Thermal History and Deep Overpressure Modelling in the Northern Carnarvon Basin, North West Shelf, Australia

Abstract

The Northern Carnarvon Basin is the richest petroleum province in Australia. About 50 gas/condensate and oil fields, associated mainly with Jurassic source rocks, have been discovered in the sub-basins and on the Rankin Platform since 1964. The basin is located at the southern end of the North West Shelf of Australia. It can be mainly subdivided into the Exmouth, Barrow, Dampier and Beagle Sub-basins, the Rankin Platform and Exmouth Plateau. The sub-basins are rift-related grabens and half-grabens developed during the Jurassic to the earliest Cretaceous and contain over 10 kilometres of Mesozoic and Cainozoic sedimentary rocks, among which are several thousand meters of Jurassic rocks. The formations of the Jurassic and the lower part of the Barrow Group of Early Cretaceous age in the sub-basins of the Northern Carnarvon Basin were found to be overpressured with excess pressures of 5-29 MPa at depths of 2900-3600 m indicated by repeat formation tests (RFTs) and drill stem tests (DSTs). The characteristics of organic matter, thermal history and thermal maturity, pressure seal and overpressure evolution in the sub-basins are crucial to a proper understanding of the nature and dynamic processes of hydrocarbon generation and migration in the basin. Based on organic geochemical data, the important source rocks in the basin are Jurassic organic-rich fine-grained rocks including the Murat Siltstone, the rift-related Athol Formation and Dingo Claystone. The Mungaroo Formation of the Middle-Upper Triassic contains gas-generating source rocks. These formations recognised to be organic rich based on 1256 values of the total organic carbon content (TOC, %) from 17 wells. Average TOC values (calculated from samples with TOC < 15 %) are about 2.19 % in the Mungaroo Formation, about 2.09 % in the Murat Siltstone and about 1.74 % in the Athol Formation and Dingo Claystone.Data from kerogen element analysis, Rock-Eval pyrolysis, visual kerogen composition and some biomarkers have been used to evaluate the kerogen type in the basin. It appears that type III kerogen is the dominant organic-matter type in the Triassic and Jurassic source rocks, while the Dingo Claystone may contain some oil-prone organic matter. The vitrinite reflectance (Ro) data in some wells of the Northern Carnarvon Basin are anomalously low. As a major thermal maturity indicator, the anomalously low Ro data seriously hinder the assessment of thermal maturity in the basin. This study differs from other studies in that it has paid more attention to Rock-Eval Tmax data. Therefore, problems affecting Tmax data in evaluating thermal maturity were investigated. A case study of contaminated Rock-Eval data in Bambra-2 and thermal modelling using Tmax data in 16 wells from different tectonic subdivisions were carried out. The major problems for using Tmax data were found to be contamination by drilling-mud additives, natural bitumen and suppression due to hydrogen index (HI) > 150 in some wells. Although the data reveal uncertainties and there is about ±3-10 % error for thermal modelling by using the proposed relationship of Ro and Tmax, the "reliable" Tmax data are found to be important, and useful to assess thermal maturity and reduce the influence of unexpectedly low Ro data.This study analyzed the characteristics of deep overpressured zones and top pressure seals, in detail, in 7 wells based on the observed fluid pressure data and petrophysical data. The deep overpressured system (depth greater than 2650-3000 m) in the Jurassic formations and the lower part of the Barrow Group is shown by the measured fluid pressure data including RFTs, DSTs and mud weights. The highly overpressured Jurassic fine-grained rocks also exhibit well-log responses of high sonic transit times and low formation resistivities. The deep overpressured zone, however, may not necessarily be caused by anomalously high porosities due to undercompaction. The porosities in the deep overpressured Jurassic rocks may be significantly less than the well-log derived porosities, which may indicate that the sonic-log and resistivity-log also directly respond to the overpressuring in the deep overpressured fine-grained rocks of the sub-basins. Based on the profiles of fluid pressure and well-log data in 5 wells of the Barrow Sub-basin, a top