Sedimentary Petrology - Diagenesis of Carbonate Sediments

Please note

• This web page is adapted from an online lecture delivered as part of a general first year university Petrology module. It serves as an introduction to this topic.
• Links and references have not yet been fully updated.

Introduction

Diagenesis refers to the physical and chemical changes which take place after the deposition of a sediment; it may include:

• dissolution and alteration of existing grains
• precipitation of new materials in pore spaces

during diagenesis:

• unstable minerals may be destroyed
• new minerals may grow
• existing crystals may become enlarged

Carbonates are much more susceptible to diagenesis than terrigenous clastic sediments, and original structures and textures are often completely destroyed.

Diagenesis of carbonates is of great economic importance. Many important oilfields are produced from carbonate reservoirs - e.g. the giant onshore oilfields of the Middle East. Diagenesis controls the porosity and permeability of carbonate reservoirs.

	Tucker, 1991, pp. 133-154
	Note: all references to Boggs, 1995 and Tucker, 1991 in these pages refer to: BOGGS, S. 1995. Principles of Sedimentology and Stratigraphy (2nd edition). Prentice Hall. TUCKER, M.E. 1991. Sedimentary Petrology (2nd edition). Blackwell.

 Diagenetic processes in carbonates

Can be summarised as:

 1. solution of more unstable minerals (especially aragonite), creating secondary porosity

 2. pore-filling by cements

 3. alteration of original minerals to new ones, especially replacement by neomorphic calcite:

 e.g.

• hi-Mg calcite ---> low-Mg calcite
• aragonite ---> low-Mg calcite
• fine-grained ---> coarse-grained (aggrading neomorphism)
• dolomitization
• silicification

Calcium carbonate cement and its recognition

chemically precipitated material, whether a new mineral, or an addition to an existing mineral, may form a cement, which binds the grains of the sediment together to form a rock.

It is important to distinguish between sparite cement (pore fill) and neomorphic spar, an in situ replacement of calcium carbonate in the solid state.

Cement is deposited in pores: these may be primary - voids between or within grains, or they may be secondary, formed during diagenesis by solution, or other chemical or physical changes.

 Some of the features which characterize cements include:

• generally clear and clean appearance, with well-defined, often (but not always) straight crystal boundaries;
• spar between the grains which does not penetrate into or cut across grains;
• sharp contacts between spar and particles;
• micrite matrix is present but unaltered; aragonite fragments 'replaced' by calcite cement;
• the presence of two or more generations of spar;
• straight crystal edges and frequent triple junctions with 180° angles (enfacial junctions);
• long axes of crystals normal to grain surface;
• increasing crystal size away from grain surface.

 The composition and crystal habit of calcium carbonate cements are highly dependent on variations in physical and chemical conditions.

Carbonate cement types

The type of cement which forms depends particularly on Mg/Ca ratio and salinity of the solution.

• In solutions where Mg/Ca ratios are low, calcite can crystallise freely, forming large, rather equant crystals.
• Where Mg/Ca ratios are high, calcite and aragonite crystals are prevented from growing sideways, so that fibrous, acicular, micritic and pelletal forms are common.

Carbonate cements can be divided into several important types

Early submarine and beach cementation

is common in modern warm water carbonates:

• fringing or drusy cements formed in submarine environment; often fibrous aragonite
• micrite cements, formed in submarine environment. Often high-Mg calcite, sometimes pelletal.

Freshwater (Meteoric Water) Cements

• blocky or mosaic cements, formed in freshwater, phreatic environment: often the main pore-filling cement;
• vadose freshwater cements, formed above the water table, where pore spaces were not completely filled with water;
• syntaxial or rim cements, growing in optical continuity with large single calcite crystals, especially echinoderm grains

Compaction

• Carbonate grains are more susceptible to compaction than quartz grains.
• Grains may behave in either a brittle or a ductile fashion. Early cementation, common in carbonates, protects grains from compaction.
• Carbonate rocks and particles are very susceptible to pressure solution. During compaction, strain is localized at grain contacts. Pressure increases solubility. If pore water is present, this results in localized dissolution at the grain contact.

As the stress is usually vertical, due to overburden pressure, there is preferential solution of the upper & lower surfaces of grains, across a solution film. This results in irregular, sutured contacts between the grains.

Grain-to-grain pressure solution must occur before the filling of all the pore spaces by cement - a pore-filling cement protects the grains against compaction and pressure solution.

Stylolites

in more cemented rocks, pressure solution takes place along much more extensive surfaces - these are stylolites. Dissolution often starts along a bedding surface, due to overburden pressure. A burial depth of at least several tens of metres is required for stylolite formation.

• Stylolites cross the whole rock, cutting grains, matrix and cement.
• They often have a zigzag form, due to differences in solubility of the different components, e.g. skeletal fragments may be less soluble than micrite matrix.
• The less soluble parts form 3-dimensional 'fingers', extending into the more soluble parts.
• Amplitude of stylolites may be from <1mm to >1m. The amplitude shows the minimum thickness of dissolved material.
• Insoluble material becomes concentrated along the stylolite surface - often clay & hydrocarbons. Thus, the stylolite surface may be an important permeability barrier, affecting the flow in aquifers and petroleum reservoirs.
• Major effects: stylolites commonly result in up to 35% reduction of thickness of a sequence.
• In areas of tectonic compression, vertical stylolites can contribute to considerable lateral shortening.
• Where does the dissolved calcium carbonate go? Ions may migrate along the solution film, or into pore spaces. The supply of CaCO₃ from pressure solution along stylolites is thought to be a major source of late diagenetic cement.

Dolomitization

• Most dolomite is of replacement origin:
• It replaces existing calcium carbonate
• Dolomite often destroys and cuts across the original textures of the carbonate sediment.
• Replacement may be complete or partial: if partial, dolomite may form isolated grains or patches in zones of greater permeability.

This page was written by Roger Suthren

Last Modified: 24 April, 2015 21:10