Occurrence and Preservation of Overpressure in Paleozoic and Older Reservoirs
Johannes Wendebourg¹, François Lorant¹, and Eric Mure²
¹Total E&P, Paris, France
²Total E&P Indonesia, Djakarta, Indonesia
Sedimentary basins filled with Paleozoic and older strata oftentimes have undergone a long and complicated tectonic history, associated with major uplift phases, and therefore these basins are found today mainly in hydrostatic pressure conditions. However, under certain circumstances overpressures can be preserved in such basins. Indeed, they are observed in Lower Paleozoic and Precambrian reservoirs and seals, and when they are observed, they oftentimes come as a surprise to operators and drilling crews.
An example is the Late Ordovician shales in the Arafura Sea, Indonesia, where equivalent mud weights of > 14 ppg have been encountered despite a major uplift and a long hiatus. Another well known example is the Neoproterozoic Ara Stringer Play in the South Oman Salt Basin with reservoir pressures reaching near-lithostatic (22 kPa/m) gradients. Here salt plays an important role to preserve high overpressures as the reservoirs are entirely surrounded by salt.
Overpressures can be recent, originating from ongoing HC generation. An example is the Late Devonian Bakken Formation of the Williston basin, onshore USA and Canada, where overpressures are observed in actively generating source rocks. A similar situation can be observed in the Middle Ordovician Utica shale of the Eastern USA where overpressures are observed in a source rock that is currently in the oil and gas window. Such overpressures are critical for production of oil and gas from these shales.
But overpressures can also occur, despite uplift, in so called tight gas formations which are low-permeability clastic reservoirs where overpressure is linked to gas saturation. However, these are transient pressure systems where the overpressure is observed in the deeper parts of the basin whereas in the shallower parts pressures are depleted and can even reach subhydrostatic conditions. This is the case in Lower Silurian formations of the Appalachian foreland. The source for the gas is the above mentioned Utica shales, creating a continuum of overpressured Lower Paleozoic source and reservoir rocks.
In this paper, we will discuss the conditions under which overpressures could be preserved in Lower Paleozoic and older formations, what are common features such as very low permeabilities, and what are potential causes such as HC generation or late diagenesis, and degree of confinement.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013