Unraveling Intra-Zechstein Structuration and Understanding the Relationship with Geo- Drilling Hazards: By means of detailed seismic mapping in the Dutch Eastern Offshore (2019)

Deze MSc scriptie is geschreven door oud-stagiair Ward Teertstra.

Carbonates and anhydrites of the Leine formation, the third evaporitic cycle of the Zechstein group, are widely distributed in the Southern Permian basin and these are often drilled through by wells targeting the underlying Rotliegend. 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”. Fully salt encased ZEZ3 Stringers can be heavily overpressured. Drilling through overpressured zones can lead to formation fluids entering the borehole, causing a “kick” situation in the well during the drilling phase. This type of geo drilling incident might lead to delays, increased drilling costs and even safety risks.
Commonly ZEZ3 stringers, also referred to as “Plattendolomite”, have 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 seismic of the Hansa 4Quad 3D survey (re-processed 2017), 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 stringer 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. Recently deposited water-rich gypsum behaves very ductile and can create 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 that 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.
In this study 154 wells that observed thickened stringers (i.e. > 60-meter of ZEZ3A), were investigated for Zechstein geodrilling events. Wells with thickened stringers showed significantly lower percentages of kicks in the stringer, compared to kicks encountered in “normal thickness” stringers as described a previous study done by P. Schilder (2019, EBN internship). By combining the results of seismic mapping and well analyses, a new geological explanation for the thickened stringers is proposed. Planning and drilling new well trajectories might benefit from the improved understanding of stringer thickness variability based on this research.