Poly-phased fluid flow in the giant fossil pockmark of Beauvoisin, SE basin of France

Abstract

The giant Jurassic-aged pockmark field of Beauvoisin developed in a 800 m wide depression for over 3.4 Ma during the Oxfordian; it formed below about 600 m water depth. It is composed of sub-sites organized in clusters and forming vertically stacked carbonate lenses encased in marls . This fine-scale study is focused on a detailed analysis of petrographical organization and geochemical signatures of crystals that grew up in early to late fractures of carbonate lenses, surrounding nodules, and tubes that fed them. The isotopic signature (C, O and Sr) shows that at least three different episodes of fluid migration participated to the mineralization processes. Most of the carbonates precipitated when biogenic seepage was active in the shallow subsurface during the Oxfordian. The second phase occurred relatively soon after burial during early Cretaceous and thermogenic fluids came probably from underlying Pliensbachian, Late Toarcian or Bajocian levels. The third phase is a bitumen-rich fluid probably related to these levels reaching the oil window during Mio-Pliocene. The fluids migrated through faults induced by the emplacement of Triassic-salt diapir of Propiac during the Late Jurassic and that remained polyphased drain structures over time.

Future challenges on focused fluid migration in sedimentary basins: Insight from field data, laboratory experiments and numerical simulations

“…In numerical experiments, three types of models are currently developed 1) based on hydraulic fracturing hypothesis where overpressured gas in the source rock induces fractures in the overlying rocks, and a network of hydraulic fractures propagates towards the surface as high-permeability conduits [121], 2) based on porosity waves where pipes propagate spontaneously due to complex nonlinear coupling between fluid buoyancy, asymmetric compaction-decompaction of pores, and viscoplastic deformations of sediment matrix [111,112], and 3) based on fluidization leading to brecciation and erosion processes within the conduit [122,123]. However, they all account for a complete or partial loss of stratigraphy within focused fluid flow conduits, which is not the case shown in fossil pipes [61].…”

“…Surprisingly, given the number and the size of pipes described offshore and given the large number of exhumed fossil paleo-seafloor seep structures reported onshore [58][59][60], underlying feeding pipes have never been identified in the fossil record. Only a few examples of carbonate tubes have been reported worlwide: in France [57,61], in Greece [40,62,63] and in New-Zealand [64][65][66][67][68]. The tubes are separated from 5 to 30 m, locally connecting to their neighbours and, taken all together, they are organized in a 150-250 m sub-circular area without disrupting the general layering of marls (Fig.…”

“…They have also shown that periods of active fluid seep alternated with periods of apparent quiescence, about 200 kyr each. Underlying fossil fluid tubes have the same mineralogic and geochemical phases, indicating that they have formed contemporaneously due to the same fluids [61]. Once the carbonate tubes are formed within the underlying feeding pipe, they remain open for a very long period of time (156 Ma), which is of primary importance for the sealing capacity of such "impermeable" intervals in the case of CO 2 or nuclear waste storage.…”