Tubular Carbonate Concretions from North Island, New Zealand: Evidence for Hydrocarbon Migration and the Subsurface Plumbing System of Cold Seeps

Among the cold seep research community, it is now appreciated that tubular carbonate concretions are important indicators of hydrocarbon migration in the subsurface. In the last few years, several publications have documented tubular concretions and interpreted a subsurface seep origin. However, the nature, timing, and relative importance of tubular concretions as records of fluid flux and chemistry over time is yet to be fully appreciated. In North Island, New Zealand, tubular concretions are geographically widespread but occur in localised clusters in several Cenozoic (especially Miocene) sedimentary formations of the East Coast Basin (Hikurangi Margin) and Taranaki Basin. They formed by precipitation of micritic dolomite and calcite cement within host siliciclastic mudstone and contain 50 to 85% carbonate (dolomite dominated), indicating precipitation in shallow burial (0 to 300 m). Several concretion morphologies occur, especially pipe, bulbous, doughnut, corkscrew, and conical shapes, and they include some of the largest examples known worldwide, ranging up to 10 m or more in length (limited only by exposure) and 0.1 to 1 m in diameter. The concretions typically support near-central conduits, from 1 to 40 cm in diameter, which may be open or variably filled with sediment and/or late cements. As well as diverse morphologic types, the tubular concretions also show, within limits, variable mineralogic, petrographic, and geochemical characteristics. Additionally, some examples display association with slope instability, fault control on fluid migration, and stratigraphic placement directly below ancient seafloor seep carbonates. δ13C values of the cement forming the concretions range from -52 to +13 PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. A trend from strongly negative to strongly positive δ13C values reflects either a mixing of methane and methanogenic CO2 and/or the extensive anaerobic oxidation of methane (AOM) as supported by lipid biomarkers. δ18O values range from −3 to +5 PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation. The tubular carbonate concretions are interpreted to represent the subsurface plumbing pathways of methane expulsion in ancient hydrocarbon seep systems in North Island. Additionally, they suggest that gas hydrates may have been forming and dissociating along the Hikurangi Margin off eastern North Island for the past 23 Ma. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system. The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin, and possibly for many modern and ancient hydrocarbon seep systems in general

Tubular carbonate concretions as hydrocarbon migration pathways? Examples from North Island, New Zealand

Cold seep carbonate deposits are associated with the development on the sea floor of distinctive chemosyn¬thetic animal communities and carbonate minerali¬sation as a consequence of microbially mediated anaerobic oxidation of methane. Several possible sources of the methane exist, identifiable from the carbon isotope values of the carbonate precipitates. In the modern, seep carbonates can occur on the sea floor above petroleum reservoirs where an important origin can be from ascending thermogenic hydrocar¬bons. The character of geological structures marking the ascent pathways from deep in the subsurface to shallow subsurface levels are poorly understood, but one such structure resulting from focused fluid flow may be tubular carbonate concretions. Several mudrock-dominated Cenozoic (especially Miocene) sedimentary formations in the North Island of New Zealand include carbonate concretions having a wide range of tubular morphologies. The concretions are typically oriented at high angles to bedding, and often have a central conduit that is either empty or filled with late stage cements. Stable isotope analyses (δ13C, δ18O) suggest that the carbonate cements in the concretions precipitated mainly from ascending methane, likely sourced from a mixture of deep thermogenic and shallow biogenic sources. A clear link between the tubular concretions and overlying paleo-sea floor seep-carbonate deposits exists at some sites. We suggest that the tubular carbonate concretions mark the subsurface plumbing network of cold seep systems. When exposed and accessible in outcrop, they afford an opportunity to investigate the geochemical evolution of cold seeps, and possibly also the nature of linkages between subsurface and surface portions of such a system. Seep field development has implications for the characterisation of fluid flow in sedimentary basins, for the global carbon cycle, for exerting a biogeochemical influence on the development of marine communities, and for the evaluation of future hydrocarbon resources, recovery, and drilling and production hazards. These matters remain to be fully assessed within a petroleum systems framework for New Zealand’s Cenozoic sedimentary basins

Miocene tubular concretions in East Coast Basin, New Zealand: Analogue for the subsurface plumbing of cold seeps

The uplifted accretionary prism of East Coast Basin, in Hikurangi Margin, North Island, New Zealand, exposes late Miocene slope mudrocks (Whangaehu Mudstone, < 10% carbonate) in coastal cliffs north of Cape Turnagain that contain conspicuous tubular carbonate concretions (50–85% carbonate) supporting near-central conduits. Pipe and bulbous morphologies dominate, ranging in exposed length up to 5 m and up to 1 m in diameter. The concretions were formed by the precipitation of micritic dolomite (and calcite) cement within the host mudstone at shallow burial depths (< 100 m). δ13C values of the cement range from − 22 to + 13‰ PDB and are interpreted to reflect carbonate precipitation from either the extensive anaerobic oxidation of methane (AOM) and/or mixing of microbial methane and methanogenic CO2. AOM is confirmed by lipid biomarker evidence indicating that methane oxidation occurred in the sediments at the time of carbonate precipitation. The mixed dolomite/calcite mineralogies and the trend of δ13C in the tubular concretions from strongly negative to strongly positive values are interpreted to reflect methane oxidation from the onset of ascent through to the end of a migration event. Depleted and enriched δ18O values suggest an evolved fluid source influenced by the dissociation of gas hydrates. Collectively, our results indicate that the tubular concretions within the upper slope mudstones delineate parts of the subsurface plumbing network of a cold seep system on the late Miocene paleo-Hikurangi Margin in which the fluids were sourced from ascending methane. The intermediate location of the Whangaehu concretions between older (early Miocene) seep carbonates to the west and modern ones offshore to the east indicates a progressive eastwards shift with time of a long-lived, if only periodically active, seep system. The concretionary plumbing features at Whangaehu provide a conceptual model for subsurface fluid pathways and seep-related processes beneath the modern Hikurangi Margin seabed, and possibly also for other modern and ancient cold seep carbonate systems