Carbonate cement dissolution

The scales of carbonate redistribution, and thus reservoir quality enhancement, are difficult to constrain. Several workers have argued that the reservoir properties of sandstones are greatly enhanced due to large-scale carbonate dissolution (Lonoy et al., 1986 Schmidt McDonald, 1979). As the undersaturated waters have to circulate through large volumes of permeable sediment to cause economically important carbonate cement dissolution, it is expected that such secondary porosity develops in partially rather than pervasively cemented sand-... 

The best reservoir quality potential expected for the Serraria Formation is in structural blocks in the distal and middle domains affected by porosity enhancement through extensive feldspar and carbonate cement dissolution in connection with the post-rift exposure and telogenetic infl ux of meteoric waters. 

Carbonate cement dissolution during a cyclic CO2 enhanced oil recovery treatment... 

Carbonate cement dissolution caused by cyclic C02-enhanced oil recovery treatments can provide information in several areas. From this study, which examined mineral dissolution, well-bore scale and reservoir heterogeneity/geometry, the following conclusions are drawn. 

Further conversion of mixed-fay-er clays towards illite authigenesis. Authigenic kaolinite precipitation. Authigenic chlorite development. Mixed-layer s ordering corrensite,allevardite. Feldspar dissolution.Significant quartz overgrowths development, chert dissolution. Carbonate cement dissolution/predpi-tation. Some secondary porosity development. 

Table 1. Observed sequence of carbonate cementation/dissolution events in clastic units during continuous burial, and suggested causes and effects...

Systematic Cycling of Carbonate Cementation/Dissolution Events... 

One of the most important aspects of carbonate diagenesis is the net movement of carbonate minerals. This mass transfer can be accomplished on a small scale by diffusive transport or on a large scale by the flow of subsurface waters. It is the basic process by which secondary porosity is created and cementation occurs. In most cases, it involves carbonate mineral dissolution at one site and precipitation at another. While this can be simply accomplished where mineralogic transformations from a metastable phase to a more stable phase are involved, more complex mechanisms may be required within mineralogically homogeneous carbonate bodies. 

Secondary porosity results from the dissolution of carbonates in the subsurface environment. It can occur both in limestones and in sandstones where carbonate cements of original labile detrital minerals are dissolved. Because the formation of secondary porosity can substantially enhance the reservoir properties of sediments, it has received considerable attention from the petroleum industry. 

Some of the basic processes in the formation of secondary porosity are similar to those for formation of carbonate cements. A solution of proper composition must be generated by subsurface processes, and this solution must also flow through the formation in which the dissolution reaction takes place in sufficient quantities to transport the dissolved carbonate. The primary differences between cement and secondary porosity formation are that an undersaturated solution must be generated rather than a supersaturated solution, and that while cement formation reduces porosity and can inhibit flow, formation of secondary porosity increases porosity and can result in enhanced flow of subsurface fluids. 

Walter L.M. (1985) Relative reactivity of skeletal carbonates during dissolution implications for diagenesis. In Carbonate Cements (eds. N. Schneidermann and P.M. Harris), pp. 3-16. Soc. Econ. Paleontologist and Mineralogists, Spec. Pub. 2. Tulsa, OK. 

During deeper burial of newly deposited carbonate sediments, the primary and secondary porosity is decreased by cementation and chemical compaction. At these deeper hurial depths pressure solution causes the sedimentary grains to dissolve and cement, and stylolites to form. Stylolites may start to form at depths of 1 to 2 km (Bjorlykke, 1989). Early formed carbonate cement may hamper later pressure solution, i.e. carbonate sediments which have been subject to relatively early cementation may retain their remaining porosity better with depth (Bj0rlykke, 1989). Aqueous dissolution of carbonates may also create secondary porosity in carbonate rocks at deeper burial. The complex evolution of porosity in carbonate sediments and rocks is reflected in the extreme lateral and vertical heterogeneity of carbonate rocks (Mazzullo and Chilingarian, 1992). 

At shallow depths carbonate cements may cause sands to become brittle and hard. Carbonate which precipitates on the sea floor may also form hard grounds in dominantly clastic sequences. Sandstones may become calcite cemented due to dissolution of biogenic aragonite at relatively shallow depth (less than a few hundred meters). Calcareous sediments flushed by meteoric water at shallow depth or exposed during regression may become rapidly ce-... 

The faulted, non-carbonate cemented sandstones which were sampled from Haltenbanken and Tampen Spur, show clear evidence of diagenetic modifications after the deformation had occurred. Feldspar dissolution, illite precipitation and stylolitization are examples of such diagenetic processes. The pre- and post-faulting diagenetic reactions have been demonstrated to be a useful tool for the purpose of dating fault movements relative to basin subsidence (Sverdrup and Bjprlykke, 1992 Saigal et al., 1995). 

When carbonate cements are subjected to physicochemical conditions that vary considerably from those under which they formed, they may dissolve and re-precipitate at various scales. Carbonate dissolution and the creation of secondary porosity may occur during eodiagenesis or telodiagenesis or in response to progressive burial. Eogenetic secondary pores may survive subsequent burial and compaction in sandstones that have been subjected to early overpressuring or hydrocarbon emplacement, or if dissolution is incomplete, and leave evenly distributed remnants of carbonate cement. 

In addition to dissolution, the destabilization of carbonate cements may result in recrystallization and replacement by other carbonates. Microcrystalline calcite and dolomite are sensitive to recrystallization at various burial depths. The recrystallization of dolomite has been reviewed by Mazzullo (1992). Burial recrystallization of micritic/microsparitic cements in sandstones may result in the formation of poikilotopic calcite (Saigal Bjorlykke, 1987). However, poikilotopic calcite is also a common primary cement in calcretes (e.g. Knox, 1977 Tan-don Narayan, 1981). Recrystallized calcite and dolomite are recognized as patchily distributed, coarsened crystals. In contrast, precipitational vari-... 

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