AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop

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Factors Controlling Faults as Barriers to Dynamic Lateral and Vertical Leakage

Abstract

Much of the world-wide exploration for hydrocarbons is in active petroleum basins with continuing generation and expulsion from source rocks, and hence ongoing migration into traps and through lateral and vertical seals. With some exceptions, these seals, including fault zones, are usually “imperfect” on the geological timescale. Hydrocarbons can migrate up/through the seal zones causing leakage, with the bulk permeability of the seal, along with pressure potential gradients, controlling this leakage rate. Overall, there is a complex set of geological factors controlling whether charge occurs into the trap faster than it leaks through the seals, and hence whether the net result is the presence or absence of an economic hydrocarbon accumulation in the trap. This presentation investigates the main controls on Darcian leakage through and/or up the fault zone seals, and their relative impacts, in order to provide guidelines to assessing the key uncertainties in seal risking for exploration prospect analysis. The factors controlling leak rates governed by single-phase Darcy flow are considered in terms of the geometric parameters of the leakage pathways through or up the fault zone, and the hydraulic and fluid parameters of pressure potential difference, permeability and viscosity. The two main leakage scenario out of a trap through the fault zone seal are considered are: (1) Across-fault flow, and (2) up-fault flow. For typically (hypothetical) trap parameters and an in-house fault permeability algorithm (Wibberley et al., 2017), a series of hypothetical tests are performed to explore the impact of these parameters on leakage flow rates and their impact on trap accumulation.