Lithification

Key Takeaways

• Compaction is the first stage of lithification and occurs continuously as sediment is buried under the weight of progressively thicker overlying rock, with the overburden stress closing pore space, dewatering the sediment (expelling connate water upward), and rearranging grains into more compact configurations that increase grain-to-grain contact areas and reduce the original depositional porosity: sand deposited in a beach or river environment may have initial depositional porosity of 40 to 50 percent, which is reduced by compaction to 30 to 35 percent at depths of 1,000 to 2,000 meters as the weight of the overburden crushes and rearranges the grains; carbonate sediments (calcareous ooze, coral rubble, and bioclastic sands) compact more rapidly than quartz sands because the softer calcite mineral is more easily deformed at low stresses; in deeply buried sequences (below 3,000 to 5,000 meters), compaction-driven porosity reduction is supplemented by pressure dissolution (chemical compaction, where stressed grain contacts dissolve into the pore fluid and are reprecipitated elsewhere as cement), which can reduce sandstone porosity to less than 5 percent in overpressured deep intervals where the fluid pressure is insufficient to fully support the grain contacts against dissolution.
• Cementation is the precipitation of mineral cements from formation water that circulate through the pore space, filling the remaining porosity after compaction with solid mineral material that binds grains together and further reduces porosity: common cements in sandstones include quartz overgrowths (precipitation of additional silica on detrital quartz grains, syntaxially continuing the grain's crystal lattice), calcite cement (precipitation of calcium carbonate from bicarbonate-rich formation waters, which can completely fill pore space and create nearly impermeable concretions or cement bands), feldspar overgrowths (rare, occurring in very low-temperature diagenetic environments), kaolinite (precipitation from aluminum-silica-bearing waters, which forms pore-filling booklets that reduce permeability more than porosity), and chlorite (precipitation on grain surfaces as pore-lining coats that dramatically improve porosity preservation by blocking quartz overgrowth nucleation); cementation patterns in sandstones are highly heterogeneous because they depend on the local fluid chemistry and temperature history, producing patchy cementation where well-cemented and poorly-cemented intervals alternate at scales of centimeters to meters, creating the permeability heterogeneity that governs flow performance in producing reservoirs.
• Porosity-depth relationships established from lithification studies in different sedimentary basins provide predictive tools for petroleum exploration, allowing estimates of reservoir quality before drilling based on the anticipated burial depth and temperature history: the Sclater-Christie compaction curve for shales and the Scherer porosity-depth relationship for sandstones quantify the expected porosity reduction with depth for different lithologies and burial rates, calibrated from measured core porosity data from many wells; in regions with anomalously rapid burial (young, high-sedimentation-rate basins like the deepwater Niger Delta or the Gulf of Mexico Pliocene-Pleistocene section), porosities may be higher than predicted for the depth because insufficient time has elapsed for full compaction and cementation equilibration; conversely, in uplifted and exhumed sequences (such as the Cretaceous sands of the North Sea that were buried to greater depths than their current elevation and then uplifted), the porosity may be lower than predicted because the enhanced cementation that occurred at the maximum burial depth is preserved after uplift; these deviations from expected porosity-depth trends are both risks (unexpectedly poor reservoir quality) and opportunities (unexpectedly good reservoir quality in basins with deep hot fluids that dissolved early cements) that careful basin analysis attempts to predict.
• Diagenetic mineral reactions during lithification create secondary porosity that partially compensates for primary porosity loss, providing an additional reservoir quality variable that cannot be predicted from depth and burial history alone: the most important secondary porosity mechanism in sandstones is the dissolution of unstable mineral grains (feldspars, rock fragments, and calcite cement) by acidic formation waters that become undersaturated with respect to these minerals during burial heating; feldspar dissolution during late-stage burial (at temperatures of 80 to 150 degrees Celsius) can create significant secondary porosity (2 to 10 percent additional pore space) in feldspathic sandstones that would otherwise be cemented closed by quartz overgrowths; the resulting pore space is typically oversized (larger than the original grain pore throats) but poorly connected (because the dissolved grains leave behind kaolinite residue that partially blocks pore throats), making secondary porosity incremental to production but often insufficient on its own to make an otherwise non-reservoir-quality sand a commercial producer without the primary porosity framework.
• Carbonate lithification differs fundamentally from siliciclastic (quartz and feldspar) lithification because carbonates are chemically reactive at near-surface conditions, undergo aragonite-to-calcite mineral conversion in the early diagenetic environment (marine and meteoric water diagenesis), and are particularly susceptible to both dissolution (creating vuggy and karst porosity) and recrystallization (dolomitization, calcite replacement by dolomite, which can either preserve or destroy porosity depending on the mechanism): early cementation of carbonate sediments in marine and freshwater environments can preserve depositional fabric and porosity by providing a framework that resists compaction during burial, with shallow-water carbonates cemented in the marine phreatic zone sometimes retaining framework porosity to depths where compaction would have destroyed it in an uncemented carbonate; dolomitization by circulating Mg-rich brines can increase reservoir quality (by creating intercrystalline porosity that exceeds the original calcite porosity) or decrease it (by completely occluding pore space in fabric-destructive dolomitization), making the diagenetic history of carbonate reservoirs a complex and geologically rewarding subject for petroleum geologists evaluating carbonate play concepts.