Insights into the permeability of polygonal faults from their intersection geometries with Linear Chimneys: a case study from the Lower Congo Basin

Abstract

Layer-bound arrays of polygonal compaction faults have long been considered as important migration routes for hydrocarbon fluids leaking to the surface across thick shale sequences. A classic example is the deep offshore of the Lower Congo Basin where numerous fluid-venting structures are present above a Pliocene polygonal fault system. In this paper we present a detailed seismic analysis of a newly recognised system of Quaternary-aged Linear Chimneys and their intersection geometries with pre-existing Pliocene-aged polygonal faults (PF). Most (73%) of the 209 chimneys analysed intersect the lower portions of polygonal faults and almost half of these are rooted in strata below the PF interval. This indicates that fluid (in this case gas) migrated vertically, cross-cutting polygonal faults as it ascended through the tier. This is a strong indicator that PFs did not provide viable migration pathways otherwise chimneys would terminate at the upper tip of the fault, which would be the most likely migration exit point. Only twice in the whole system of Linear Venting Systems did this occur. A sub-set of chimneys stems from or above PF planes but these are restricted to either the lower footwall or from the apex area of hanging wall. At best they are evidence of fluids migrating up the lower part of polygonal faults and exiting deep within the tier, then migrating through most of the tier in their own vertical leakage vents. These results provide strong indicators that at least within this part of the Lower Congo Basin polygonal faults were the least effective/favoured migration pathway and that it was more energy-efficient for migrating gas to hydrofracture its fine-grained overburden than to re-open polygonal faults.Key Words: Linear chimneys; hydrocarbon; polygonal faults (PFs); permeability; methane-related carbonates; Lower Congo Basin; Angola. Résumé : La perméabilité des failles polygonales déduite de la géométrie de leurs intersections avec des cheminées linéaires : une étude de cas dans le bassin du Bas-Congo.-Les réseaux de "failles polygonales" (failles de compaction restreintes à un intervalle stratigraphique) ont été longtemps considérés comme d'importants chemins pour la migration vers la surface des hydrocarbures à travers d'épaisses séries argileuses. Un exemple classique est fourni par l'offshore profond du bassin du Bas-Congo où de nombreuses structures d'échappement de fluides sont présentes au-dessus d'un système de failles polygonales affectant l'intervalle pliocène. Ce papier présente l'analyse sismique détaillée d'un système récemment reconnu de cheminées linéaires d'âge quaternaire, ainsi que de la géométrie de leurs intersections avec des failles polygonales pliocènes (donc préexistantes). La plupart (73 %) des 209 cheminées analysées recoupent le mur des failles polygonales, et près de la moitié d'entre elles s'enracinent dans l'intervalle sous-jacent à l'intervalle faillé. Cette disposition indique que le fluide (gaz en l'occurrence) a migré verticalement, recoupant les failles pol...

Part 2 - authors reply to Review 1 of the Discussion Paper titled “Formation of linear planform chimneys controlled by preferential hydrocarbon leakage and anisotropic stresses in faulted fine-grained sediments, Offshore Angola” by Ho et al.

“…2b). These intervals are deformed by polygonal faulting, which conforms to two distinct tiers, named here as Tier-1 and Tier-2 (Ho et al, 2012a(Ho et al, , 2016(Ho et al, , 2018Ho, 2013). The deepest Tier-1 ranges from 70-130 m thick and contains the Late Miocene units, whilst the shallower Tier-2 contains the Pliocene units and has a maximum thickness of ca.…”

Geological fluid flow in sedimentary basins

“…In addition, mineralized fluids intensify diagenesis in the fracture planes, rendering this interval all the more impermeable (Laurent et al, 2012). The detailed analysis of the geometric relationships between highimpedance seismic anomalies and polygonal fault pairs reveals that they can act as a seal during early stages of burial (Ho et al, 2016). In the Congo Basin, the Pliocene-present interval contains widespread fluid escape features, at seabed and in the subsurface, indicating past and ongoing bypass of the regional seal which is intensely affected by a major polygonal fault system (Gay et al, 2004).…”

Formation of linear planform chimneys controlled by preferential hydrocarbon leakage and anisotropic stresses in faulted fine-grained sediments, offshore Angola

“…The morphology of structures formed during fluid leakage records the style and intensity of fluid expulsion and is thus useful for deciphering the fluid migration history (Roberts et al, 2006;Blouet et al, 2017;Imbert et al, 2017;Imbert and Ho, 2012;Ho et al, 2012bHo et al, , 2018. As 3-D seismic reflection data have played an increasingly important role in visualisation and identification of fluid flow features (Heggland, 1997), by conducting seismic analyses for vertical successions of fluid leakage expressions around faults, such as gas chimneys feeding pockmarks and seep carbonates, it is possible to unravel the timing and pathways of migrating fluids and the sealing efficiency of faults (Ligtenberg, 2005;Plaza-Faverola et al, 2012;Ho et al, 2016).…”

“…Recent studies from the upper slope of the Lower Congo Basin have revealed the existence of a new type of chimney (Ho et al, 2016;Ho, 2013). Chimneys are usually circular in planform.…”