UPLIFT OF ONSHORE BASINS, NATURAL FRACTURING AND HYDROCARBON LIBERATION – TIME TO RETHINK THE CRITICAL MOMENT CONCEPT

The Critical Moment (CM) concept, widely used to assess the probability of hydrocarbon charge, asserts that trap formation must predate generation. We propose that this viewpoint should be modified in highly uplifted basins, and that in such cases there are two CMs – one associated with Generation Charge, and one with Exhumation Charge. This hypothesis, which can further be substantiated using global databases and is testable in the laboratory, completely changes the perception of risk associated with exhumation, and with late-formed traps.

The Norwegian experience of exhumed basins is conditioned by results in the Barents Sea, where traps are often underfilled, have residual columns, and dominantly contain gas. However, most exhumed petroleum basins worldwide do not show these characteristics, and hydrocarbon charge is not a limiting factor. In many documented cases conventional traps are full, despite peak generation occurring early and the basin being exhumed and cooled such that primary generation can no longer occur. Thus, in such basins, either retention time for hydrocarbons is much longer than previously envisaged (up to hundreds of MY), or new hydrocarbons have entered the traps long after generation.

We demonstrate that, in a prolific petroleum basin, hydrocarbons are introduced into traps by the process of exhumation. Well-known mechanisms include exsolution of gas from oil and brine as pressure and temperature falls, and redistribution of hydrocarbons by remigration. Potentially more important, however, is the release hydrocarbons adsorbed on to kerogen or in the pore system of source rocks. This significant resource can be exploited by “unconventional” methods, e.g. from artificial hydraulic fractures. In the case of gas, it has been shown that the capacity of the source rock to retain gas decreases as pressure and temperature falls, and gas is thus released into the system. We show that exhumation, by reducing effective stress, can also mimic the “unconventional” fracturing process. Oil and gas release will be much slower than production from an “unconventional” well, but the time over which this can occur is significantly longer, and the scale is basin-wide and hence volumetrically significant. Natural “propping” mechanisms are recognized that further enhance hydrocarbon release from tight rocks.