A Review of the Global Polygonal Faults: Are They Playing a Big Role in Fluid Migration?

Abstract

Polygonal faults (PFs) have been widely found in over 100 sedimentary basins worldwide, mainly in marine setting on continental margins and intracratonic sedimentary basins. PFs are characterized by layer-bound minor normal faults arranged in polygonal patterns and multi-direction strikes, most of which were developed in fine-grained sediments rich in claystone. The genesis mechanisms of PFs have been recognized as non-tectonic, mainly including shear failure, syneresis, density inversion, low coefficients of friction and gravity sliding theory. Previous studies often regarded PFs as conduits playing a role in favouring fluid migration, which were also regarded as fluid source expelled during PF generation. However, some researchers proposed that the PF-bearing layer is generally impermeable and may act as a seal. To better understand the role of PFs in fluid migration and their contrition to the shallower hydrocarbon accumulation, a geophysical review of global PFs case studies is conducted in this paper. A comprehensive analysis is carried out based on abundant case studies, with key parameters analyzed including the PFs characteristics (e.g., buried depth, lithology, size, etc.), formation mechanisms of PFs, tectonic-sedimentary setting, and the spatial relationship of PFs, nearby major fluid migration pathways and high amplitude anomalies. The compilation of global PF case studies of shows that hydrocarbon fluids from the deep is more likely to migrate upward through normal big faults, gas chimneys or unconformities surface around PFs. In most cases, PFs play limited roles in fluid migration, such as in South China Sea where hydrocarbon accumulations have been observed under the PFs and the PF-bearing layer may act as a seal trapping the free gas underneath. Generally, the role of PFs in fluid migration should be determined through a comprehensive analysis of the characteristics of PFs, surrounding amplitude anomalies, hydrocarbon distribution, and even the sealing ability of PFs which needs further study. It is more likely that the PFs play a limited role in vertical fluid migration and related shallower hydrocarbon accumulation and distribution.

Seismic stratigraphy of the Guinea Plateau before, during and after the opening of the Equatorial Atlantic Gateway

“…A large, laterally extensive, polygonal fault field in the SU12 unit of the NW Guinea margin slope domain (Figs. 4,9) indicates biosiliceous sediments, with polygonal faults forming due to volume loss during dewatering and compaction (Cartwright, 1993;Xia et al, 2022).…”

Geologic controls on the genesis of the Arctic permafrost and sub-permafrost methane hydrate-bearing system in the Beaufort–Mackenzie Delta

“…Hydrocarbon fluids from the deeper sequences are likely to migrate upward through present fault systems (Xia et al, 2022), gas chimneys (McNeil et al, 2011), mud diapirs, mud volcanoes (Zhang et al, 2021), and unconformities (Levell et al, 2010). However, gas chimneys and unconformities, mud diapirs, and mud volcanoes have not yet been reported near the onshore subpermafrost GH reservoirs in the MD.…”

A Review of the Migration of Hydrogen from the Planetary to Basin Scale

“…−100 m throw) faults restricted to individual sedimentary layers, known as polygonal fault systems(PFS) have been identified as having important impacts on basin-scale fluid flow. The impact of PFS on fluid migration is debated within the literature, with authors attributing PFS for both enhancing (e.g.,Ireland et al (2021);Cartwright (2011);Cartwright et al (2003)) and restricting (e.g.,Xia et al (2022);Andresen and Huuse (2011)) fluid flow. Whilst PFS have been identified as a mechanism for seal bypass(Ireland et al, 2021), permeability may be effectively destroyed by clay smearing along fault planes and thus increase the sealing capacity of PFS(Xia et al, 2022;Andresen & Huuse, 2011).…”

“…The impact of PFS on fluid migration is debated within the literature, with authors attributing PFS for both enhancing (e.g.,Ireland et al (2021);Cartwright (2011);Cartwright et al (2003)) and restricting (e.g.,Xia et al (2022);Andresen and Huuse (2011)) fluid flow. Whilst PFS have been identified as a mechanism for seal bypass(Ireland et al, 2021), permeability may be effectively destroyed by clay smearing along fault planes and thus increase the sealing capacity of PFS(Xia et al, 2022;Andresen & Huuse, 2011). Basin inversion, fault reactivation or dewatering of host sediments may lead to the periodic opening of fluid, and thus hydrogen migration pathways along impermeable PFS(Xia et al, 2022).Whilst PFS must inevitably enhance the passage of fluids during their diagensis, it is unlikely that PFS and microfractures provide substantial hydrogen migration pathways over geological timescales during periods of tectonic quiescence.…”

“…Whilst PFS have been identified as a mechanism for seal bypass(Ireland et al, 2021), permeability may be effectively destroyed by clay smearing along fault planes and thus increase the sealing capacity of PFS(Xia et al, 2022;Andresen & Huuse, 2011). Basin inversion, fault reactivation or dewatering of host sediments may lead to the periodic opening of fluid, and thus hydrogen migration pathways along impermeable PFS(Xia et al, 2022).Whilst PFS must inevitably enhance the passage of fluids during their diagensis, it is unlikely that PFS and microfractures provide substantial hydrogen migration pathways over geological timescales during periods of tectonic quiescence. However, PFS may be capable of both providing a mechanism for fluid communication and opening hydrogen migration pathways on short geological timescales during periods of tectonic activity or fault slip.Whilst faulting and fluid flow have been extensively reviewed in the literature, their impact on hydrogen migration have only recently gained significant attention.…”