Lithologic Determination of Ordovician Carbonate Rocks from South Estonian Boreholes by Physical Parameters

Ordovician carbonate succession from southern Estonia is mainly represented by primary limestones and calcitic marlstones and by dolomitized carbonate rocks (dolostones and dolomitic marlstones). For case study 110 core samples from two boreholes were analysed by geochemical and petrophysical methods. Rocks were initially subdivided into groups using data of carbonate chemical analysis. It is shown that primary and dolomitized rocks could be lithologically discriminated using porosity-density cross-plot. Gamma-ray log permits to determine thin reference layers of metabentonites and claystones, and to determine clayey rocks (marlstones) from more pure carbonates. Ordovician dolostones are characterized by increase in magnetic susceptibility the most significant in the late diagenetic dolostones. Their magnetic susceptibility may be higher than in marlstones. Magnetic susceptibility is offered to apply for dolostone genesis determination together with density, porosity and gamma-ray logs, and for express determination of dolostones in drill cores and outcrops.