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Thermal History And Reservoir Property Correlations With Hydrocarbon Type, North West Shelf, Australia.

Filipi, Igor

Honours Degree, 2001

University of Adelaide

Abstract

Santos's geoscientists have noticed that oil discoveries on the North West Shelf are uncommon where present temperatures are above 120-130°C. The main aim of this project is to determine whether there is a correlation between reservoir quality parameters, such as porosity and permeability, with temperature and thermal maturity. If so, a better prediction of the probability of oil accumulations on the North West Shelf might be possible.

Core porosity and permeability values show that sandstone reservoirs in the North West Shelf are in general of high quality. More than 70% of the samples have porosity higher than 15%, and more than 65% of the samples have permeability higher than 100 mD.

Results show that porosity is correlative with reservoir temperature. Porosity decreases with increasing temperature for all sub-basins and formations in the North West Shelf. However, the strongest relationship exists between thermal maturity and porosity confirming temperature and time are effective controlling factors on reservoir quality. Areas with higher geothermal gradients have lower porosities than areas with lower geothermal gradients. A weak correlation exists between core permeability and temperature, and with vitrinite reflectance, indicating the importance of other factors on reservoir quality e.g. depositional environment. However, maximum permeability is strongly dependent on temperature.

Reservoir porosity and permeability are still good despite deep (> 3 km) burial and higher temperature (> 120°C). Temperature in combination with other factors is one of the most important controlling factors affecting reservoir quality. However reservoir deterioration is not the reason for lack of oil accumulations at temperatures higher than 120°C.

Results from the correlation of the reservoir temperature, thermal maturity and hydrocarbon type indicate that the highest probability for oil discovery on the North West Shelf exists at temperature between 60-120°C. Most of oil accumulations occur at the top or above the oil window (vitrinite reflectance range of 0.3 to 1.0).

Exploration risk in finding oil accumulations significantly increases at temperatures greater than 120°C, as more than 90 % of discovered oil accumulations occur at lower temperatures. These observations are consistent with observations from other petroleum provinces in the world (e.g. the North Sea province). However, the probability for gas discovery is still high at greater depth and temperature. This relationship can be useful in evaluation of petroleum exploration risks for similar play types both at basin scale and prospect level.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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