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What Happens When Plate Motions Change? Re-Thinking Hydrocarbon Expulsion and Migration


Standard models for hydrocarbon maturation, expulsion, and migration center on continuous processes. Potential source rocks undergo gradual burial as prolonged sedimentation fills a subsiding basin. In response to a normal geothermal gradient, deeply buried organic material matures. Expansion that accompanies cracking of hydrocarbons facilitates expulsion. Low density hydrocarbons seek higher ground, buoyed upwards until a seal is encountered. In contrast, Re-Os analyses of organic material from mature to over-mature source rocks suggest a more complex history of episodic maturation and expulsion, perhaps not always synchronous. Both Re and Os are concentrated in kerogens and asphaltenes relative to lighter hydrocarbons, and Re-Os geochronology of organic matter from source rocks commonly yields the depositional age. But not always … For example, Re-Os isotopic analyses of targeted samples from the mid-Triassic Gråklint Beds in East Greenland point to discrete events at ∼130 Ma and ∼85 Ma. These ages correspond to marked changes in the rate and direction of sea-floor spreading between Greenland and Norway (Gaina and Torsvik 2011). What happens when plate motion changes? First, the stress regime changes. Fractures that were open may close; fractures that were closed may open. Fluid pathways change; driving forces for fluid flow change. Certainly, expansion of organic material with cracking exerts pressure which drives expulsion; density contrasts drive upward migration. But, are these processes always sufficient, or is mobilization of hydrocarbons by aqueous fluid movement essential? If so, then a change in stress regime may abruptly change the hydrologic regime and thus change (increase or decrease) hydrocarbon expulsion and migration. Second, the thermal regime changes. An increase in spreading rate promotes crustal thinning, in turn increasing asthenospheric upwelling. The resulting increase in heat flow may cause abrupt onset of hydrocarbon maturation. While normal temperature increases with burial may nudge many source rocks into the oil window, episodic maturation and especially expulsion is likely a common phenomenon. Thermal events trigger maturation events. Structurally controlled expulsion gateways and migration pathways develop during discrete tectonic events. Models for evolution of hydrocarbon systems must incorporate episodic tectonic events that induce abrupt changes in thermal and stress regimes. Funded by the CHRONOS project.