ALBERT

Description: Petroleum (oil and gas) forms from the bacterial or thermal breakdown of kerogen during progressive burial in sedimentary basins. During times of petroleum generation, kerogens in organic-rich source rocks expel petroleum to form a fluid phase in the pore system, capable of migrating under hydrodynamic and buoyancy forces to ultimately escape to the surface or accumulate within petroleum traps in the subsurface. The relative timing of petroleum charge and trap formation is a vital component in the accumulation of petroleum deposits. Exhumed basins have been historically viewed as higher-risk targets for conventional petroleum exploration because of, inter alia, the switch-off of petroleum generation in the source rock at the commencement of cooling during exhumation. However, even at the switch-off point, the source rock may retain a significant volume of petroleum sorbed in kerogen and within its pore system. Herein we demonstrate that if the source rock is exhumed to shallower depths after peak burial, pore pressure reduction and the associated volumetric expansion of the petroleum—particularly of the gaseous—phase in the pore system will result in the discharge of additional petroleum into the adjacent carrier bed or reservoir formations. Because most onshore sedimentary basins are characterized by major exhumation events at some point in their history, this represents an additional and underappreciated mechanism for a late-stage petroleum charge in exhumed sedimentary basins. The modeling also indicates that both the initial, pre-exhumation, total gas storage capacity and the exhumation gas charge are likely to be volumetrically more significant for gas-bearing source rocks that have been exposed to higher initial pressures and lower thermal gradients. The concepts presented here also have implications for petroleum resources retained within unconventional shale reservoirs because high-graded shale plays may be associated with systems where the magnitude or rate of relative overpressure dissipation has limited exhumation charge from the unconventional to conventional reservoirs within the basin.

Description: Intracratonic sag basins commonly have relatively simple tectonic histories; however, later tectonic activity involving exhumation can make reconstructing the burial history a challenging task. This is important because the relative timing of hydrocarbon generation and trap formation can be a key factor in risk assessment. If trap formation postdates peak hydrocarbon generation, exploration plays are typically downgraded. Mechanisms for charge in such exhumed basins are critical factors for understanding exploration risk. This study uses data collected from an Ordovician gas-condensate field in the Illizi Basin of Algeria to document the charging of a trap formed, or modified, during exhumation of the basin following maximum burial. Integrated analysis of sonic compaction data, thermal history indicators, and stratigraphic well data was used to constrain the burial and thermal history of the region. Hydrocarbon generation in the lower Silurian source rock is interpreted to have occurred during the Carboniferous (prior to Hercynian exhumation) and during the Late Cretaceous–early Eocene maximum burial (prior to Eocene exhumation). Structural reconstructions indicate that the field was initially located on the southern flank of a long-lived, intrabasinal, Paleozoic paleohigh. The large, low-relief structural closure that defines the present-day accumulation formed as a result of northward tilting of the Illizi Basin during Eocene uplift of the Hoggar massif. The study demonstrates that the timing of trap formation at the Ordovician field postdates the main local hydrocarbon generation events within the basin, suggesting that alternative hydrocarbon charge mechanisms are required. This study indicates considerable potential to charge updip traps on the flanks of exhumed petroliferous basins via redistribution of the preexisting hydrocarbons within the basin.