Thermal History Analysis Of The Barrow And Dampier Sub-Basins, North West Shelf, Western Australia
Kaiko, Alexander Ronald
Doctor of Philosophy, 1998
University of South Australia
The Barrow and Dampier Sub-basins, offshore Western Australia, contain major oil and gas accumulations associated with Triassic to Cretaceous reservoirs and source rocks. Jurassic-Cretaceous formations of predominantly marine origin have vitrinite reflectance values that are often lower than expected from present day thermal conditions, attributed to either a recent heating event or vitrinite reflectance suppression. In order to constrain the timing of hydrocarbon generation it is important to understand the cause of these anomalously low vitrinite reflectance values.
A simple geohistory model combined with multiple maturity parameters shows that the sediments in the majority of wells are at their maximum temperature today and that the anomalously low vitrinite reflectance values are due to suppression, not to recent heating. Because the majority of sediments are at maximum temperature today, corrected bottom hole temperature data have been used to update the maturity estimates, leading to significant reductions in the depth to the 0.7 %Ro isoreflectance surface. The re-evaluation has also highlighted the fact that in some areas potential source rocks have been ignored because of their perceived low maturity.
The use of a simple burial model has also indicated that higher temperatures have occurred in the past in some areas along the eastern margin of the sub-basins and around the Alpha Arch. Along the eastern margins the higher temperatures are mainly due to deeper burial. However, along the Alpha Arch the higher temperatures are due to elevated heatflow related to rifting or to a combination of deeper burial and elevated heatflow related to rifting.
The match between the calculated and measured maturity profiles for wells along the Rankin Trend shows that this region is at maximum temperature today and that any temperature history related to Jurassic rifting and missing section has been overprinted by the present thermal conditions. Thus any of a broad range of values for missing section, e.g. 0 to 1700m at Gorgon #1 shows a good match between calculated and measured maturity data sets.
Vitrinite reflectance, spore colour and apatite fission tracks record the maximum temperatures since deposition. Modelling of thermal histories in wells which are at maximum temperatures today is limited in that any thermal effects associated with cycles of deeper burial and uplift and erosion are overprinted by present conditions. In such cases siderite diagenesis and illite dating may help to constrain thermal histories. Conversely thermal history studies can also provide absolute timing of diagenetic events.
To overcome the problems associated with measuring vitrinite reflectance, a technique using combined fluorescence and reflectance (CFR) measurements on dispersed organic matter was investigated and a set of guidelines developed for the interpretation of the raw data. The CFR technique uses fluorescence and reflectance to identify the perhydrous vitrinite, normal vitrinite and inertinite maceral populations amongst the dispersed organic matter. CFR analysis has been shown to be a useful technique for identifying and correcting for vitrinite reflectance suppression.
The correlation of several lines of evidence suggests that vitrinite reflectance suppression, in the Barrow and Dampier Sub-basins, as possibly due to several processes. These processes include the variation in floral precursors, the concentration of hydrogen-rich material by biodegradation during transport, and possible chemical alteration during transport and early diagenesis.