RECRYSTALLIZATION OF LIMESTONE TO DOLOMITE

Presented by: Zohreh BaghbanProfessor: Dr. Shafiee10/30/2014

Content

Recrystallization vs. dolomitization

Carbonate rocks

Limestone

Dolostone

Limestone vs. dolomite

Porosity and permeability

References

RECRYSTALLIZATION VS. DOLOMITIZATION

Recrystallization vs. dolomitization recrystallization: rearrangement of crystal mass or crystal defects (including grain

boundaries)  If accompanied by deformation, recrystallization can produce a crystallographic

preferred orientation, lineation, foliation, and porphyroclasts. If no phases appear or disappear during the recrystallization, only grain growth is

occurring; this coarsening is driven by the free energy of grain boundaries, which is the energy resulting from two crystals being in contact along an imperfect boundary.

Dolomite is very common in the rock record but the mineral dolomite is rarely observed forming in sedimentary environments. For this reason it is believed that most dolomites form when lime muds or limestones are modified by postdepositional chemical change. 

Recrystallization vs. dolomitization

Two mechanisms of dolomitization of limestones have been proposed based on field and laboratory studies.

Evaporative Reflux. This mechanism involves the evaporation of seawater to form a brine that precipitates gypsum. After precipitation of gypsum, the brine is both enriched in Mg relative to Ca and has a higher density. If the brine then enters the groundwater system and moves downward into buried limestones. This Mg-rich brine then reacts with the calcite in the limestone to produce dolomite. 

Mixing of Seawater and Meteoric Water.  This mechanism involves the mixing of groundwater derived from the surface with saline groundwater beneath the oceans. Dolomitization is thought to occur where the two groundwater compositions mix with each in the porous and permeable limestone within a few meters of the surface.

The dolomitization process results in a slight volume reduction when limestone is converted into dolomite. 

Dolomite is thought to form when the calcite (CaCO2) in carbonate mud or limestone is modified by magnesium-rich groundwater. The available magnesium facilitates the conversion of calcite into dolomite (CaMg(CO2)3). This chemical change is known as “dolomitization.”

Recrystallization

A specimen of coarsely crystalline dolomitic marble from Thornwood, New York. This specimen is approximately 3 inches (6.7 centimeters) across.

CARBONATE ROCKS

Carbonate Rocks

The carbonate rocks make up 10 to 15% of sedimentary rocks.  They largely consist of two types of rocks.

Limestones which are composed mostly of calcite (CaCO3) or high Mg calcite [(Ca,Mg)CO3], and 

Dolostones which are composed mostly of dolomite [CaMg(CO3)2]

Because carbonate minerals in general are soluble in slightly acidic waters, they often have high porosity and permeability, making them ideal reservoirs for petroleum. 

LIMESTONE

Photomicrograph showing skeletal oolitic limestone, with clean calcite cement, from the Lower Triassic Period (magnified 18×). Courtesy of A. Bosellini

Limestone

Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite.

Limestones are for the most part primary carbonate rocks. 

They consist of 50 percent or more calcite and aragonite (both CaCO3). 

It most commonly forms in clear, warm, shallow marine waters.

 Limestone is forming in the Caribbean Sea, Indian Ocean, Persian Gulf, Gulf of Mexico, around Pacific Ocean islands and within the Indonesian archipelago. 

The limestone that makes up these cave formations is known as "travertine" and is a chemical sedimentary rock. A rock known as "tufa" is a limestone formed by evaporation at a hot spring, lake shore, or other area. 

All limestones contain at least a few percent other materials. These can be small particles of quartz, feldspar, clay minerals, pyrite, siderite and other minerals.

DOLOSTONE

Photomicrograph showing pisolitic dolomite from the Upper Triassic Period (magnified 5×). Courtesy of A. Bosellini

Dolostone

Dolostones are carbonate rocks composed almost entirely of dolomite - (Ca,Mg)CO3.  

Dolomites are mainly produced by the secondary alteration or replacement of limestones; i.e., the mineral dolomite [CaMg(CO3)2] replaces the calcite and aragonite minerals in limestones during diagenesis. 

