Black shales are fine-grained sedimentary rocks, usually composed mainly of clay materials that have a high amount of carbonaceous and organic material within the rock's matrix. They're the same types of rich hydrocarbon source rock found in many parts of the world, including the Bakken in North Dakota and Montana, and the Kimmeridge Claystone in the North Sea.
Widespread, Oil-Rich Source Rocks
Waipawa source rock is a fractured, poorly bedded, organic-rich siltstone of Paleocene age, deposited in an anoxic, limited circulation, marine environment. Outcrop sampling indicates average total organic carbon (TOC) values of 5% and average Hydrogen Index (HI) of 245, with maxima of 12% TOC and 550 HI, respectively. The formation occurs throughout TAG Oil's East Coast Basin acreage and has also been identified in other areas of New Zealand. The Waipawa is normally 30m or more thick and achieves a maximum thickness of 70m in Southern Hawke's Bay. The organic carbon is primarily of marine origin, although significant terrestrial organic matter is evident in some samples.