquifers in the onshore Carpentaria Basin indicate that suitable reservoirs are present. Sandstones in the Cretaceous Gilbert River Formation, the Jurassic Eulo Queen Group and the Hutton Sandstone have the best reservoir potential. The Gilbert River Formation is the primary objective because it was extensively deposited throughout the basin, whereas Jurassic sandstones are restricted to topographic lows.

The Gilbert River Formation is sourced and sealed by shales of the Wallumbilla Formation. Onshore this formation is too shallow to be mature but seismic indicates that offshore it is sufficiently deeply buried to be producing and expelling hydrocarbons.

The hydrocarbon prospects of the Carpentaria Basin have in the past been considered low because the Mesozoic sequence onshore is thin and even offshore the total thickness of Mesozic and Cainozoic sediments is only about 1760 m at its maximum. This judgement ignored the fact that, although no economic hydrocarbon discoveries have been recorded from the Carpentaria Basin and/or the underlying Palaeozoic Basins, several shows have been reported. Shows reported in onshore wells suggest hydrocarbon generation within the basin. Geochemical and palynological studies carried out on samples from onshore wells and bores indicate the basal Mesozoic sequence is within geochemical Zone II, where the actual generation of hydrocarbons occurs but where significant mobilization and effective oil expulsion do not take place. These data suggest the deeper offshore portion of the basin should be within the oil window.