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ALIZARINE RED S TEST AND IMAGE ANALYSIS AS DOLOMITIZATION DEGREE INDICATORS FOR ZECHSTEIN LIMESTONE CARBONATES

Adam Fheed

Abstract

Carbonate rocks, bearifng the majority of world?s hydrocarbon reserves often exhibit variable lithology. In carbonates, different proportions of calcite and dolomite, accompanied by other minerals can be expected. Dolomitization of carbonate rocks usually leads to porosity enhancement, since dolomite crystals occupy smaller volume in comparison to calcite. This paper considers the integration of Alizarine Red S staining method and image analysis in order to qualitatively state the content of both calcite and dolomite. The studied material consists of the Zechstein Limestone (Ca1) carbonate reef rocks from West Poland. Based on core description and polarized-light microscopy, six samples representing different proportions of carbonate minerals were chosen. Chemical solution was prepared by dissolving 0.1 g of Alizarin Red S in 100 ml of 1.5 % hydrochloric acid. In order to digitize the obtained results, the rocks were scanned prior to and after the staining procedure. The natural rock?s color treated as a background was subtracted from the greyscale images of the stained samples to give a differential color value. The obtained values were compared with the corresponding X-ray diffraction-derived mineral composition of the samples studied. It was concluded that there is a general relation between the observed color values and the apparent mineralogy of the samples. The rocks become increasingly limy as one approaches the pale grey color. The greyscale values oscillating around 100 denote almost pure limestones, whereas the values falling below 50 are related either to pure or limy dolomites. However, the natural color of matrix seems to significantly constrict the procedure. Therefore the approach is only valid for the rocks of similar matrix colors. The impact of the matrix color on final results will be the subject of further research on greater sample population.

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DOI resolver: doi.org/10.5593/sgem2018/1.4/s06.030

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