Migration Mechanisms and Pathways in the Nile Delta Fan

Unraveling natural gas migration mechanisms and pathways in the Nile Delta fan is a major challenge due to the ubiquitous occurence of locally generated microbial gas. We applied a coupled isotope-and-concentration model in order to substract this methanogen fingerprint from our hydrocarbon gas data. Variations in thereby isolated thermogenic gas properties relate to formation temperature and result from biodegradation rather than maturity. Our results strongly indicate that there are three distinct categories of thermogenic gas: unaltered, biodegraded, and mixed unaltered-biodegraded. Unaltered gas has been sampled mostly beyond 90°C, whereas degraded gas is found within reservoir and mud-rich caprock sections between 40-58°C. Stratigraphy with intermediate temperatures hosts a remarkably constant mixture of biodegraded and unaltered gas, implying re-charge of formerly biodegraded gas with fresh thermogenic gas from below. We conclude that the reservoirs are in part being charged through the pore space of the sub-reservoir sections, and we can see leakage via the pore network of the caprock as well as via caprock bypass systems. Our observations support the concept of a dynamic charge-leak scenario.