Diagenesis to very low-grade metamorphism in lower Palaeozoic sediments: A case study from deep borehole Tobolka 1, the Barrandian Basin, Czech Republic

Abstract

The diagenetic grade of a 2.7km thick sequence of lower Palaeozoic marine sediments in the Tobolka 1 deep exploration borehole was investigated by analysing organic and mineral components and fluid inclusions embedded in vein minerals.The reflectance of organic matter (Rr %) increases down-section from values of about 0.77% at the top of the Devonian Srbsko Formation to values around 3% at the base of the Ordovician Letná Formation. In the Devonian and Silurian sediments the reflectance values of vitrinite-like and graptolite fragments (0.77–1.13% Rr and 1.09–1.46% Rr respectively) and the composition of fluid inclusions entrapped in vein minerals (CH4+CO2+complex saline solutions±light gaseous hydrocarbons±liquid petroleum; Th=170–230°C) collectively indicate organic maturation within the high-temperature part of the oil window to the wet gas stage. In the Ordovician sediments below 1000m, chitinozoan reflectance values (1.38–3.03% Rr) and the CH4-dominated composition of the fluid inclusions (Th=250–310°C) indicate a wet gas/condensate stage to an advanced dry gas stage.The clay mineral assemblage remains uniform throughout the whole succession and consists of illite, mixed-layered illite–smectite (IS), chlorite and kaolinite. Below 1400m the illite content gradually increases down-hole, forming up to 45–55% of the clay fraction in the deeply-buried Ordovician sediments. Throughout the borehole, the IS displays uniformly low expandability (7–12±3% of smectite layers) and an ISII ordering. The illite crystallinity broadly correlates with organic matter reflectance values and improves with depth (from 1.1 to 0.54° Δ2θ according to the Crystallinity Index Standard (CIS) scale) indicating high-grade diagenesis to incipient anchimetamorphism. The chlorite crystallinity (ChC index), however, shows no systematic correlations with depth.The combined evidence from organic matter, clay minerals and fluid inclusions points to maximum paleotemperatures of ~100–230°C in the Silurian–Devonian strata, and up to ~250°C or even higher in the Ordovician strata. Apatite fission track analysis (AFTA) data suggests that maximum heating occurred between 340 and 380Ma, i.e. during Variscan orogenic deformations. These paleotemperatures and the degree of diagenesis may have been produced by a post-Middle Devonian burial or tectonic load that was subsequently eroded, or by higher paleogeothermal gradients, or by a combination of both.