Chloritization of mixed siliciclastic-carbonate rocks: case study from Gydan Peninsula, Russia

The chlorite group of minerals (chlorites) are well-known phyllosilicates, which have been described from magmatic, metamorphic and sedimentary rocks. Chlorites often appear in the cement of sandstones and can affect their reservoir properties. Here, we present the results of a study on the distribution and composition of chloritized mixed siliciclastic-carbonate rocks from the Middle Ordovician of the Gydan Peninsula, northern Siberia, Russia. This study is based on the first deep well in the area to penetrate the entire 4500-meter-thick sedimentary succession. A number of different carbonate rock textures are found in this area, including rudstone-floatstone (composed almost entirely of calcite), wackestone and packstone (composed of calcite bioclasts and clayey micrite matrix), marl (containing calcite bioclasts, dolomite / ankerite, illite / muscovite, quartz, feldspar and chlorites), and dolomitic marl (composed of illite / muscovite and dolomite / ankerite). Chlorite is represented by chamosite and contains up to 33.5 wt% FeO and up to 0.7 wt% TiO₂. Chlorite content positively correlates with illite / muscovite content, reaching a maximum in marls and dolomitic marls (up to 13 vol%). Chlorite crystals are micron-sized and appear as authigenic grains, where they can be distributed in the matrix or form secondary rims around calcite bioclasts and dolomite / ankerite grains. The two main processes leading to chloritization in the studied rocks include illite / muscovite replacement by chlorite and a reaction between illite / muscovite and dolomite / ankerite resulted in chlorite crystallization. Paleotemperatures during chlorite crystallization reached as high as 295–318 °C. Chloritization of the studied carbonate rocks resulted in a porosity reduction, as chlorite rims filled micropores at the contacts with the siliciclastic matrix.