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Petrophysics of the Northern Browse Basin

Irma Savitri Widiasri
Master of Science (Petroleum Geoscience) - 2007
Australian School of Petroleum
The University of Adelaide


Abstract

The Northern Browse Basin, located on the North West Shelf of Australia, is a gas province
with minor oil accumulations. Significant gas discoveries have been made within the Caswell
Sub-basin, including the Scott Reef, Brecknock, Brecknock South and Brewster Fields. This
area is a focus of exploration for the Chevron Australia Business Unit (Chevron ASBU). With
the increasing potential for commercialization of the long-discovered gas in the Northern
Browse Basin, Chevron ASBU's interest, and the aim of this project, is to identify any
additional unrecognized gas reservoirs in the Late Triassic to Late Cretaceous rocks of the
Northern Browse Basin. The section of interest covers the Nome, Plover, Lower Vulcan, Upper
Vulcan, Echuca Shoals, Jamieson, Woolaston, Fenelon/Gibson and Puffin Formations.
Petrophysical analysis of well logs was undertaken to identify unrecognized potential
reservoirs.
A stepwise methodology was undertaken in this study. Log editing, pre-calculation and
environmental corrections were first performed. A deterministic method was then used in the
petrophysical analysis. The lowest of the estimates from the gamma ray and the density-neutron
combination was taken as the shale volume. The Raymer-Hunt-Gardner sonic porosity
transform was then used to calculate the porosity, with calibration to core data. Water saturation
was determined using the dual water method for shaly sand.
A major uncertainty of this study was the determination of formation water resistivity (Rw).
Formation water resistivity was determined using Pickett plots and assumed to be the same
through the entire Late Triassic-Late Cretaceous sequence analyzed in each well. The resultant
Rw values describe the salinity distribution within the study area. Salinity generally increases
from southwest to northeast, with a range from 12,000 to 80,000 ppm.
Twelve out of the 16 analyzed wells were interpreted to contain potential net pay, which was
encountered in the Puffin, Jamieson, Echuca Shoals and Upper Vulcan Formations. In the
Puffin Formation, significant potential net pay was interpreted in Gryphaea-1, Discorbis-1 and
ii
Kalyptea-1ST1, with the greatest thickness of 127 m being identified in Kalyptea-1ST1.
Potential net pay in this formation has good reservoir quality, with an average of 17% shale
volume, 20% porosity and 50% water saturation. Potential net pay in the Jamieson Formation
averaged 25 m in thickness and 20% porosity. A half of the wells that encountered the Echuca
Shoals Formation are interpreted to contain potential net pay in that unit, with the maximum
thickness being 61 m in Adele-1. A very significant potential net pay of 430 m was interpreted
in the Upper Vulcan Formation in Heywood-1. The Upper Vulcan Formation has good
reservoir quality with an average of 125 m net pay thickness and 10% porosity in five out of
seven wells.
This project was a reconnaissance study for possible unrecognized reservoirs in the Northern
Browse Basin. It has identified many possible potential reservoirs and follow-up study should
focus on re-analyzing the intervals highlighted here. A reservoir by reservoir analysis,
especially when picking the parameter, such as Rw, would reduce the uncertainties. A
probabilistic method of log analysis e.g. Multimin in Geolog (which was briefly attempted
here), should be investigated as a replacement for deterministic methods such as the one used in
this study.
Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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