Open Geosciences 2020; 12: 851–865 Research Article Sukonmeth Jitmahantakul, Piyaphong Chenrai*, Pitsanupong Kanjanapayont, and Waruntorn Kanitpanyacharoen Seismic characteristics of polygonal fault systems in the Great South Basin, New Zealand https://doi.org/10.1515/geo-2020-0177 during the diagenesis process. Interpretation of the received January 30, 2020; accepted June 12, 2020 polygonal fault in this area is useful in assessing the Abstract: Awell-developed multi-tier polygonal fault migration pathway and seal ability of the Eocene mudstone system is located in the Great South Basin offshore New sequence in the Great South Basin. Zealand’s South Island. The system has been characterised Keywords: seismic interpretation, polygonal fault using a high-quality three-dimensional seismic survey tied system, Great South Basin 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 1 Introduction Eocene) with small polygonal faults. Tier 2 is restricted to ( ) - the Paleocene-to-Late Eocene interval with a great number Since polygonal fault systems PFSs have been dis of large faults. In map view, polygonal fault cells are covered in sedimentary basins worldwide, many PFSs outlined by a series of conjugate pairs of normal faults. The have been studied with respect to petroleum exploration, - polygonal faults are demonstrated to be controlled by such as their seal capacity and as a paleo stress [ – ] ( ) fi depositional facies, specifically offshore bathyal deposits indicator 1 5 . Henriet et al. 1991 rst described PFS characterised by fine-grained clays, marls and muds. Fault in the North Sea Basin as fracture networks in the - ( ) throw analysis is used to understand the propagation Palaeogene clays based on two dimensional 2D seismic [ ] ( ) history of the polygonal faults in this area. Tier 1 and Tier 2 interpretation 6 . Later, Cartwright 1994 analysed - ( ) initiate at about Late Eocene and Early Eocene, respec- three dimensional 3D seismic data in the same area tively, based on their maximum fault throws. A set of and illustrated polygonal fault geometry on seismic time [ ] three-dimensional fault throw images within Tier 2 shows slices 7 . In general, polygonal faults are a dense that maximum fault throws of the inner polygonal fault cell pattern of normal faults formed by compaction and [ ] occurs at the same age, while the outer polygonal fault cell dewatering of a sedimentary formation 8 . They are exhibits maximum fault throws at shallower levels of characterised by vertically and laterally extensive arrays fi different ages. The polygonal fault systems are believed to in the host rock and are usually formed in the rst few [ ] be related to the dewatering of sedimentary formation hundred meters of burial 3,9,10 . 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]. Lateral and * Corresponding author: Piyaphong Chenrai, M.Sc. Program in vertical propagations of polygonal faults are defined by Petroleum Geoscience, Department of Geology, Faculty of Science, changes in lithology, especially muddy properties within Chulalongkorn University, Bangkok 10330, Thailand, the interval, which may relate to depositional environ- e-mail: [email protected] ( [ ]) Sukonmeth Jitmahantakul: M.Sc. Program in Petroleum ments e.g. 3,11 . Tier boundaries are recognized by the Geoscience, Department of Geology, Faculty of Science, disappearing of polygonal faults in the seismic data, Chulalongkorn University, Bangkok 10330, Thailand; Basin Analysis which may mark the changing of lithological properties and Structural Evolution Special Task Force for Activating Research or the changing of individual polygonal fault geometry ( ) BASE STAR , Department of Geology, Bangkok 10330, Thailand [12,13]. Thus, PFSs might be useful to highlight Pitsanupong Kanjanapayont, Waruntorn Kanitpanyacharoen: Basin - Analysis and Structural Evolution Special Task Force for Activating lithological variation related to depositional environ Research (BASE STAR), Department of Geology, Bangkok 10330, ments. For instance, in frontier exploration areas of Thailand petroliferous basins where well information is lacking, it Open Access. © 2020 Sukonmeth Jitmahantakul et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 852 Sukonmeth Jitmahantakul et al. is difficult to map out the depositional environments and lithological variation from seismic data independently. PFSs can be recognized both laterally and vertically and could be used to delineate and assign as a facies model for geological or petroleum systems modelling. PFSs have been described elsewhere from New Zealand and