Title: Unraveling Zechstein 3 Anhydrite Structuration and possible implications for Geo-Drilling Hazards
Ward Teertstra (1) & Guido Hoetz (2)
VU Amsterdam, The Netherlands (1); EBN, The Netherlands (2);
Event: Abstract GeoUtrecht2020
Anhydrites Carbonates of the Leine formation, the third evaporitic cycle of the Zechstein group, are widely distributed in the Southern Permian basin. These strata are often drilled through by wells targeting underlying Rotliegend gas prospects. The ZEZ3C/A members constitute brittle rocks sandwiched by the ductile halites from the underlying ZEZ2H and the overlying ZEZ3H. The brittle sheet is frequently rifted (boudinage) and the resulting fragments are often referred to as stringers. In many areas ZEZ3 stringers (consisting of Haupt Anhydrite/ Platten Dolomite), show a fairly constant thickness of around 50 meters. Deviations from these general observations, in particular anomalously thick stringers, were known from wells but their origin was unclear.
This poor understanding was also related to limitations in seismic quality, which generally did not allow detailed stringer mapping. With the arrival of new 3D seismic from the dutch offshore G/H blocks (adjacent to the German sector), the stringer interval could be studied in great detail. Seismic-to-well ties in this area, show that thickened zones consist of a locally thickened Zechstein III anhydrite member.
Isopach mapping of the stringers resulted in new insights of the thickened zones distribution which tend to have a characteristic appearance consisting of linear, branching and closing segments. Based on these observations the gypsum dome structure model is proposed in this research, which is largely based on comparable observations described from the Harz mountains. In this model recently deposited water-rich gypsum behaves ductile and creates doming features comparable to halite salt domes, albeit on a smaller scale. Subsequent dewatering of gypsum bodies after some burial, causes the transition to anhydrite which has a higher specific density and is more brittle. Also, the associated volumetric shrinkage creates new accommodation space which effect can be observed in the overlying strata. In addition, the dewatering process might create fluid pathways to the surface and result in the leak-off of over pressures in these zones. Planning and drilling new well trajectories might benefit from the improved understanding of stringer thickness variability based on this research.