Abstract
The initial sediment lithification starts with complex interactions involving minerals, surface water, decomposing organic matter and living organisms. This is the eogenesis domain (0 to 2 km below the seafloor) in which the sediments are subject to physical, chemical and mechanical transformations defining the early fabric of rocks. This interval is intensively prospected for its energy/mining resources (hydrocarbons, metal deposits, geothermal energy). In most basins worldwide it is composed of very fine-grained sediments and it is supposed to play the role of a seal for fluids migration. However, it is affected by polygonal faulting due to a volume loss during burial by contraction of clay sediments with a high smectite content. This process is of high interest for fractured reservoirs and/or cover integrity but it is not well constrained giving an uncertainty as this interval can either promote the migration of deeper fluids and the mineralized fluids intensifies diagenesis in the fracture planes, rendering this interval all the more impermeable. The next challenge will be to define where, when and how does this polygonal fault interval occur and this can only be done by understanding the behavior of clay grains and fluids during early burial.
Highlights
On the Gjallar Ridge, the 800 m thick Middle OligoPliocene sequence is affected by small, closely-spaced intraformational faults, showing small throw values with a maximum of 30 msTWT and spacing ranging from 200 to 1000 m (Fig. 1)
Such fault systems have been recognised in many basins all over the world: in the North Sea
Four hypotheses are proposed to explain the formation of these polygonal faults: i) syneresis related to colloidal properties of such fine-grained sediments [8], ii) density inversions and associated hydrofracturing [19], iii) smectite-rich clays causing residual friction at low burial depth [20,21] and iv) grain dissolution in uncemented media inducing a decrease in horizontal stress that leads to shear failure and shear strain localization [22,23]
Summary
On the Gjallar Ridge, the 800 m thick Middle OligoPliocene sequence is affected by small, closely-spaced intraformational faults, showing small throw values with a maximum of 30 msTWT and spacing ranging from 200 to 1000 m (Fig. 1). Spaced polygonal faults form in fine-grained clay-rich sediments and they display a polygonal pattern in plane view [6,7,8] Such fault systems have been recognised in many basins all over the world: in the North Sea The kinematic model of polygonal fault growth in which the propagation of faults is discontinuous during basin infilling leads to a 4D interpretation of the whole fault system which will be one of the challenge in basin modelling
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