Almost all dolomites are believed to be produced by recrystallization of preexisting limestones, although the exact details of this dolomitization process continue to be debated.

Most dolostones appear to result from diagenetic conversion of calcite or high-Mg calcite to dolomite, after primary deposition of the original calcium carbonate bearing minerals. 

Dolomite originates in the same sedimentary environments as limestone - warm, shallow, marine environments where calcium carbonate mud accumulates in the form of shell debris, fecal material, coral fragments and carbonate precipitates.

LIMESTONE VS. DOLOSTONE

Limestone vs. dolostone

Limestone can be easily recognized in hand specimen or outcrop because of its high solubility in HCl.

whereas limestones tend to weather to a white or gray colored rock. 

A dolostone, on the other hand, will not fizz until a fine powder is made from the rock or mineral.

Also, dolostones tend to weather to a brownish color rock.

The brown color of dolostones is due to the fact that Fe occurs in small amounts replacing some of the Mg in dolomite.

Dolomite is slightly harder than limestone.

POROSITY AND PERMEABILITY

Porosity and permeability

The dolomitization process results in a slight volume reduction when limestone is converted into dolomite.

This can produce a porosity zone in the strata where dolomitization has occurred.

These pore spaces can be traps for subsurface fluids like oil and natural gas. 

This is why dolomite is often a reservoir rock that is sought in the exploration for oil and natural gas.

Dolomite can also serve as a host rock for lead, zinc and copper deposits. 

Porosity and permeability in depth

Carbonate Porosity Versus Depth: A Predictable Relation for South Florida

James W. Schmoker and Robert B. Halley

This study examines the porosity of limestones and dolomites in the South Florida basin.

Two data subsets with carbonate compositions of 75 to 100% limestone (489 intervals) and 75 to 100% dolomite (336 intervals) were derived from the original data set.

Dolomite porosity is lower than limestone porosity in the near surface, but does not decrease as rapidly with depth.

 The figure reinforces the widely held supposition, based on practical experience and supported by the fact that dolomites account for about 80% of North American carbonate reservoirs (Zenger et al, 1980, p. iii), that dolomite is commonly a better reservoir rock than limestone.

Porosity and permeability

Frequency cross-plot of neutron and density log responses from zones in the Viola of the Belcher A-1 well.

Porosity and permeability

Continuum of rock fabrics and associated porosity-permeability transforms. (A) Rock-fabric numbers ranging from 0.5 - 4 defined by class-average and class-boundary porosity-permeability transforms. (B) Fabric continuum in nonvuggy limestone. (C) Fabric continuum in nonvuggy dolostone.

Porosity and permeability

Schematic drawings of dolomite crystal growth and porosity evolution through time. (A) Initial dolomitization of limestone would result in the most porous dolomite. Porosity is decreased as additional dolomite was precipitated on existing rhombs (B), then (C).

REFERENCES

References

http://www.britannica.com/EBchecked/topic/532232/sedimentary-rock/80283/Shales-of-economic-value

http://www.tulane.edu/~sanelson/eens212/carbonates.htm

http://geology.com/rocks/dolomite.shtml

http://geology.com/rocks/limestone.shtml

http://www.geol.ucsb.edu/faculty/hacker/geo102C/lectures/part2.html

http://sofia.usgs.gov/publications/papers/carb_porosity/index.html U.S. Geological Survey, 600 Fourth Street South, St. Petersburg, FL 33701.  R. P. Steinen, 1980, Mississippian non-supratidal dolomite, Ste. Genevieve Limestone, Illinois basin: Evidence for mixed-

water dolomitization, in D. H. Zenger, J. B. Dunham, and R. L. Ethington, eds., Concepts and models of dolomitization: SEPM Spec. Pub. 28, p. 163-196.

Zenger, D. H., J. B. Dunham, and R. L. Ethington, eds., 1980, Concepts and models of dolomitization: SEPM Spec. Publ. 28, 320 p.

http://www.kgs.ku.edu/Publications/Bulletins/PS3/

http://www.beg.utexas.edu/lmod/_IOL-CM07/old-4.29.03/cm07-step04.htm

http://aapgbull.geoscienceworld.org/content/85/3/530/F2.expansion.html#F1