The Role Of Focused Fluid Flows In Improved Understanding Of Subsurface Pressure: A Case Study From The Northern Carnarvon Basin, Northwest Australia

Abstract

Focused fluid flows are phenomena where subsurface fluids are remobilized to shallower depths through focusing mechanisms. The nature of migrating fluids can vary from hydrocarbons, to biogenic gasses, brine or even hydrothermal fluids. Focused fluid flows are often associated with differential pressures or overpressured sediments. In the deeper subsurface, fluids that experience sufficiently high pressures are able to hydraulically fracture overlying sediments and migrate vertically through the strata. Similarly, geological features that locally reduce the overburden pressure, such as faults or sediment slumping, are able to induce focused fluid flows. As such, identification of generation mechanisms and fluid sources that form focused fluid flows can lead to improved understanding of the local subsurface pressure regime. This is especially valuable in frontier hydrocarbon exploration areas where seismic data is more readily available and well data is often limited or absent. The Exmouth Plateau of the Northern Carnarvon Basin, northwest Australia is a submerged platform that structurally defines the northwestern portion of the basin. While the proximal part of the basin is one of Australia’s premier hydrocarbon exploration and production regions, the Exmouth Plateau is underexplored in comparison. Investigation of three seismic reflection surveys from the plateau reveal the occurrence of paleo-focused fluid flows (n=315) in the Triassic and Jurassic sequences. These fluid flow features are expressed as paleo-pockmarks, all of which had formed along an unconformity surface that represents the top of the Jurassic sequence. Underlying the paleo-pockmarks are vertical zones of disrupted seismic reflections, interpreted to be fluid feeder pipes, which extend down and terminate within the Triassic sequence. Heights of the feeder pipes range between 75–750 m. The paleo-fluid flow features in the three survey areas occur in linear trends (NW-SE in survey 1, N-S in survey 2, and NNE-SSW in survey 3), suggesting an underlying control on their spatial distributions. Further investigation of the seismic data reveals arrays of low throw (~20-70 m) extensional faults with strikes that are parallel to the linear trends of the paleo-fluid flows. The tops of these faults are along the top of the Jurassic sequence and they extend down into the Triassic sequence where they terminate or drop below seismic resolution. The upper tips of the faults are laterally offset from the paleo-pockmarks with distances ranging from 170–1000 m with the paleo-pockmarks occurring only within the hanging walls of the faults. When analysed on cross-sections, the bases of the fluid feeder pipes are found intersecting the normal fault planes within the Triassic sequence. This observation, along with the lack of fluid flow features identified overlying the tops of faults, suggests that the faults did not act as fluid migration pathway but instead acted as a triggering mechanism for the fluid flow features. The fault planes are interpreted to have intersected an overpressured unit within the Triassic sequence. The point of intersection locally reduced the overburden pressure and focused the drainage of the fluids that subsequently migrated vertically through the strata and were expelled along the top of the Jurassic sediments. The timing of the fluid flow event was constrained to have occurred at the end of the Jurassic, when regions of the Exmouth Plateau were uplifted, and prior to the deposition of the Cretaceous sediments. Though we are unable to determine the source and extent of fluid overpressure, this potential dissipation of pressure through focused fluid flow could have implications for hydrocarbon prospectivity in the underexplored Exmouth Plateau.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018