HIGH RESOLUTION 3D SEISMIC IMAGING OF SUBSURFACE FLUID FLOW SYSTEMS AND SEDIMENT REMOBILIZATION IN SEDIMENTARY BASINS

Abstract

Comprehensive characterization of gas hydrate systems and subsurface fluid flow phenomena offshore The Gambia, offshore Niger Delta and along the Norwegian margin has been performed through an integrated analysis of geophysical data. The presence of a gas hydrate system is recognized by prominent bottom simulating reflectors (BSR) in both the MSGBC (Mauritania, Senegal, Guinea, Bissau, Conakry) Basin and ultra-deepwater Niger Delta. Assuming steady-state conditions, BSR depths are employed in the estimation of geothermal gradient (GTG) and consequently, heatflow. BSR depths in conjunction with velocity and established empirical relations for gas hydrate stability yields shallow geothermal gradient estimates of 3.6 – 6.5°C/100m and an overall mean heatflow of 56mW/m2 for the MSGBC basin. Present-day thermal maturity of the Neocomian and Jurassic source intervals was assessed by applying the derived mean heatflow. The result indicates mature source intervals. Heatflow ranging from 54 – 79mW/m2 and average shallow GTG of 4.0 – 5.3°C/100m derived from twenty interpreted BSRs was obtained for the Niger Delta study area. In the absence of calibration data, associated errors are evaluated at 38% and 40% for the MSGBC Basin and Niger Delta respectively. Fluid flow features and their associated plumbing systems provide useful information on the sources feeding the Niger Delta gas hydrate system. High permeability flow conduits in the form of pipes, faults and gas chimneys provide suitable paths for the upward migration of subsurface fluids. The spatial distribution of these migration conduits is believed to greatly influence the development of a gas hydrate system in the study area. Seismic interpretation across the Norwegian Margin presents new results for focused fluid flow phenomena and shallow gas accumulation in the Nyegga and Helland Hansen Arch areas. In the Nyegga area, a large population of seabed pockmarks linked to underlying fluid pipes occur in close proximity to the northern escarpment of the Storegga slide. The distribution of fluid pipes is controlled by the local topography of the area. Fluid escape through pipes terminating as pockmarks and occasionally as mounds is attributed to rapid and increased sediment loading caused by the giant Storegga Slide. Shallow gas accumulations evidenced by numerous high amplitude anomalies is common in the vicinity of the Helland Hansen Arch (HHA). Fluid migration in HHA occurs through leakage point of faults and via polygonal fault systems. Detailed analysis and data integration present new observations and evidences for an improved understanding of the gas hydrate systems, thermal regime and subsurface fluid flow phenomena across the investigated areas.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Redfern (Supervisor) & Mads Huuse (Supervisor)