TSK 11 Göttingen 2006 Hilgers et al.
 Fossil overpressures compart-
 ments? A case study from the
 Eifel area and some general
 aspects Vortrag
 Christoph Hilgers1 Carsten Bücker2
 Janos L. Urai1
 Introduction
 Fluid overpressures are well known from
 hydrocarbon exploration in many sed-
 imentary basins. They can reach al-
 most lithostatic values, and may cause
 the fracturing of rock. Fracturing allows
 the discharge of fluid overpressure, and
 fluid flows along a hydraulic gradient to-
 wards a low pressure reservoir. Differ-
 ent mechanisms may cause the precip-
 itation from the fluid, such as a fluid
 pressure drop, a variation of tempera-
 ture at the low pressure reservoir, or
 a different rock type inducing differ-
 ent Eh-pH conditions. Such precipitates
 in fractures are called veins, which of-
 ten display paleo-fluid overpressures in
 rocks. In this study, we present some re-
 sults from Devonian clastic sedimentary
 rocks of the Eifel area. Results are com-
 pared with other sedimentary basins to
 highlight some general aspects.
 Geological setting
 The lower Devonian rocks exposed along
 the shore of the Rursee water reservoir
 are Siegenian Upper Rurberger beds
 and Emsian Klerfer and Heimbacher
 beds to the NE. They are located on the
 SE-flank of the NE-plunging Variscan
 Venn-Anticlinorium. Both units expose
 shales, siltstones and sandstones de-
 posited in the subsiding Devonian Eifel
 Basin.
 1 Geologie-Endogene Dynamik, RWTH-
 Aachen, Germany 2 Shell E&P, Houston,
 USA
 Meso- and microstructural data
 Two different vein sets are oriented
 subnormal (#1) and parallel (#2) to
 bedding, respectively. Both sets are
 filled with quartz. Vein set #1 is re-
 stricted to sandstone layers, and rarely
 continues into the enclosed shale beds.
 Their shape is sigmoidal on fold limbs,
 and their orientation in accordance
 with flexural slip along the bedding
 planes. The vein microstructure of #1
 shows an elongate-blocky to fibrous mi-
 crostructure. Fibrous grains continue
 across the vein (stretched crystals) and
 contain solid and fluid inclusions ar-
 ranged parallel to the vein wall inter-
 face. Meso- and microstructural ob-
 servations indicate vein formation prior
 or syn-folding, veins opened in incre-
 mental steps (crack-seal mechanism).
 Vein set #2 is located at the shale-
 sandstone interface and can be traced
 for several tens of metres. It gener-
 ally truncates #1 and thus post-dates
 the bedding-normal veins. Locally, vein
 set #2 cuts trough the hanging-wall and
 is associated with small thrusts. The
 blocky quartz grains of vein set #2
 extinct undulose and are recrystallised
 by grain boundary migration and sub-
 grain rotation. The quartz grains of the
 host rock, however, are elongated (over-
 growth, fringes and dissolution along
 the cleavage planes), but generally op-
 tically undeformed. This indicates that
 pressure solution was the dominant de-
 formation mechanism in the host rock.
 Modelling the subsidence of the up-
 per Rurberg beds using published data
 of burial temperatures (vitrinite re-
 flectance, illite crystallinity etc., e.g.
 von Winterfeld 1994) indicates rapid
 burial down to 7–8 km depth prior to the
 onset of Variscan compression and sub-
 sequent basin inversion. This accounts
 1
Hilgers et al. TSK 11 Göttingen 2006
 Figure 1: (a) The vein microstructure of #1 shows stretched crystals, which can laterally
 evolve towards an elongate-blocky microstructure grown syntaxially. b) Vein set #2 is
 blocky and was deformed plastically by dynamic recrystallisation. c, d) Fluid inclusion
 data show maximum temperatures of about 370°C for both vein sets #1 (c) and #2 (d).
 for the up to approx. 6.5 km thick lower
 Devonian sedimentary pile deposited in
 the Eifel Basin, and fluid overpressure
 generation during subsidence and the
 onset of Variscan compression (Fig. 2).
 Discussion and Conclusion
 Field and microstructural observations
 of vein sets #1 and #2 are consistent
 with our modelled overpressure gener-
 ation due to basin evolution (Fig. 2).
 Bedding normal veins #1 are restricted
 to the competent sandstone layers and
 formed in an already competent rock,
 as shown by transgranular fractures and
 the absence of compaction features as-
 sociated with vein set #1. Repeated
 crack-seal increments and the variation
 of paleo-temperatures suggest that veins
 were subsequently opened and re-sealed
 during subsidence. Stretched crystals
 are oriented normal to the vein wall, in-
 dicating extension normal to bedding.
 Vein formation requires tensile fractur-
 ing of the sandstone with the maximum
 principal stress oriented normal to bed-
 ding, which is consistent with vein for-
 mation during subsidence at high fluid
 overpressures and low differential stress.
 Bedding-parallel vein set #2 truncates
 set #1 and cuts through the hanging
 Figure 2: Subsidence curve of the Lower
 Devonian upper Rurberg beds.
 wall associated with small thrusts. This
 indicates a reorientation of the princi-
 pal stresses between #1 and #2. Vein
 set #2 is folded in accordance with the
 Variscan folds exposed on the shore, but
 parasitic folds of the veins are absent.
 This suggests that vein set #2 repre-
 sents the first compressional event asso-
 2
TSK 11 Göttingen 2006 Hilgers et al.
 ciated with Variscan deformation. The
 vein system of the Eifel Basin is con-
 sistent with results from other sedimen-
 tary basins, which may also include mi-
 crostructural aspects.
 References
 von Winterfeld, C-H (1994) Variszische Deck-
 entektonik und devonische Beckengeometrie
 der Nordeifel — Ein quantitatives Modell.
 PhD Thesis, RWTH Aachen, Aachen, 319
 pp.
 3