Stylolite

Key Takeaways

• Pressure solution mechanism of stylolite formation involves the dissolution of mineral grains at grain contact points where the compressive stress concentration increases the chemical potential of the solid, making it more soluble than unstressed material in the surrounding pore fluid, with the dissolved material removed by diffusion through the pore fluid to sites of lower chemical potential where it may reprecipitate as cement: the driving force for pressure solution is the difference in chemical potential (free energy) between the stressed grain contact and the unstressed pore fluid, which is proportional to the applied stress, the molar volume of the mineral, and the inverse of the absolute temperature; at typical burial conditions (100-300 MPa overburden stress, 50-150 degrees Celsius temperature), the pressure solution rate is sufficient to produce significant stylolite development in 10,000-100,000 years of burial, which is geologically rapid; the rate-controlling step in stylolite development may be the dissolution reaction at the grain contact (interfacial dissolution kinetics), the diffusion of dissolved material away from the contact through the thin fluid film trapped between the grains (diffusion kinetics), or the precipitation of the dissolved material as cement at sites of lower stress (precipitation kinetics), with different steps dominating in different pressure, temperature, and fluid composition conditions; limestones are particularly susceptible to pressure solution because calcite dissolves orders of magnitude faster than quartz at low temperatures, making stylolites much more common and better developed in limestones than in sandstones at comparable burial depths.
• Stylolite amplitude (the height of the teeth or columns that interlock across the stylolite surface) provides a measure of the amount of rock dissolved at the stylolite and removed from the section, which is typically 10-50% of the original rock thickness in stylolite-rich intervals and can exceed 30% in intensely stylolitized sections: the stylolite amplitude (typically 1-30 mm for individual stylolites) approximates the vertical extent of the dissolution zone that produced the stylolite, because the dissolved material that was originally distributed through this zone is concentrated at the stylolite surface as insoluble residue; counting the stylolite density (number of stylolites per meter of core) and multiplying by the average amplitude provides an estimate of the total compaction by pressure solution, which can be subtracted from the present formation thickness to estimate the original depositional thickness before stylolitization; this decompaction calculation is used in basin models to reconstruct the original sediment thickness and the burial history, because stylolite-induced compaction can reduce carbonate section thickness by 20-40% over geological time, significantly affecting the paleogeographic reconstructions and the thermal history models used to predict hydrocarbon generation from associated source rocks.
• Stylolite orientation relative to the principal stress axes provides information about the paleostress field that drove stylolite development, because pressure solution occurs preferentially at contacts where the compressive stress is highest, causing stylolites to develop perpendicular to the maximum principal compressive stress: horizontal stylolites (the most common type in undeformed sedimentary basins) record the dominance of vertical overburden stress (lithostatic load) as the maximum principal compressive stress during burial, consistent with normal faulting or extensional tectonic regimes; vertical and steeply inclined stylolites (tectonic stylolites) record intervals where the horizontal compressive stress from tectonic compression exceeded the vertical lithostatic stress, causing pressure solution perpendicular to the horizontal compression direction; the orientation of tectonic stylolites in outcrop and core can be used to reconstruct the direction of paleotectonic compression if the rock has not been subsequently rotated, providing paleostress information that complements the structural geology of the deformation belt; the coexistence of horizontal burial stylolites and vertical tectonic stylolites in the same rock unit records a complex stress history with different stress regimes at different times during the rock's burial, tectonic, and exhumation history.
• Stylolite effects on carbonate reservoir permeability are complex and depend on the continuity and thickness of the insoluble residue layer, the associated fracture development, and the position of the stylolite within the reservoir flow unit: a continuous, clay-rich stylolite seam (1-5 mm thick) with high clay content (above 30-50% by weight of the residue) and lateral continuity extending across the reservoir width can act as an effective permeability barrier (vertical permeability across the stylolite below 0.01 millidarcies) that compartmentalizes the reservoir and prevents the vertical migration of oil or water during production and fluid injection; discontinuous stylolite seams, or stylolites with low clay content (below 10% clay in the residue), do not significantly impede vertical fluid flow and may be essentially transparent to reservoir fluid movement; the fractures that commonly accompany stylolites (both synthetic fractures sub-parallel to the stylolite and antithetic fractures perpendicular to it) can reverse the permeability effect of the clay-filled stylolite by creating high-permeability channels through the stylolite zone that allow fluid crossflow; the net effect of stylolites on reservoir permeability therefore depends on the integrated effect of both the clay barrier function and the associated fracture conduit function, which varies on a stylolite-by-stylolite basis and requires characterization from both core and image log data to quantify adequately for reservoir simulation.
• Stylolites as hydrocarbon migration conduits and concentrators in carbonate reservoirs have been documented in several major oil fields where stylolite networks have focused oil migration and accumulation: the same pressure solution chemistry that concentrates insoluble organic residue in stylolites also concentrates migrating hydrocarbons that travel along stylolite-associated fractures, and the organic richness of stylolites (which may reach 5-20 wt% TOC in organic-rich carbonate rocks) provides a record of both the original organic content of the rock and the subsequent hydrocarbon flux through the stylolite network; in some fields, stylolite-associated fractures have been identified as the primary migration pathway for hydrocarbons from deeper source rocks to shallower reservoirs, with the regularity of the stylolite-fracture network (controlled by the systematic orientation of the compressive stress field during stylolite formation) providing a predictable geometry for the migration pathway and the hydrocarbon accumulation; the observation of fluorescent hydrocarbons along stylolites in core samples is a direct indicator of paleo-oil migration along the stylolite network and is used in oil migration pathway analysis to track the charge history of the reservoir from source to trap.