in the Great South Basin (GSB)(Figure 1). PFSs in the Great South and Canterbury Basins have recently been reported by several studies (e.g. [14–17]). Morley et al. (2017) studied the honeycomb structures associated with the PFS in the GSB [15]. The honeycomb structures are characterised by extensive circular to polygonal depres- sions. Morley et al. (2017) observed two PFS tiers, which are referred to as Tier 1 (southern area) and Tier 2 (northern area), but they did not focus on details of the PFS characteristics such as fault throw analysis and depositional environment at the PFS interval [15].Li et al. (2020) also studied the characteristics of the PFSs in the GSB using fault enhancement and skeletonization processes [17]. However, they focused on the south- eastern part of the 3D seismic data; hence, the north- western area is not well-documented. Morley et al. (2017) and Li et al. (2020) suggested that the honeycomb structure and PFS in this basin are related to the opal-A/ CT transformation, which is characterised by a high amplitude reflection [15,17]. Without well data to constrain the lithology of the sedimentary succession, Figure 1: Location map of the GSB and study area. The GSB 3D seismic survey is highlighted as a black polygon and the 2D seismic lacking depositional environment interpretation and lines are highlighted as blue lines (modified from [48]). lacking temperature calculation at the opal-A/CT trans- formation interval, the proposed opal-A/CT transforma- tion may be somewhat uncertain. to highlight the possible lithology at the interested Nowadays, the GSB contains several potential interval. The goal of this study is to image and recognize petroleum plays [18], and even though the petroleum geologically and geomorphologically meaningful pat- fields are not at an economic stage, they are still terns of the PFS from seismic data. Fault throw analysis attractive for further exploration. To increase the under- is performed for understanding the PFS evolution with standing of petroleum resources in this basin, it is discussion on fault initiation, propagation and linkages necessary to investigate the basic geological information in three dimensions. such as seal ability, fluid migration and trap. PFSs usually occur within fine-grained sedimentary succes- sions, which often form seals for petroleum reservoirs. In this study, 3D seismic data and seismic attributes are 2 Geological setting utilized to map and characterise the PFSs in the GSB. The PFSs in this study area are mainly confined within the The GSB is situated in the southeast offshore New Paleocene to Eocene fine-grained sedimentary succes- Zealand’s South Island. The basin is located beneath the sions (Figure 2). Sand injections are reported to occur modern shelf area and covers an offshore area of within Late Paleocene succession beneath the PFSs in approximately 85,000 km2 with water depths of this study area [19]. Sandstone injections constitute 300–600 m (Figure 1). Rifting of Zealandia from Aus- prolific petroleum reservoirs. This study presents the tralia and Antarctica was initiated from the break-up of quantitative method for throw–depth (T–Z) plots along eastern Gondwana at approximately 105–100 Ma that the polygonal faults in the GSB. The depositional eventually led to the formation of sedimentary basins environment and stratigraphic interpretations are useful across the older pre-rift basement rocks in New Zealand Seismic characteristics of PFS in GSB 853 Figure 2: Regional seismic line shows the seismic stratigraphy and age correlation of the study area. Polygonal faults are restricted to the Eocene mudstone sequence of the Laing Formation. See the seismic profile location in Figure 1. [20–25]. A pre-Eocene movement of Alpine fault was Rakiura Group from the Penrod Group sediments. Laing proposed to develop before 45 Ma may have formed a Formation was deposited in a shelf to the upper bathyal paleo-high that contributes the sediment sources into environment, and it extends over most of the basin with the Canterbury Basin through early Oligocene channels a thickness of approximately 2 km in some places. [26,27]. During this rifting event, the GSB was dominated During the Early Eocene, the basin was shallowed by a series of grabens and half-grabens trending in the toward the northwest with the formation of a thick northeast–southwest direction [28,29]. The basement prograding clastic wedge. By the end of the Eocene, the rocks include silicic to intermediate plutonics and basin became a deeper marine setting according to a metasedimentary rocks [30,31].