What are Stylolites ?
Stylolites are seams of insoluble residue that record the removal of the soluble component of the rock as a result of dissolution. They often have a serrated appearance, sometimes resembling a wiggly pencil mark on rock core. (Fig. 1). They are usually described on rock outcrops or core as individual structures are too fine to be resolved on logs.
Do they exist in Sandstones ?
I’ve heard these being called pressure solution seams in sandstones and stylolites in carbonates [1] but I think the structures we observe in core can be called stylolites irrespective of lithology. If they are parallel to bedding we can generally assume they have formed during burial (Fig. 1, 2). Those that form later i.e., as a result of tectonic processes, tend to cut across the bedding.
Burial depths ?
In sandstones, significant quartz dissolution requires temperatures in excess of 90 deg C. The sandstones in Fig. 1 have been buried to at least 4 km. These sandstones have clay content up to 20% and it has been shown that relatively small amounts of clay may encourage pressure solution at grain contacts (Fig. 3), [2]. This process can result in fault rocks (phyllosilicate framework) actually transforming into stylolites during later burial.
Impact upon reservoir permeability
In my experience, burial stylolites are more common than tectonic stylolites. Therefore, vertical permeability tends to be impacted more than horizontal permeability. When the two occur together they can form a network that is likely to be permeability reducing as opposed to permeability enhancing. There are also situations where the presence of stylolites can improve permeability i.e., where related tension gashes (open fractures) protrude vertically from the stylolite (Fig. 2).
Figure 1 (a) Stylolite in medium to coarse grained sandstone, Mid Jurassic (Brent), Northern North Sea (b) stylolites in limestone, Cupido platform, NE Mexico.
Fig. 2. Burial stylolite in sandstone with associated fractures/tension gashes. These are likely to enhance the reservoir permeability.
Figure 3. Thin section taken across a deformation band in Mid Jurassic sandstones from the Volve Field, North Sea. There may be some enhanced grain contact quartz dissolution.100x magnification. Equinor public data.
https://www.ogilviegeoscience.co.uk/blog/2022/11/22/deformation-bands-in-the-volve-field-north-sea
References
[1] Koepnik, R.B. 1987. Distribution and permeability of stylolite-bearing horizons within a Lower Cretaceous carbonate reservoir in the Middle East. SPE Form. Eval, June, 1987, 137-142.]
[2] Fisher, Q, Knipe, R. 1998. Fault Sealing processes in siliclastic sediments. From Jones, G, Fisher, Q & Knipe, R (eds) Fault Sealing and Fluid Flow in Hydrocarbnon reservoirs. Geol Soc Spec Publ 147, 117-134.
Key Takeaways
Stylolites occur in both carbonates and sandstones although are often more commonly associated with carbonates.
They can form during both burial (often parallel to bedding) and due to later tectonic movements (when they usually cross-cut the bedding).
Stylolites are usually permeability reducing as opposed to permeability enhancing although associated tension gashes can enhance the reservoir permeability.