Compressional deformation and exhumation in sedimentary basins at ‘passive’ continental margins, with implications for hydrocarbon exploration and development.

Abstract

There is growing recognition that extensive phases of compressional deformation and exhumation have interrupted the post-rift subsidence histories of some economically important ‘passive’ continental margins. Understanding the distribution, magnitude, chronology and causes of exhumation and compressional deformation at these margins can reduce exploration uncertainty. The Otway and Faroe-Shetland basins along the southern Australian margin and northwest European Atlantic ‘passive’ margins, respectively, provide ideal natural laboratories to further understand syn and post-rift compressional deformation, inversion and exhumation. Post-Albian exhumation in the Otway Basin was quantified to be ~400-3600 m across the eastern and northern parts of basin using a new sonic transit time-depth trend, which represents normal compaction of volcaniclastic shales deposited within a fluvio-lacustrine environment – unlike any other such trends previously published. These estimates are consistent with those from complementary thermal, palynological and seismic datasets. Whilst the impacts of exhumation are well known for conventional hydrocarbon systems, this study is amongst the first to highlight the implications of exhumation on unconventional hydrocarbon systems, in particular related to petrographical and geomechanical rock properties. Exhumation in the Otway Basin is mainly related to mid-Cretaceous and Neogene neotectonic compressional deformation and inversion episodes, with the latter strongly governed by the contemporary stress state. Using complementary geophysical datasets and considering lithological heterogeneity, basement fabrics and variations in structural style with depth – factors generally neglected in previous geomechanical-focused studies in this region – it is possible to better understand the relationship between neotectonic deformation and stress. Comparisons between bulk crustal strain rates based on Neogene shortening estimates, and present-day strain rates of based on earthquake data and geological observations demonstrates that strain rates in the Otway Basin have declined since the onset of Neogene compressional deformation and exhumation. Neogene bulk crustal strain rates determined independently from shortening estimates and exhumation magnitudes yield similar results, suggesting that Neogene exhumation in the eastern Otway Basin can be accounted for solely by crustal shortening within a mildly compressional intraplate stress field, with ~30% of the total present-day strain rate accounted for by aseismic deformation. In the central parts of the offshore Otway Basin, where there is very thick preserved Upper Cretaceous sequence and few indications of major post-Albian tectonic activity, significant and previously unreported overpressures are examined. Pore pressure gradients exceed ~16 MPa/km within the fine-grained Upper Cretaceous Belfast and Flaxman formations, and are most likely due to a disequilibrium compaction associated with Pliocene burial by a proto-Murray River discharge. Estimating exhumation in the Otway Basin using sonic log data provided consistent values with thermal-based techniques, indicating that heating can be related (in part) to deeper burial. However, this may not hold true in all basins. More than ~400m of post-Danian exhumation was quantified using sonic data along the Rona High in the Faroe-Shetland region where thermal history data indicates anomalous heating due to transient hot fluid flow and is problematic for exhumation analyses. This exhumation likely occurred during the Oligocene to Mid-Miocene in response to a major reorganisation of the northern North Atlantic spreading system.