Ferroan dolomite cement in Cambrian sandstones: burial history and hydrocarbon generation of the Baltic sedimentary basin

The conditions and timing of carbonate cementation in Cambrian sandstones of the Baltic sedimentary basin were determined by oxygen and carbon stable isotope and chemical data in combination with optical and cathodoluminescence petrographic studies. Studied samples represent a range in present burial depth from 340 to 2150 m.

The carbonate cement is dominantly ferroan dolomite that occurs as dispersed patches of poikilotopic crystals. Temperatures of dolomite precipitation, based on delta O-18 values, range from 27 degrees C in the shallow buried to 95 degrees C in the deep buried sandstones. The burial history modelling points to development of most of the dolomite cement during rapid Silurian-Devonian subsidence and Carboniferous-early Permian uplift.

A wide range of precipitation temperatures indicate that temperature was not a major factor in triggering the carbonate cementation. Dolomite precipitation is related to early stages of organic matter maturation and thus to the oil generation history in the basin. delta C-13 values vary from +0.03% to -6.2%( PDB), suggesting limited addition of carbon from an organic source, with the major part derived from marine bicarbonate.

The sourcing of carbon from the organic-rich Cambrian shales is identified from the distribution of delta C-13 values in the dolomite cement within the Cambrian section. The chemical composition of the dolomite cement shows a depth-controlled trend that is coincident with the present-day hydrochemical zonation of the Cambrian aquifer.

The increase in the Fe content of the dolomite towards the deeper buried part of the Baltic basin is related to increasing sourcing of ions ( Fe and Mg) from adjacent shales.