Seismic Characteristics Of Paleo-Pockmarks At Great South Basin, New Zealand

Abstract

Paleo-pockmarks and present-day pockmarks are founded in several varieties both onshore and offshore worldwide. These features can be used as an indicator of fluid flow process which expulses through an unconsolidated sediment within the basin. The Great South Basin, New Zealand is one of the basins that paleo-pockmarks are observed covering the southeast offshore of South Island at around 1.5 s (TWT) depth beneath the surface. This study aims to identify seismic characteristics of paleo-pockmarks in the Great South Basin and predict the possible fluid origins and their mechanism. The numerous paleo-pockmarks are identified and imaged by using 3D seismic interpretation. Paleo-pockmarks in the study area are observed on the fine-grained sedimentary succession of middle Eocene as known Laing formation. The paleo-pockmark distribution is aligned from the southwest toward northeast direction coincident with fan shaped geometry. Paleo-pockmark area are divided within 3 major zones to study morphology and density of paleo-pockmarks. There are zone 1 located in the proximal fan, zone 2 located in the middle fan, and zone 3 located in the distal fan. The 94 paleo-pockmark samples are randomly chosen from 3 zones for size measurement. Paleo-pockmarks in the study area are identified as “normal pockmark” with sub-round to round on the top view, V-shape and U-shape in profile view. Their size ranges from 131 to 481 m in diameter and 15 to 45 ms (TWT) in depth. Paleo-pockmarks are observed to have high density around 13.4 paleo-pockmarks per a square kilometer at the central part of the study area (zone B). Furthermore, location distribution of paleo-pockmarks is interpreted to be caused by two possible fluid origins. Microbial methane is the main fluid origins which generate more widespread middle Eocene paleo-pockmarks in low temperature condition. While pore water in compacted fine-grained sedimentary layers, including late Cretaceous to early Paleocene Wickcliffe formation and early to middle Eocene Laing formation, are possible to be one of the fluid sources of paleo-pockmarks in the study area as well.