Fluid venting phenomena in the Eastern Mediterranean

Thesis

Fluid venting poses clear risks to various industrial applications including carbon dioxide sequestration. To assess these risks, a detailed understanding of the conditions for fluid venting is required. While in-situ measurements of these conditions are scarce, recent observations of natural fluid vents in the Eastern Mediterranean provide robust constraints for the validation of predictive models. In this thesis, I develop a general theory of fluid venting and test it against these observations.

I begin by examining episodic fluid venting. Venting is widely interpreted to occur via hydraulic fracturing, requiring near-lithostatic pore pressures. I propose that fluid expulsion causes a local pressure drop that is gradually recharged by ongoing basin-wide pressurisation and by pressure diffusion, whereby fluid flows from neighbouring regions. Pressure diffusion is predicted to be a major contributor to episodic venting globally.

I compare the predictions from my theory with the geological record of episodic fluid venting from the Levant Basin, where venting has been estimated to occur every ~100 kyr. I invert these venting observations for a pressure recharge rate of ~30 MPa/Myr. To explain this rate, I quantify and compare a range of candidate mechanisms. I find that pressure diffusion from neighbouring mudstones provides the most plausible explanation.

Remobilised mudstones are commonly expelled from fluid vents known as mud volcanoes, a significant geohazard. However, the mechanisms by which mudstones are remobilised remain unclear. Observations of mud volcanoes from the Nile Deep Sea Fan reveal thinning of the mudstone unit in bowl-shaped zones at the base of each vent, resulting from mud depletion. I present a model of mud volcanism, whereby rapid fluid expulsion drives mudstone remobilisation. The model predicts that remobilisation spreads radially from the base of the vent, forming a bowl-shaped mud-depletion zone. I show that the predicted size of depletion zones is consistent with observed sizes.

Authors: Kearney, LM

• DOI: 10.5287/ora-zgvkzrbvw
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: sedimentary basin; fluid-escape pipe; overpressure; fluid venting; fluidisation; hydraulic fracturing; mud volcano
• Subjects: Applied mathematics; Geophysics; Fluid mechanics; Solid mechanics; Earth sciences