pressure seal was interpreted to be consistent with the transitional pressure zone in the Barrow Sub-basin. This top pressure seal was observed to consist of a rock layer of 60-80 % claystone and siltstone. The depths of the rock layer range from 2650 m to 3300 m with thicknesses of 300-500 m and temperatures of 110-135 °C. Based on the well-log data, measured porosity and sandstone diagenesis, the rock layer seems to be well compacted and cemented with a porosity range of about 2-5 % and calculated permeabilities of about 10-19 to 10-22 M2.This study performed thermal history and maturity modelling in 14 wells using the BasinMod 1D software. It was found that the thermal maturity data in 4 wells are consistent with the maturity curves predicted by the rifting heat flow history associated with the tectonic regime of this basin. The maximum heat flows during the rift event of the Jurassic and earliest Cretaceous possibly ranged from 60-70 mW/m2 along the sub-basins and 70-80 mW/m2 on the southern and central Exmouth Plateau. This study also carried out two case studies of thermal maturity and thermal modelling within the deep overpressured system in the Barrow and Bambra wells of the Barrow Sub-basin. These case studies were aimed at understanding whether overpressure has a determinable influence on thermal maturation in this region. It was found that there is no evidence for overpressure-related retardation of thermal maturity in the deep overpressured system, based on the measured maturity, biomarker maturity parameters and 1D thermal modelling. Therefore, based on the data analysed, overpressure is an insignificant factor in thermal maturity and h hydrocarbon generation in this basin.Three seismic lines in the Exmouth, Barrow and Dampier Sub-basins were selected and converted to depth cross-sections, and then 2D geological models were created for overpressure evolution modelling. A major object of these 2D geological models was to define the critical faults. A top pressure seal was also detected based on the 2D model of the Barrow Sub-basin. Two-dimensional overpressure modelling was performed using the BasinMod 2D software. The mathematical 2D model takes into consideration compaction, fluid thermal expansion, pressure produced by hydrocarbon generation and quartz cementation. The sealed overpressured conditions can be modelled with fault sealing, bottom pressure seal (permeabilities of 10-23 to 10-25 M2 ) and top pressure seal (permeabilities of 10-19 to 10-22 m2). The modelling supports the development of a top pressure seal with quartz cementation. The 2D modelling suggests the rapid sedimentation rates can cause compaction disequilibrium in the fine-grained rocks, which may be a mechanism for overpressure generation during the Jurassic to the Early Cretaceous. The data suggest that the present-day deep overpressure is not associated with the porosity anomaly due to compaction disequilibrium and that compaction may be much less important than recurrent pressure charges because most of the porosity in the Jurassic source rocks has been lost through compaction and deposition rates have been very slow since the beginning of the Cainozoic.Three simple 1D models were developed and applied to estimate how rapidly the overpressure dissipates. The results suggest that the present day overpressure would be almost dissipated after 2 million years with a pressure seal with an average permeability of 10-22 M2 (10-7 md). On the basis of numerous accumulations of oil and gas to be expelled from the overpressured Jurassic source rocks in the basin and the pressure seal modelling, it seems that the top pressure seal with permeabilities of 10-19 to 10-22 M2 (10-4 to 10-7 md) is not enough to retain the deep overpressure for tens of millions of years without pressure recharging. Only if the permeabilities were 10-23 m2 (10-8 md) or less, would a long-lived overpressured system be preserved. This study suggests that hydrocarbon generation, especially gas generation and thermal expansion, within sealed conditions of low-permeability is a likely major cause for maintaining the deep overpressure over the past tens of millions of years. Keywords: Thermal history; Deep overpressure; Type III kerogen; Rock-Eval Tmax; Thermal maturity; Palaeoheatflow modelling; Pressure seal; 2D deep overpressure modelling; Pressure behaviour modelling; Overpressure generation; Northern Carnarvon Basin.