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Heat‐model analysis of wall rocks below a diabase sill in Huimin Sag, China compared with thermal alteration of mudstone to carbargilite and hornfels and with increase of vitrinite reflectance

This paper presents an application of heat flow modeling in a study on the thermal alteration of the underlying sedimentary rocks caused by an isolated intrusive sill in Huimin Sag, Bohai Bay Basin, China. It is found that during the cooling of the sill, the assignment of the heat transferred into its both sides can be acquired according to the ratio of the thickness of the overlying and underlying thermally altered rocks. Heat‐transfer modeling indicates that the transition temperatures for the alteration from calcareous mudstone to carbargilite and successively to hornfels are identified as 245 ± 9°C and as 449 ± 19°C, respectively. The peak temperature calculated from vitrinite reflectance higher than 3.0% of the altered wall rocks is generally lower than expected values. Especially in the hornfels zone, vitrinite reflectance ever decreased with increased temperature. The thermal alteration of mudstone is possibly an important factor causing such a decrease.