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Seismic characterisation of fluid leakage in marine sediments

Hajana, Muhammad 2015. Seismic characterisation of fluid leakage in marine sediments. PhD Thesis, Cardiff University.
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Hydrocarbon migration is one of the key processes that takes place in the development of a successful petroleum system. Understanding when fluids migrated, how they migrated and which routes they took either into primary reservoirs or via transient seals into shallow reservoirs is paramount for successful extraction. Fluids in the sub-surface can be imaged in seismic reflection data as anomalously high reflection amplitudes owing to their contrasting acoustic properties (density and p-wave velocity) with sedimentary rocks. This thesis uses 3-Dimensional seismic reflection data from the Møre Basin, offshore mid-Norway and the Lower Congo Basin, West African margin to investigate the migration of fluids from primary reservoir intervals through overlying successions of fine-grained sediments. These shallow intervals are typically considered as regional seal layers and understanding how seals were breached and fluids migrate through them is vital to risking exploration targets. The Møre Basin case study investigates a gas-associated amplitude anomaly at the crest of a domal structure cored with alternating fine-grained biosiliceous and calcareous ooze sediments. The anomaly has a rather unique convex-upward basal contact which is explained by the superposition of lateral velocity variations through the gas-filled dome. The centre of the dome has more gas than the flanks resulting in a lower velocity which pushes the basal contact to deeper positions with respect to the flanks. The domal trap was charged from gas migrating from depth via capillary entry pressure and possibly via sub-vertical pathways created by compaction-derived polygonal faults which pervasively deform the host stratigraphy. The Lower Congo case study investigates a range of high-amplitude seismic amplitude anomalies in a thick sequence of hemipelagites (the waste zone) above a deep-seated turbidite reservoir. Anomalies take many forms and include; Linear anomalies, Sub-circular anomalies, Patchy anomalies at which finger-shaped anomalies emanate from their lateral edges, and Discrete filamental anomalies. The Sub-circular and Patchy anomalies were interpreted as being related to the presence of hydrocarbons. Detailed analysis of a sub-set of the hydrocarbon-bearing amplitude anomalies suggest leakage occurred through two means; 1) vertical leakage through feeders and 2) via deep-seated extensional faults formed during gravity-driven gliding of vi an underlying salt detachment. Vertical leakage is expressed in the form of Vertical Anomaly Clusters which comprise vertically stacked assemblages of high-amplitude anomalies. A common aspect of the two case studies are that high-amplitude anomalies within fine-grained sedimentary successions are linked to vertical or sub-vertical migration pathways provided either by faults or pipe-like structures formed during overpressure. These results have implications for our understanding of how seals are breached when reservoirs are overpressured.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Date of First Compliant Deposit: 30 March 2016
Last Modified: 18 Jun 2018 01:30

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