Collected reprints: Abstract 2798
Collected reprints
Abstract
Henriet, J.-P.; De Batist, M.; Verschuren, J. (1991). Early fracturing of Palaeogene clays, southernmost North Sea : relevance to mechanisms of primary hydrocarbon migration. Special publication of the European Association of Petroleum Geoscientist 1: 217-227
Argillaceous petroleum source-rocks often show evidence of fracturing, which is considered to be an important factor for the primary migration of hydrocarbons. Several authors consider overpressuring of the enclosed pore fluids at great burial depths as a major factor for the generation of microfractures, through the mechanism of hydrofracturing.
Various scales of fracture networks have been observed in Palaeogene clays which have never reached burial depths, greater than a few hundreds of meters. High-resolution seismic reflection investigations carried out in the southern North Sea have revealed large-scale fracturing patterns, confined to the London/Ieper clays. Equivalent deformation and microfracturing have been observed at outcrops on land.
Field evidence suggests an early fracturing of these clays by overpressuring of the poor water at relatively shallow depths and at an early stage of the clay diagenesis. A locally observed wavelike deformation pattern may prove a relict form of a Ray-leigh-Taylor instability, related to the temporary density inversion caused by undercompaction. The observed deformations can to some extent be compared with sand-box models, developed at Rennes University in France.
Argillaceous potential source-rocks, having perhaps already undergone general hydrofracturing at shallow depths may thus have reached the depths required for catagenesis in a pre-fractured state. This would have a bearing on the timing of the onset of primary migration.
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