Changes

===Environment of deposition===

===Environment of deposition===

The initial pore network of newly deposited sediments and the quality of shallow buried reservoirs are generally determined by the environment of deposition (see [[Lithofacies and environmental analysis of clastic depositional systems]]). This dictates the grain characteristics, which in turn control porosity and permeability. In clastic rocks, these characteristics include grain size and [[Core_description#Maturity|sorting]], sphericity, angularity, packing, and the abundance of matrix materials. The best reservoir quality rocks are well-sorted, have well-rounded grains, and contain no matrix material.

The initial pore network of newly deposited sediments and the quality of shallow buried reservoirs are generally determined by the environment of deposition (see [[Lithofacies and environmental analysis of clastic depositional systems]]). This dictates the grain characteristics, which in turn control porosity and permeability. In clastic rocks, these characteristics include [[grain size]] and [[Core_description#Maturity|sorting]], sphericity, angularity, packing, and the abundance of matrix materials. The best reservoir quality rocks are well-sorted, have well-rounded grains, and contain no matrix material.

Sedimentary structures affect initial reservoir quality by imparting a preferential flow pattern in the reservoir. Planar bedding, laminations, or other stratification features can create stratified planar flow, especially if permeability barriers such as clay partings, finer-grained laminae, or graded beds are present. Slump structures may reduce permeability by creating a tortuous flow path, or may increase permeability (and porosity) by causing a looser grain packing and by producing small faults. [[Bioturbation]] typically decreases reservoir quality by mixing adjacent sands and clays, introducing the clay into the interstices among the sand grains.

Sedimentary structures affect initial reservoir quality by imparting a preferential flow pattern in the reservoir. Planar bedding, laminations, or other stratification features can create stratified planar flow, especially if permeability barriers such as clay partings, finer-grained laminae, or graded beds are present. Slump structures may reduce permeability by creating a tortuous flow path, or may increase permeability (and porosity) by causing a looser grain packing and by producing small faults. [[Bioturbation]] typically decreases reservoir quality by mixing adjacent sands and clays, introducing the clay into the interstices among the sand grains.

===Compaction===

===Compaction===

Compaction reduces the porosity and permeability of a rock by causing the following: (1) grain rotation and rearrangement into a tighter packing configuration, (2) plastic [[deformation]] of [[Ductility|ductile]] grains that flow into adjacent pores and pore throats, (3) [[Fracture|fracturing]] and crushing of [[Brittleness|brittle]] grains, and (4) pressure solution in the form of grain suturing and stylolitization.<ref name=pt06r84>McBride, E. F., 1984, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0068/0004/0500/0505.htm Compaction in sandstones—influence on reservoir quality]: AAPG Bulletin, v. 68, p. 505.</ref> Rocks that contain mechanically labile grains, such as clay clasts, altered rock fragments, or delicate fossils, are likely to experience a reduction in porosity and permeability as the ductile grains plastically flow into adjacent pore spaces. Brittle grains will fracture, shatter, or in the case of some fossils and porous grains, collapse. A rock that consists of a framework of strong minerals, such as quartz, tends to undergo only minor porosity and permeability reduction during compaction due to grain rotation and rearrangement into a tighter packing configuration.

Compaction reduces the porosity and permeability of a rock by causing the following: (1) grain rotation and rearrangement into a tighter packing configuration, (2) plastic [[deformation]] of [[Ductility|ductile]] grains that flow into adjacent pores and pore throats, (3) [[Fracture|fracturing]] and crushing of [[Brittleness|brittle]] grains, and (4) pressure solution in the form of grain suturing and stylolitization.<ref name=pt06r84>McBride, E. F., 1984, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0068/0004/0500/0505.htm Compaction in sandstones—influence on reservoir quality]: AAPG Bulletin, v. 68, p. 505.</ref> Rocks that contain mechanically labile grains, such as clay clasts, altered rock fragments, or delicate fossils, are likely to experience a reduction in porosity and permeability as the ductile grains plastically flow into adjacent pore spaces. Brittle grains will fracture, shatter, or in the case of some fossils and porous grains, collapse. A rock that consists of a framework of strong minerals, such as [[quartz]], tends to undergo only minor porosity and permeability reduction during compaction due to grain rotation and rearrangement into a tighter packing configuration.

===Cementation===

===Cementation===

! Cement || Common Crystal Form

! Cement || Common Crystal Form

|-

|-

| Quartz || Syntaxial overgrowth, prismatic

| [[Quartz]] || Syntaxial overgrowth, prismatic

|-

|-

| Calcite || Fibrous, bladed, granular, blocky, poikilotopic, syntaxial rim

| Calcite || Fibrous, bladed, granular, blocky, poikilotopic, syntaxial rim

|-

|-

| Dolomite || Rhombohedral, blocky, granular

| [[Dolomite]] || Rhombohedral, blocky, granular

|-

|-

| Anhydrite || Blocky, bladed

| [[Anhydrite]] || Blocky, bladed

|-

|-

| Gypsum || Blocky, bladed, prismatic

| [[Gypsum]] || Blocky, bladed, prismatic

|-

|-

| Feldspar || Syntaxial overgrowth, prismatic

| Feldspar || Syntaxial overgrowth, prismatic

[[Category:Geological methods]]

[[Category:Geological methods]]