Fluids, migration systems and diapirism, East Coast North Island, New Zealand

Understanding fluid migration systems is especially important for companies exploring for hydrocarbons in the East Coast region because the best potential hydrocarbon source rocks (Late Cretaceous Paleocene) are separated from the best reservoir rocks (Neogene) by a thick section of mudstones with good fluid seal potential. The Cretaceous-Cenozoic rocks the East Coast region of New Zealand comprise three main packets: a Cretaceous sandstone-mudstone succession, a Late Cretaceous-Paleogene mudstone-dominated unit, and a Neogene sandstone-mudstone-limestone succession deposited on the Hikurangi subduction margin. High sedimentation rates in the Neogene and vertical fluid migration from subducted sediments have contributed to over-pressured formation fluids, causing mud volcanoes, mud diapirs and problems for drillers. Avenues for fluid movement include faults, joints, lateral connectivity of adjacent clastic facies (e.g., slope channel conduits) and across unconformities at which low-permeability units ha ie been removed by erosion, and bouyancy-created conduits (e.g., diapirism).