Influencing factors and fracability of lacustrine shale oil reservoirs
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Lithology
Carbonate minerals
Dolostone
Carbonate minerals
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ABSTRACT The mineralogy and isotope geochemistry of carbonate minerals in the Coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water. The different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral. In addition, the dolomite was not formed by simple solid state cation exchange. The occurrence of two types of dolomite was shown by isotope analysis and SEM observations. The dolomite, which is isotopically light (δ 13 C = ‐1 to ‐2% 0 ; δ 18 O=+3 to +5% 0 ) and of fine grain size (˜ 0·5 μm) probably precipitated under the influence of evaporitically modified continental water. Coarser grained dolomite (up to 4 μm) is isotopically heavier (δ 13 C=+3 to +4% 0 ; δ 18 O=+5 to + 6% 0 ) contains Mg in excess of Ca and was formed in or close to equilibrium with atmospheric CO 2 probably by the dolomitization of aragonite.
Dolomitization
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ABSTRACT The “dolomite problem” is the product of two distinct observations. First, there are massive amounts of ancient marine limestone (CaCO3) deposits that have been replaced by the mineral dolomite (MgCa(CO3)2). However, recent (Holocene and Pleistocene) marine deposits contain relatively minuscule amounts of dolomite, although the occurrence of small quantities of dolomite is observed in many modern settings, from deep marine to supratidal. Second, low-temperature synthesis of dolomite in laboratory settings has been elusive, particularly in comparison to the ease with which common marine calcium carbonate minerals (aragonite and calcite) can be synthesized. Since low-temperature solid-state diffusion can be discounted as a method for Mg incorporation into calcium carbonate (as it operates on time scales too long to matter), the replacement of CaCO3 by dolomite is one of dissolution followed by precipitation. Therefore, an often overlooked but required factor in the replacement of limestone by dolomite is that of undersaturation regarding the original calcium carbonate mineral during replacement. Such conditions could conceivably be caused by rapid dolomite growth relative to aragonite and calcite dissolution–precipitation reactions, but laboratory studies, modern systems analyses, and observations of ancient deposits all point to this possibility being uncommon because dolomite growth is kinetically inhibited at low temperature. Pressure solution by force of dolomite crystallization is a second possible driver for CaCO3 undersaturation, but requires a confining stress most likely attained through burial. However, based on petrographic observations, significant amounts of ancient dolomite replaced limestone before burial (synsedimentary dolomite), and many such platforms have not suffered any significant burial. Because these possibilities of undersaturation caused by dolomite precipitation and crystal growth can be largely discounted, the undersaturation required for “dolomitization” to proceed is most likely to be externally forced. In modern natural systems, undersaturation and selective CaCO3 dissolution in marine porewaters is very common, even in warm-water environments, being forced by the breakdown of organic matter. Such dissolution is frequently attended, to varying degrees, by precipitation of a kinetically-less-favored but thermodynamically more stable phase of CaCO3. Laboratory studies as well as observations of modern systems show that when undersaturation is reached with respect to all common marine CaCO3 phases, dolomite assumes the role of this kinetically-less-favored precipitate. This degree of undersaturation is uncommon in modern shallow marine pore systems in warm-water settings, but it was more common during times of elevated atmospheric CO2, and ocean acidification. Furthermore, because oxidation of organic matter drives dolomite formation, near-surface organic-rich deposits such as the remains of microbial mat communities, were more predisposed to dolomite replacement in the acidified oceans of the ancient past relative to contemporaneous deposits that contained less organic matter. These observations lend to a more harmonious explanation for the abundance and occurrence of dolomite through time.
Carbonate minerals
Dolomitization
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Cementation (geology)
Thin section
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Contact karst is a type of karst formed where allogenic waters from the surface influence the karst geomorphic system. Contact karst may be considered in both a strict sense and in a wide sense. In a strict sense, contact karst is the karst phenomena and forms influenced by the contact between a karstifiable rock and a non-karstifiable rock. In a wide sense, contact karst may also be the karst phenomena and forms influenced by the contact between two different karstifiable rocks, for example limestone and dolomite. This thesis focuses on the geomorphological characteristics of contact karst on limestone-dolomite contacts in Slovenia. The purpose of the research was to determine which processes contribute to the development of contact karst on the contact between limestone and dolomite, to define their dynamics, and to identify which surface and underground landforms are developed.
The spatial distribution of contacts between limestone and dolomite in Slovenia was determined in a GIS. Using existing lithological data as a data layer, the extent of carbonate rock cover in Slovenia was calculated. Carbonate rocks cover 47 % of Slovenia’s territory (27 % limestone, 14 % dolomite, and 6 % clastic carbonate or impure carbonate rocks). And, there are 1,353 limestone-dolomite contact lines in the country, totalling a length of 2,625 km.
Study areas were selected based on GIS analysis of the limestone-dolomite contacts. A total of 17 areas in Slovenia were studied in detail. Fieldwork at the study areas consisted of the collection and analysis of rock, sediment, and water samples, allowing each study area to be geomorphologically mapped.
General factors contributing to contact karst development on the lithological contact between limestone and dolomite were determined. The most important factor appears to be the characteristics of the inflow part, formed on the dolomite. Where dolomite functions as a karst rock, the water is dispersedly drained into the karst. In that case, the limestone-dolomite contact does not function as contact karst. Alternatively, where the dolomite functions as fluviokarst, a point recharge, or sinking stream, is formed. In that case, contact karst may be formed. The fluviokarstic character of the dolomite depends on its chemical and mechanical properties. The dolomite bedrock must be positioned at a higher elevation than the neighbouring limestone bedrock. To meet this requirement, dolomite beds, which in Slovenia are generally older than limestone and hence stratigraphically positioned below the limestone beds, need to be positioned above limestone by either folding that leads to inverse stratification, overthrusting, or by displacement along faults. Along faults, the dolomite is more prone to mechanical weathering due to tectonic crushing in addition to its chemical properties. Hence, contact karst is more likely to form at thrust contacts between thrust limestone and dolomite. Limestone-dolomite contact karst develops predominately at higher elevations due to increased precipitation (where allogenic inflow is higher) and greater frost action due to lower temperatures. Intense mechanical weathering of dolomite over limestone directly affects contact karst processes and significantly contributes to the spatial distribution of these types of surfaces. The location of the water table close to the surface is also a leading factor in limestone-dolomite contact karst formation due to enhanced border corrosion.
Landforms typical of contact karst were identified in the study areas during geomorphological analyses. However, they are not as clearly recognizable as those on contact between carbonate and non-carbonate rocks. The reason for this is the fact that allogenic waters from dolomitic catchment areas are by far not as corrosive as those from non-carbonate catchment areas.
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