A sedimentary basin is a depression in the earth’s crust into which sediments have been deposited over millions of years. The sedimentary basins in New Zealand that are likely to contain oil and gas are young (less than 80 million years old), and most have many faults that offset the rock layers.
When did sedimentary basins form?
New Zealand’s key sedimentary basins started forming after the breakup of Gondwana, about 85 million years ago, and the opening of the sea floor in the Tasman Sea. Rivers, eroding the nearby land, transported sediments containing organic material into these basins. This allowed shoreline sands to be deposited, followed by marine silts and mud many kilometres thick. These sands were compacted by the weight of the overlying sediment. Being both porous and permeable, they made ideal reservoir rocks. Impermeable overlying silts, mud and carbonates formed seals.
A drowned spirit
The Sugar Loaf Islands and the New Plymouth foreshore in Taranaki were known for their oily waters and strong smell of hydrogen sulfide, as The New Zealand mining handbook of 1906 reported. A Māori story has it that an atua (spirit) drowned there, and is still decomposing.
Different sedimentary basins
In most New Zealand basins, oil and gas formed only over the past 20 million years. During this period the New Zealand land mass was uplifted and eroded. Sediments kilometres deep were deposited, and rock layers were moved by faults, which produced many underground structural traps (places where rocks containing petroleum are capped and sealed by impermeable rocks).
Around New Zealand, there are eight sedimentary basins, onshore and underlying the continental shelf, with known or potential hydrocarbons. In addition, there are several deep-water basins offshore. Commercial quantities of oil and gas have only been produced from structural traps in the Taranaki Basin. There has been considerable exploration in other basins: Northland, Canterbury, the Great South Basin, western Southland and Westland, but none of the finds have been commercially viable.
In the Taranaki and East Coast basins, the most promising traps consist of structures formed by rock compression. Other traps were formed by changes in the types of sediment deposited, which allowed reservoir rocks to be trapped in impermeable siltstones and mudstones. This occurred particularly in the Northland, East Coast and Great South basins.
In New Zealand basins, the paths that oil and gas have taken from source rocks to reservoir rocks are poorly understood – in some, faults may have acted as pathways.