Katrine Juul Andresen

Hydrocarbon plumbing systems of salt minibasins offshore Angola revealed by three-dimensional seismic analysis

Analysis of three-dimensional seismic data from the lower Congo Basin, offshore Angola, reveals numerous fluid-flow features in the Miocene to Holocene succession and the potential for large, shielded traps underneath basinward overhanging salt structures. The fluid-flow evidence includes present-day sea floor pockmarks clustered above salt structures, Pliocene–Pleistocene stacked paleopockmarks and Miocene pockmark fields. Other fluid-flow features include high-amplitude cylindrical pipe structures 60 to 300 m (197–984 ft) wide and 25 to 300 m (82–984 ft) high within lower and middle Miocene strata, thick (150 m [492 ft]) high-reflectivity zones within the Pliocene succession associated with bottom-simulating reflections, and subvertical low-amplitude chimneys originating from the deeper section (1 km [0.6 mi] beneath the sea floor). The Miocene pockmark fields occur at a specific horizon, suggesting a regional fluid expulsion event at ca. 12 Ma, and the Miocene fluid-flow regime is interpreted to be dominated by thermogenic fluids supplied via carrier beds and leaking vertically above structural highs. The Pliocene–Pleistocene fluid-flow regime was dominated by short-distance vertical fluid migration and expulsion related to early stage diagenetic processes involving biogenic methane and pore water. The present-day fluid-flow regime is inferred to be dominated by thermogenic fluids primarily controlled by kilometer-scale salt-flank-controlled migration. The study emphasizes the use of seismically imaged fluid-flow features in hydrocarbon systems analysis by documenting the evolution of an overburden plumbing system through time, involving several fluid types and flow regimes, depending on the spatiotemporal availability of thermogenic and diagenetic fluids and the tectonostratigraphic occurrence of aquifers, traps, and seals.

A composite mud volcano system in the Chalk Group of the North Sea Central Graben

Abstract: The study describes a large (kilometre-scale) composite mud volcano system within the North Sea Chalk Group, containing both intrusion and extrusion features. The upper part of the system consists of three stacked constructive components. At the top, a Late Palaeocene mound-shaped extrusion with subtle onlaps is observed. Below, a Danian near-surface intrusion caused forced-folding of the c. 100 m thick overburden. This is underlain by a shallow intrusion. The lower part of the mud volcano system is defined by a depletion-related depression and the fluid conduit. Seismic data suggest that the source unit for the linked intrusion–extrusion is to be found either within a Lower Cretaceous diapiric siliciclastic structure or within the fine-grained carbonates of the Chalk Group. The latter is more probable, implying that chalk may generate hitherto undocumented intrusions and extrusions comparable with siliciclastic mud volcano systems. Remobilization is inferred to have been driven by fluidization caused by the influx of mainly pore water expelled from the Lower Cretaceous succession. A distinct depositional variation within the Chalk Group caused early diagenetic differences in porosity preservation and mechanical properties of the chalk units, allowing the remobilization of certain chalk units even at burial depths as great as 200 m.

Source rock maturity from seismic data: low-velocity anomalies as potential indicators of hydrocarbon generation

In this study we investigate the source rock maturity of the organic-rich Upper Jurassic Farsund and Lola formations in the southern Danish Central Graben. We do this by analyzing an interval velocity volume derived through seismic processing flows for discrete low-velocity anomalies occurring within the source rock succession. We have mapped the anomalies and calibrated them with source rock data within several wells in the study area. Although not unambiguous, our results indicate that some of the low-velocity anomalies potentially can be ascribed to variations in source rock composition and maturity, and hydrocarbon generation. Two critical steps are to 1) quality-check the utilized interval velocity volume i.e. rule-out any geophysical effects and artefacts and 2) calibrate the mapped low-velocity anomalies to source rock effects i.e. rule-out effects from other geological features such as lithological variations, porosity-permeability fluctuations, diagenesis, anisotropy, overpressure and clay content. Besides of using information from well logs and laboratory measurements on core and drill cuttings samples, calibration is based on utilizing seismic attributes and fluid flow features which address the overall migration routes in the conventional hydrocarbon plumbing system of the study area.