Palaeogeothermal Gradient Research Articles

TROBRIAND BASIN EXPLORATION, PAPUA NEW GUINEA

Regional studies suggested that the Solomon Sea would be underlain by upper Tertiary sediments inluding possible Miocene reef carbonates similar to those found in the Gulf of Papua and Irian Jaya. As the Trobriand area of the Solomon Sea lies in a zone of interaction between the Australian and Pacific tectonic plates, it was considered likely that Tertiary basins prospective for petroleum would be present. In 1969 the East Papua aeromagnetic survey revealed a magnetic low which was interpreted as a basin, here named the Trobriand Basin. A sedimentary section of some 3000 m, situated under shallow water was indicated.Amoco, Australian Oil & Gas and Southern Pacific Petroleum made application for and were granted exploration permit PNG/15P in June, 1971. The Group initially undertook field geological, aerial photographic and hydrographic surveys which revealed the presence of numerous Pliocene to Recent coral reefs throughout the permit and also indicated the likely nature of economic basement. Between April 1972 and May 1973, three marine seismic surveys by Western Geophysical produced 2250 km of reflection profiles. The seismic data suggest that the Trobriand Basin is an east-west trending graben filled with up to 5000 m of probable Miocene and younger sediments. Positive structures, of which several were interpreted as mid-Miocene reefs, were mapped.Two subsidised exploratory wells, Goodenough No. 1 and Nubiam No. 1, were drilled in 1973. Only minor and questionable hydrocarbon shows were encountered and both wells bottomed in Miocene volcaniclastics. The wells penetrated immature upper Tertiary sediments with low present and palaeo-geothermal gradients and consequently the sediments might be considered an unfavourable environment for petroleum generation. Nevertheless, the Trobriand Basin has not been adequately explored for hydrocarbon accumulations as only a portion of the Tertiary section has been evaluated in two widely-spaced wells.

Palaeogeothermal gradients derived from xenoliths in kimberlite

GREAT problems exist in the determination of an ancient geothermal gradient. An attempt has been made by Boyd1, based primarily upon the equilibration conditions of coexisting pyroxenes in garnet lherzolite xenoliths from the Thaba Putsoa, Letseng and Mothae kimberlite intrusions in northern Lesotho. He derived the equilibration temperature of the xenoliths from the amount of enstatite in solid solution in the diopside2, and estimated the pressure from the amount of Al2O3 in the orthopyroxene3. He argued that, for a group of lherzolite xenoliths, a line joining a series of pyroxene-derived pressure/temperature points should define the geothermal gradient in the mantle below northern Lesotho during the Cretaceous when the xenoliths were entrained by the kimberlite during its ascent. He found a Cretaceous gradient similar to that in present-day shield areas between depths of 100 and 150 km, though from 150 to 200 km depth it was considerably steeper. Moreover, whereas the lherzolites outlining the shallower ‘normal’ part of the gradient are of granular texture, those outlining the high-temperature, steeper and deeper segment exhibit a range of deformation and recrystallisation textures (the porphyroclastic and mosaic textures in the classification scheme of Bouiller and Nicolas4).

DIAGENESIS OF ORGANIC MATTER AND OCCURRENCE OF HYDROCARBONS IN SOME AUSTRALIAN SEDIMENTARY BASINS

It is important that petroleum exploration geologists know the critical depth limits where oil is generated from original organic matter in sediments and where the oil changes to natural gas. Organic matter is very sensitive to temperature. The maximum temperature experienced is related to its depth of burial.CSIRO has used the composition and physical properties of various types of organic matter in shaly rocks as indicators for the degree of diagenesis caused by this heat alteration. The reflectance of vitrinite in associated coals is used as the primary standard, and carbon content of such coals as the secondary parameter to distinguish various stages of oil and gas generation. Depth-reflectance curves are useful 1., for estimating palaeogeothermal gradients, 2., for determining the degree of diagenesis at a particular depth and also 3., for estimating the approximate thickness of sediments subsequently lost after deposition.The petroleum potential of some Australian sedimentary basins is reviewed in the light of this knowledge. In the Northwest Shelf area and in the Capricorn and Otway Basins, the oil generation zone is deeper than in the Cooper, Galilee and Surat Basins. In the Bowen and Sydney Basins and several other small basins along the eastern coast of Australia, this zone is very shallow, and in some areas the oil generation zone has been completely lost by erosion. The areas most promising for oil fields are those where little erosion of sediments has taken place subsequent to deposition and diagenesis, provided that all other geologic factors for hydrocarbon accumulation are present.