Sandstone Diagenesis

During diagenesis a sandstone may undergo compaction, cementation and dissolution. Detail these processes and explain why they are important factors that must be considered by petroleum geologists.

Diagenesis is the lithification of loose sediment into solid rock through compaction, cementation, and dissolution.

Compaction is increased pressure during burial physically squeezing sedimentary grains together, compressing void pore space. Different materials compact to different extents: clays compact more, while sandstones compact less. The result is differential compaction dependent on material. Not all material can be lithified through compaction — a mix of sand and gravel without a clay matrix lacks sufficient grain-to-grain contact for even heavily compressed sediments to lithify by compaction alone.

During compaction, the visible fabric is deformed and distorted into new textures. Pebbles and fossils can be flattened; fine grained fragments deformed or even broken into clay particles. Grains may be subject to pressure solution: pressing concavities into each other at contact points. Textural characteristics, including an increase of fractures, can help with estimating the maximum depth reached during compaction.

Additional clues can come from minerals changing properties as the temperature increases with depth, with some fossils changing colour, organics growing increasingly shiny, and clay minerals transforming into other minerals.

Cementation is the precipitation of new minerals to hold together grains. Cementation is more common in the near surface, as groundwater flow slows with depth. Common cements are silica, calcite, and iron oxide. Magnetic cements (such as iron oxide) lock in the magnetic field at the time of formation, potentially allowing the timeframe and latitude of cementation to be determined.

Dissolution is the weathering of sandstone through the removal of minerals or grains by fluid transport.

Petroleum geologists love finding porous sandstone capped with an impermeable surface like clay that can trap natural gas. Shoreline regression and transgression forming jagged vertical switches between sandstone and offshore clays are a favourite target.

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