Hydrocarbon Preservation in Cambrian and Neoproterozoic Petroleum Systems: Potential Risks for the Reward in Conventional and Unconventional Plays

Everett, Melanie A.

Until recently, interest in Cambrian and Neoproterozoic petroleum systems has been dampened by relatively small resource densities and the availability of younger, less complex systems. As conventional exploration plays in productive basins become more "mature", there is increasing attention on older and deeper plays, including unconventional resource plays (shale gas, shale oil). With the greatly increased age of these systems, the risks associated with hydrocarbon preservation (volumes and quality) are amplified relative to younger systems.

Hydrocarbon preservation in conventional plays necessitates entrapment of hydrocarbons in reservoirs separate from the source rock and is thus partially controlled by basin tectonic evolution. Basins with relatively simple tectonic histories, such as East Siberia and those of the West African Craton, have reduced risk of losing accumulated hydrocarbons due to faulting, which may have led to leakage in extensional systems such as the Mid-Continent Rift System (USA) and McArthur Basin (Australia). If a simple tectonic history cannot be invoked, seal lithology becomes an important factor, as with the thick salt seals of the Oman Basin and Indian sub-continent.

In unconventional plays, preservation of the source rock is a key risk. Neoproterozoic and Paleozoic orogenic events, such as the Pan-African Orogeny and closing of the Appalachian and Uralian oceans, have led to the erosion or metamorphosing of potential source rocks. Conventional plays may survive this risk through pre-orogenic expulsion and migration to preserved strata, as in the case of the supposed source responsible for large accumulations in the Nepa-Botuoba anteclise (East Siberia).

For both conventional and unconventional plays, long residence times of hydrocarbons in reservoirs or source rocks may increase risk of alteration processes. Liquid hydrocarbons in deeply-buried source rocks are likely to undergo cracking, affecting the potential for shale oil recovery. Severe biodegradation is a risk if the basin is cool and/or reservoirs are shallow, although this process may be slowed or halted if pore water conditions are not conducive to microbial growth, such as the hyper-saline pore waters of the Vendian reservoirs in East Siberia.

A comparison of known Cambrian and Neoproterozoic productive petroleum systems to non-productive basins may help to identify key risk trends, such as those above, and steer the successful exploration of these unique systems.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013