Hydrocarbon Generation and Hydrochemical Basin Zonation from Dolomite Cement in Cambrian Sandstones

摘要

The conditions and timing of carbonate cementation in Cambrian sandstones in the Baltic Basin were constrained by stable oxygen and carbon isotope and chemical data in combination with optical and cathodoluminescence petrographic studies. Studied samples represent a range in present burial depth 340-2150 m. The carbonate cement is dominantly ferroan dolomite that occurs as dispersed patches of poikilotopic crystals. Temperatures of dolomite precipitation, based on 18O values, range from 27oC in the shallow to 82oC in the deep buried sandstones, indicating that temperature is not a major factor in dolomite cementation. The burial history modelling points to development of most of the dolomite cement during rapid Silurian-Devonian subsidence of the Cambrian succession, partly concurrent with quartz cementation. Dolomite precipitation is related to early stages of organic matter maturation and thus to the oil generation history. 13C varies from +0.03‰ to -6.24‰ (PDB), suggesting limited carbon addition from an organic source with major part of ions from marine bicarbonate. The dolomite Fe content increases towards the deeper part of the basin and finally ankerite developed. The chemical composition of the dolomite cement shows a depth-controlled trend that is concurrent to the present day hydrochemical zonation of the Cambrian aquifer.