Linear Trends of Paleo-Pockmarks and Fluid Flow Pipes in the Jurassic and Triassic Sediments of Offshore Northwest Australia

Abstract

This study records 315 paleo-pockmarks with associated focused fluid flow pipes within the Jurassic and Triassic sediments over three study areas on the Exmouth Plateau, offshore Northwest Australia. Paleo-pockmarks are identified along a surface that represents the top of Jurassic sediments, while the fluid flow pipes extend into Triassic sediments from the base of these pockmarks. The pockmarks and pipes form in linear trends that are parallel to and laterally offset from the tops of extensional faults intersecting an interval from the top of Jurassic sediments into Triassic sediments, where they are seen to terminate. Bases of the fluid flow pipes are observed to intersect and terminate along these extensional faults within the Triassic sediments. The pockmarks and associated fluid flow pipes are interpreted to have formed when extensional faults developed that intersected an overpressured unit within Triassic sediments. This caused a localized reduction of lithostatic pressure along the overpressured sequence at the intersection which then acted as a focal point for fluid escape and vertical migration. The source of the fluid overpressure could not be confirmed in this study. The Triassic sequence is a known hydrocarbon source and 1D modelling shows that at the time of fluid flow and pockmark formation, these Triassic sediments were entering the hydrocarbon generation window. However, no evidence of hydrocarbons associated with the pockmarks has been observed. Our findings identify fluid migration pathways that are seal risks for hydrocarbon reservoirs, but could also potentially be fluid migration pathways that were previously untested.