FORMATION OF POLYGONAL FRACTURE SYSTEMS AS A RESULT OF HYDRODYNAMICAL INSTABILITIES IN CLAY-RICH DEPOSITS

Clay-rich deposits are characterised by the development of polygonal fractures systems (PFS) in terrestrial [1] and marine conditions [2]. On continents, PFS with a size ranging from centimetre to hundred meters large are described. They form by tensile stresses due to the deposit contraction during dehydration. PFS are also observed in sub-marine deposits and their widths can reach few kilometres. Since these PFS are in basins with no clear evidences of tectonic stresses, the fracturing is attributed to stresses due to horizontal density variations generated during the basin subsidence. Two classic hypotheses are used to explain their formation: the syneresis (a spontaneous horizontal contraction) and the low coefficient of friction of clay. However, new understandings in clay rheology and compaction process permit us to propose an alternative hypothesis.

Considering the two phases formalism describing the compaction process in a consolidated media [3], we propose that compaction in clay-rich deposits can be described by the formation of spherical waves with a fluid-rich core. The size of these spherical waves are proportional to the compaction length L that depends on the permeability and viscosity of the deposit. When the deposit is still deconsolidated, compaction leads to the formation of agglomerates by chemical and/or physical bonds. The size of these agglomerates ranges between few micrometres to millimetres. The agglomeration occurs until the yield stress of the deposit is reached. Once the deposit is consolidated, stresses generated by the buoyancy of overpressured horizons and/or Rayleigh-Taylor instabilities can overcome this yield stress and leads to a new stage of compaction. In that case, the formation of spherical waves impact macroscopically the deposits and form PFS, which sizes are proportional to the compaction length. Without fracture, upward migration of fluids in clay-rich formations is extremely slow due to their small permeabilities (≤10^-16m²). The PFS permit the expulsion of fluids during compaction and may remain as fluid escape pathways [4].

[1] Cartwright and Lonergan (1998), GSA Bulletin, 110, 1242–1257. [2] Neal et al. (1968), GSA Bulletin, 79, 69-90. [3] Bercovici et al. (2001), JGR, 106, 8887-8906. [4] Mazzini et al. (2006), Norwegian Sea. Mar. Geol., 231, 89–102.