Stylolite-controlled diagenesis of a mudstone carbonate reservoir: a cse study from the Zechstein_2_carbonate (Central European Basin, NW Germany)

Abstract

Stylolites are rough dissolution surfaces that form due to intergranular pressure-solution resulting from burial compaction or tectonic stress. Despite being ubiquitous in most carbonate rocks, their potential impact on structural diagenesis and fluid flow remains unclear. The Zechstein 2 Carbonate (Ca2) is a diagenetically complex reservoir in the Southern Permian Basin and represents one of the most prolific gas reservoirs in NW Germany. This investigation focuses on evaluating the relationship between stylolites, fractures/veins and their subsequent influence on the spatial variations in reservoir quality. We utilise drill core samples to carry out a combined analysis of cross-cutting relationships between different structures and diagenetic products. We therefore use a combination of petrography and statistical analyses on stylolite networks, focusing on their occurrence, morphology and sealing capacity. In the study area, the Ca2 carbonate mudstone was deposited in a slope environment and dolomitised under shallow burial conditions, followed by bedding-parallel stylolitisation during burial. Results indicate that calcium-rich fluids percolated from neighbouring evaporite units causing widespread calcitisation within the more distal environments of deposition. Some stylolites locally acted as barriers to affect the migration of the calcitising fluids, resulting in a macroscopic diagenetic stratification of relatively porous dolomite and areas of calcitised dolomite with lower porosity. However, pressure-solution continued during burial and bedding-parallel stylolites also appear postdating calcitisation. During inversion, horizontal stylolites were reopened to act as conduits to enable fluid migration that precipitated metal sulphides. This indicates that stylolites acted as both barriers and conduits for fluid flow depending on variations of the overburden pressure and regional stress regime. Stylolites present a range of sealing capacities between 63 and 89%, depending on their morphology, and can result in partial leakage and subsequent invasive calcitisation in their vicinity. This study highlights the importance of understanding the impact of stylolites on structural diagenesis and spatial variations in petrophysical rock properties that determine reservoir quality.