TUBULAR CARBONATE CONCRETIONS AS INDICATORS OF COLD SEEPS AND HYDROCARBON MIGRATION PATHWAYS: EXAMPLES FROM TARANAKI AND EAST COAST BASINS, NEW ZEALAND

Modern cold seep carbonate deposits have been documented from passive and tectonically active continental margins worldwide. To date, limited investigations of ancient cold seeps have been undertaken yet, unlike modern seeps, fossil seeps may afford a unique view of the three-dimensional subsurface geometry of cold seep fields.

Several of the Cenozoic sedimentary formations in the North Island of New Zealand include locally prominent development of calcitic and dolomitic concretions lacking a typical subspherical shape. These concretions exhibit a variety of tubular morphologies, usually with their long axis normal to bedding. A central conduit may be empty or filled with sediment or multiple generations of later carbonate cements, indicating at times a complex diagenetic history. Internal networks of cement-filled radial cracks in many concretions may be associated with hydrofracturing as a consequence of build up of fluid/gas pressures. Stable oxygen and carbon isotope data suggest that many of the cements were sourced primarily from ascending methane fluids/gases.

We suggest the tubular concretions in the Urenui Formation, for example, mark the subsurface plumbing network of a late Miocene cold seep system, so that they have relevance to the migration of hydrocarbons in Taranaki Basin, at this site perhaps focused along the Taranaki Fault. The presence of dislodged and mass-emplaced tubular concretions in the axial conglomerate of channels within the formation suggests a connection between the loci of seep field development and slope failure and canyon cutting on the late Miocene Taranaki margin.

Field reconnaissance at sites with tubular concretions in the East Coast Basin of the North Island shows several similarities to the Taranaki examples, but also local differences likely reflecting their tectonic setting, stratigraphic position, and available fluid migration pathways during seep development. In some cases there is a clear association of the tubular concretions with overlying paleo-sea floor seep-carbonate deposits. If tubular concretions often represent fluid conduits of a plumbing system derived primarily from hydrocarbon fluids/gases within a sedimentary basin, then they have direct implications for evaluating hydrocarbon resources, recovery, and drilling/production hazards.