Review articleAn overview of pre-Devonian petroleum systems - Unique characteristics and elevated risks

•Significant hydrocarbon resources exist in the pre-Devonian.•The age of the petroleum system does not appear to increase exploratory risks.•Kerogen character is highly variable as a result of organic preservation level.•Assessment of thermal maturity is complicated as a result of lack of consistency.•A primary risk remains the preservation of hydrocarbons within the trap.

The radiation of land plants at the onset of the Devonian resulted in a change in the character of available biomass, which in turn, introduced changes to the nature of petroleum systems. This study examines the nature of pre-Devonian petroleum systems prior to this significant evolutionary event.Conventional hydrocarbon resources have been associated with pre-Devonian petroleum systems across the globe, including the Neoproterozoic-Infracambrian Huqf Supergroup of the Oman basin, the Riphean-Vendian sequences of East Siberia, the Cambrian of the Murzuq basin, the Ordovician of the Tarim basin, and the Silurian Qusaiba of Saudi Arabia. Although individual accumulations can be quite significant (e.g., Hassi Messaoud has ultimate recoverable reserves of ∼10 billion barrels), the relative importance of pre-Devonian-derived oils is thought to be limited compared to the global conventional resource-base. With growing interest in unconventional resources, the relative importance of these systems is expected to increase as resource plays such as the Ordovician Utica Shale of the Appalachian basin, the Cambrian of China, and possibly the Silurian of Central and Eastern Europe develop.The advanced ages of these systems result in some unique properties and amplify risks that exist in younger petroleum systems. Differences in the nature of the biomass contributing to pre-Devonian source rocks may give rise to oils that display unique geochemical characteristics. For example, there are some Precambrian oils where C29 steranes dominate even though land plants were absent. There are also Ordovician source rocks dominated by Gloeocapsamorpha prisca, a primitive prokaryote, which yield oils containing limited amounts of C20+ components and nearly lack pristane and phytane.The potential for unconventional reservoirs in pre-Devonian systems may also be highly dependent upon the age of the system, as biological evolution influences the availability and nature of biogenic silica, an important factor controlling brittleness and fracability. Literature has shown that biogenic silica from different sources displays varying degrees of resistance to diagenesis, which allows the formation of a silica network and influences brittleness. For example, radiolaria present during the Cambrian are more resistant to diagenesis than diatoms, which did not evolve until the Jurassic.Risks associated with preservation of hydrocarbons are also amplified in pre-Devonian petroleum systems. Many such systems have been exposed to significant thermal stresses, resulting in the cracking of oil and wet gas. Others have had complex tectonic histories potentially resulting in the breaching of seals or changes in PVT conditions that can result in gas loss. Gas loss may also occur through diffusion from these older reservoirs as a result of long residence times.