Calcite barrier

In reviewing reported values of E for calcite decompositions, Beruto and Searcy [121] find that most are close to the dissociation enthalpy. They suggest, as a possible explanation, that if product gas removal is not rapid and complete, readsorption of C02 on CaO may establish dissociation equilibria within the pores and channels of the layer of residual phase. The rate of gas diffusion across this barrier is modified accordingly and is not characteristic of the dissociation step at the interface. 

Although surfece waters are supersaturated with respect to calcium carbonate, abiogenic precipitation is imcommon, probably because of unfevorable kinetics. (The relatively rare formation of abiogenic calcite is discussed further in Chapter 18.) Marine organisms are able to overcome this kinetic barrier because they have enzymes that catalyze the precipitation reaction. Because fl declines with depth, organisms that deposit calcareous shells in deep waters, such as benthic foraminiferans, must expend more energy to create their hard parts as compared to surfece dwellers. 

Smith, R. W. Walfond, C. The Effects of Calcite Solid Solution Formation on the Transient Release of Radionuclides from Concrete Barriers. Scientific Basis for Nuclear Waste Management XIV, 1991, 212, 403—409. 

Near-surface seawater is typically supersaturated by over six times with respect to calcite, and over four times with respect to aragonite (see Chapter 4). When it flows over shallow areas of the world s oceans, its chemistry can be modified by several different processes. The extent of modification is strongly dependent on the residence time of the water over the shallow areas. Morphologic controls on flow, such as embayments, barrier islands, and reefs, can often lead to significant restriction of the flow of seawater. In areas where these restrictions occur, major changes in the chemistry of the seawater are usually observed. 

The growth of calcite crystals to form speleothems is a delicately balanced process depending on the degree of supersaturation of the water and its total concentration of dissolved carbonates. Waters dripping onto speleothems require supersaturations on the order of Sk = +0.5 in order to overcome nucleation barriers (where Sic is the saturation index defined in the textbooks cited above). However, the critical supersaturation for 2-dimensional nucleation and the continued growth of a single ciystal is only slightly... 

Although many biological fluids appear to be supersaturated with calcite, spontaneous mineralization does not occur. A number of explanations have been proposed, including the effects of crystal poisons and the high solubility of small crystal nuclei. An alternative approach to the problem is to suggest that there is an energy barrier which has to be overcome by the... 

Figure 11.7 Scanning electron microscopy images of marine beachrock and meteoric cayrock cements. (A) Blades of high-magnesium calcite on constituent particles in beachrock. Northeast Sapodilla Cay, southern Belize Barrier Reef. (B) Dissolution cavity is lined with blockyand bladed low-magnesium calcite in cayrock. Cay Bokel, Turneffe Islands, Belize. (C) Low-magnesium calcite needle fibre (whisker) cement in cayrock. Harry Jones Point, Turneffe Islands, Belize.

Calcite cementation in the Zia Formation has greatly reduced potential reservoir/aquifer quality. Most permeable units are extensively cemented with phreatic calcite. Many tabular units are often laterally extensive, forming significant barriers to vertical fluid flow and conceivably resulting in compartmentalization of the reservoir/aquifer. 

Calcite-cemented layers and lensoid concretions commonly form low-permeability barriers in shallow marine reservoir sandstones. In the porous and permeable Lower Jurassic Luxemburg Sandstone such calcite-cemented lenses form permeability barriers with lateral continuities of a few decimetres to hundreds of metres. Deposition of these sandstones ( 90 m thick) occurred in a wave- and storm-reworked tidal delta that formed where a seaway through the Ardennes and Rhenish Massifs entered the shallow Paris basin. 

Haszeldine et al., 1992 Macaulay et al., 1993). In a number of studies particular attention has been paid to extensive calcite-cemented intervals present in many oilfields of the northern North Sea, as they may constitute permeability barriers to fluid flow. 

Dutton Flanders examined the diagenesis and reservoir quality of the East Ford Field, Texas, which is undergoing a carbon dioxide flood. The compartmentalization of the arkosic sandstone reservoir is chiefly controlled by authi-genic calcite layers associated with the tops and bottoms of turbidite sandstone units. Gases produced from zones below low permeability calcite cemented sandstone intervals in new infill wells have high carbon dioxide contents. These, together with geophysical log interpretations, indicate that the calcite-cemented zones are laterally continuous and act as vertical barriers in the reservoir. 

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