Three-dimensional seismic analysis of a polygonal fault system (PFS) in the Northern Carnarvon Basin, Australia: Implications for fluid flow migration and gas hydrate system

Polygonal fault system (PFS) is confined to fine-grained strata and have significant potential for hydrocarbon plumbing systems in sedimentary basins from deeper structures to shallow-depth gas hydrate systems. Based on a three-dimensional (3D) seismic reflection dataset from the Exmouth Plateau, Northern Carnarvon Basin, NW Shelf Australia, two different tiers of PFS (PFS-I and PFS-II) are characterized in section as well as in the plan views. The PFS-I is consisted of Early Jurassic-Early Cretaceous shallow marine limestone and marl, while the PFS-II is composed of Late Cretaceous to Paleogene bathyal facies with mudstone, calcareous mudstone, marl and silty mudstone. The polygonal faults of PFS-I show relatively low amplitude reflections, which are difficult to identify in seismic amplitude sections. PFS-I can be effectively depicted via coherency sections and time slices, with a good relationship between the low coherent black anomalies and the polygonal faults. In contrast, the polygonal faults of PFS-II are characterized by high-medium amplitude and low continuity seismic reflections, which can be easily identified in the seismic profiles and coherency time slices. The polygonal faults are represented by small throws (5-15 m) and multidirectional strikes. In addition, a series of gas chimneys, pockmarks, and shallow-depth high-amplitude anomalies of the gas hydrate system have also been identified above the deeper Triassic tectonic faults and uplifts. The polygonal faults might increase the connectivity and permeability of the seal and fine-grained layers, resulting in a certain local reduction of seal integrity and an increase in the possibility of fluid leakage. The typical seismic characteristics of PFS and the shallow depth high-amplitude anomalies of the gas hydrate system indicate a significant relationship among the deep-depth Triassic thermogenic gas, the fluid flow migration, and the development of the generation of gas hydrate system.