Abstract
Abstract A well-developed multi-tier polygonal fault system is located in the Great South Basin offshore New Zealand’s South Island. The system has been characterised using a high-quality three-dimensional seismic survey tied to available exploration boreholes using regional two-dimensional seismic data. In this study area, two polygonal fault intervals are identified and analysed, Tier 1 and Tier 2. Tier 1 coincides with the Tucker Cove Formation (Late Eocene) with small polygonal faults. Tier 2 is restricted to the Paleocene-to-Late Eocene interval with a great number of large faults. In map view, polygonal fault cells are outlined by a series of conjugate pairs of normal faults. The polygonal faults are demonstrated to be controlled by depositional facies, specifically offshore bathyal deposits characterised by fine-grained clays, marls and muds. Fault throw analysis is used to understand the propagation history of the polygonal faults in this area. Tier 1 and Tier 2 initiate at about Late Eocene and Early Eocene, respectively, based on their maximum fault throws. A set of three-dimensional fault throw images within Tier 2 shows that maximum fault throws of the inner polygonal fault cell occurs at the same age, while the outer polygonal fault cell exhibits maximum fault throws at shallower levels of different ages. The polygonal fault systems are believed to be related to the dewatering of sedimentary formation during the diagenesis process. Interpretation of the polygonal fault in this area is useful in assessing the migration pathway and seal ability of the Eocene mudstone sequence in the Great South Basin.
Highlights
Since polygonal fault systems (PFSs) have been discovered in sedimentary basins worldwide, many PFSs have been studied with respect to petroleum exploration, such as their seal capacity and as a paleo-stress indicator [1,2,3,4,5]
The occurrence of PFSs is often linked to very fine-grained sedimentary succession that is confined by stratigraphy or lithology, giving a polygonal fault interval or tier [8,9]
We proposed that the PFSs are formed by dewatering of sedimentary successions, it should be noted that the dewatering of sedimentary successions often occurs when montmorillonite is predominantly rich in clay
Summary
Since polygonal fault systems (PFSs) have been discovered in sedimentary basins worldwide, many PFSs have been studied with respect to petroleum exploration, such as their seal capacity and as a paleo-stress indicator [1,2,3,4,5]. Cartwright (1994) analysed three-dimensional (3D) seismic data in the same area and illustrated polygonal fault geometry on seismic time slices [7]. Polygonal faults are a dense pattern of normal faults formed by compaction and dewatering of a sedimentary formation [8]. They are characterised by vertically and laterally extensive arrays in the host rock and are usually formed in the first few hundred meters of burial [3,9,10]. Lateral and vertical propagations of polygonal faults are defined by changes in lithology, especially muddy properties within the interval, which may relate to depositional environments In frontier exploration areas of petroliferous basins where well information is lacking, it
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