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Recent Results
Hydrocarbon accumulation and seepage processes in the southern Gulf of Mexico
The southern Gulf of Mexico received our attention because of spectacular lavaflow-like, massive, natural asphalt accumulations observed in the area [MacDonald et al., 2004]. With an extent of several hundred meters in diameter, a large number of flow-like patches up to hundreds of square meters were discovered with different degrees of weathering, indicating episodic deposition of the asphalts.
2D high resolution seismic profiles, acquired during the Meteor cruise M67/2 across several of the Campeche Knolls to study seafloor asphalt occurrences, identified a highly reflective, coarse-grained sediment unit of late Miocene age as the potential shallow hydrocarbon reservoir. Local salt tectonism in the area has elevated this unit and has allowed it to be coevered by only a thin (100–200 m), fine grained sediment layer on the crest of Chapopote. High-amplitude reflector packages in both units document the presence of hydrocarbons, the leakage of which to the seafloor is facilitated by the thin seal. Seismic images suggest the asphalt site to be the currently active seep location, with its vicinity probably sealed temporarily by solid-phase hydrocarbons on the surface.
Seismic profile across the Chapopote crest to the asphalt seep site. Stratigraphic units I-V (seal) thin out towards the peak of the knoll; high reflection amplitudes observed in unit VI indicate the presence of a shallow hydrocarbon reservoir.
Shallow sediment deformation styles and their control on seepage
As a further step, 20 bathymetric highs and ridges in the Campeche Knoll area were analysed. These were assigned to three principle structural types: Passive Type, Chaopopte Type, and Asymmetric Flap Type.
The structure of knolls bears great relevance to the seepage occuring at them. In contrast to the Asymmetric Flap Type knolls, the Chapopote Type ones are symmetrical in structure. Both types reveal uplifted, coarse-grained, Miocene sediments in their cores and fairly thin syn-kinematic sediment covers above. A number of seeps are preferentially found along the crest of Chapopote Type features and above the lower flap of the Assymetric Type ones. High reflection amplitudes in the Passive Type knolls are most likely due to salt instead of fluids. Only one seep site was observed in relationship to these batymetric features.
In summary, seismic data suggest that although the hydrocarbons probably come from greater depth, it is the shallow sediment structures that control the distribution of the hydrocarbon seeps of the north-western Campeche Knolls.
Conceptional model of seepage at knolls with an Asymmetric Flap Type structure (right)
Interplay of thrust faulting and slope sedimentation and its effect on seepage
On the Makran accretionary wedge, the distribution pattern of shallow sediments, their structure and deformation processes, and their role on the migration, accumulation and seepage of hydrocarbon‐rich fluids was investigated. Seismic data from the Meteor cruise M74/2 indicate that rapid syn‐kinematic sedimentation at the frontal prism allows emerging imbricated thrust packets to override the footwall at the seafloor without significant mass wasting and destruction of fault‐related anticlines in the hanging wall. This in turn enables anticlinal growth structures to form, which act as structural traps for hydrocarbon‐rich fluids. Above the traps, fluid escape structures are generated, leading to seafloor seeps.
Seismic profile offshore Pakistan (left) and its interpretation (right). Fluids advecting from depth are redirected along sediment strata as well as along the base of gas hydrate stability zone (labelled BSR). Seep locations at the seafloor are thus likely to be laterally offset from the fluid source and primarily controlled by the depth of shallow reservoir and thickness of seal on top.
Authigenic carbonates offshore South-West Africa
In co-operation with LOCEAN (Paris, France) and IFREMER (Brest, France), Marum has collected a multi-disciplinary data set to investigate authigenic carbonates found at the pockmark seep sites of the West-African Margin. Methods included, among others, swath bathymetry, seismo-acoustic profiling, and sediment coring. First results concerning the mineralogy and isotope geochemistry of the samples have been summarized by C. Pierre (LOCEAN) and colleagues, who find AOM to have been the main control on carbonate precipitation as well as a relationship between the temporal variation of methane flux through the pockmarks on one hand and the isotopic signature of carbonates on the other.