The brachiopod shells are the pink areas. The pink color is due to a chemical stain that was applied to the rock to help identify the mineral calcite. When brachiopod shells form, they have an intricate internal structure.
Maximum diagenesis associated with fresh water occurs at the Rock -air interface and in the zone of mixing between fresh water and seawater or fresh water and brines figure below. Cements include calcite and possibly dolomite. Where pores are incompletely filled, the cement is confined to points of contact between grains or to the undersides of grains. This asymmetry in the cements does not occur below the air-water interface. Below the water table, calcite cements commonly form rims of bladed to equant crystals that completely encircle the grains. The diagenesis associated with fresh water includes the inversion of aragonite to calcite , the loss of magnesium from magnesium calcite and dolomitization left figure.
Aragonite and magnesium calcite , both minerals common to the marine environments, are unstable in magnesium-deficient waters regardless of whether these waters are fresh, brackish or saline. These minerals change to calcite by leaching and reprecipitation. In some cases the original mineral fabrics are completely preserved and in others they are completely destroyed.
The aragonite to calcite trans formation may cause partial plugging of the existing porosity by precipitation of calcite cement. In carbonate s that are tightly cemented a source of the cement other than the trans formation of aragonite to calcite is required. Most probably, the additional carbonate is derived from the weathering and dissolution of adjacent carbonate s. Cements formed in subsurface brines are commonly equant iron-rich calcite spar with crystals that have flat faces. If magnesium to calcium ratios are high, particularly in mixing zones of brines with different compositions, dolomitization may locally be important.
Although the deep burial diagenetic realm is not as well understood as near-surface conditions, it is apparent that pressure solution compaction, cementation, and dolomitization may occur. The subsurface fluids responsible for the diagenesis can be derived from a variety of sources figure above , but a likely source is the down-dip basin al shale s and fine carbonate s that expel fluids as they are compacted during burial.
Compaction in carbonate s can cause significant restructuring within the Rock. In grain-rich Rock s, the grains may be flattened, broken or dissolved at grain contacts right figure. Pre-burial cements may be similarly affected, resulting in a change of the porosity and permeability patterns in the Rock. Stylolites and other pressure solution features are commonly formed during burial or tectonic stress of mudstone s and wackestones.
The formation of such features is important because vertical permeability patterns are created and pore fluids are displaced. Because of the broad-spectrum of diagenesis that affects carbonate Rock s, the final porosity in carbonate s may or may not be related to depositional environment.
Unlike other lithologies, the original primary porosity in carbonate s may be totally destroyed during diagenesis and significant new secondary porosity may be created. The types of porosities encountered are quite varied figure below. Interparticle, intraparticle, growth-framework, shelter and fenestral porosities are depositional porosities.
More illustrations of porosity can be reached at a galllery of carbonate porosity that can be reached by clicking on the highlighted text.
Depositional porosity is a function of Rock texture, grain sorting and shape. Sorting and shape in turn are related to bottom agitation at the depositional site. Where currents and waves are particularly active, lime mud is winnowed from carbonate sands.
In contrast, lime muds tend to collect in less agitated environments or where trapped by organisms, and after the sediment dewaters little or no porosity is retained. Many of the world's larger carbonate reservoirs have porosities that are largely depositional in origin.
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The relationship between porosity and diagenesis is complex and variable. The major diagenetic processes affecting porosity are dissolution, cementation and dolomitization. Each process requires a permeable host Rock and a mechanism to flush chemically active waters through the Rock. The water movement is controlled regionally by the hydrostatic head, structure and Rock fabric.
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Dissolution creates and enhances porosity. Commonly, however, dissolution of carbonate grains is accompanied by calcite cementation in adjacent primary pores. The end product in such a case is tightly cemented carbonate sand with well-developed moldic porosity and low associated permeability. Cementation is an extremely important diagenetic process because it reduces porosity. The degree of cementation varies from thin cement coatings around the grains that partially fill the pores and alter permeability patterns to calcite spar that completely fill the pores.
Dolomitization may reduce, redistribute, preserve or create porosity. In a few carbonate reservoirs, as in the Jurassic Arab limestone s of Ghawar field in Saudi Arabia, replacement dolomite crystals extend into adjacent pores thereby reducing the primary porosity. In many dolomitized reservoirs such as the Jurassic Smackover formation of Alabama and the Leduc reef carbonate s in Alberta, porosity and permeability were redistributed during dolomitization and associated leaching.
Early dolomitization may preserve porosity by creating a rigid framework that inhibits compaction.
In still other cases dolomitization in lime muds may enhance porosity , because dolomite s are denser and so consequently take up less volume than the original calcite. Perhaps burial was rapid and the carbonate s underwent only brief diagenesis in near-surface settings left figure. Another possibility is that evaporites , shale s, red beds or dense micrite s formed a protective impermeableseal over the porous carbonate s and prevented fresh-water flushing. The same near-surface waters that are responsible for cementation of carbonate sequence s may also produce secondary porosity during dissolution.
If the right balance is met in the near surface between precipitation and dissolution, an attractive reservoir Rock with good porosity and associated permeability can form. Deep burial processes such as cementation and grain to grain interpenetration by physical compaction or pressure solution may be very important in porosity reduction. Hurley, and P. Cercone and K. Purser and J.
Diagenesis and Porosity - SEPM Strata
Riff Diagenesis. Sedimentation and deposition. Diagenese Sediment Diagenese. Sedimentary rocks. Diagenesis Reefs Sedimentation and deposition Diagenesis. Purser, Bruce H. Related item. Online version:: Reef diagenesis. Internet Resources. Back to results. Aberystwyth University Library.