Contribution of erosion and intrusive-hydrothermal activity to the depth profile of organic matter maturation in sedimentary basins

Abstract

Sharp variations in organic matter (OM) maturity with depth in sedimentary basins reflected in an abrupt increase in vitrinite reflectance (%Ro) can be caused by both the thermal effect of intrusions accelerating OM maturation in adjacent rocks and removal of a thick sedimentary layer during the extensive erosion of the basin. It is traditionally believed that the erosion of a 1.5–3.0-km-thick sedimentary layer must significantly affect the depth profile of OM maturity, resulting in a drastic increase in %Ro in the present-day sedimentary basin. Some authors used such jumps (ΔRo) to estimate the amplitude of erosion. However, our analysis showed that the depth profile of %Ro is controlled not only by the amplitude of erosion but also by sedimentation history during the posterosion stage of basin evolution. Eastern Algerian basins are good examples to examine this problem, because their evolution involved both the stage of extensive Hercynian erosion and subsequent strong hydrothermal and intrusive activity. Numerous measurements of %Ro in core samples from eastern Algerian sedimentary basins were used to analyze this problem. The calculations of %Ro(z) distributions on the basis of numerical reconstructions of the thermal history of sedimentary sections in more than 60 boreholes of the area showed that the Hercynian erosion could account for only a minor part of the observed amplitude of Δ%Ro, in spite of the significant amplitude of erosion (1.5–3.5 km) typical of the basins of the Algerian Sahara. At the same time, the intrusive and associated hydrothermal activity of the Triassic and Jurassic adequately explains the amplitude of a stepwise increase in OM maturation with depth in the present-day sedimentary sections of these basins. Thus, the reliability of erosion amplitudes estimated from jumps in depth profiles of %Ro can be assessed only after the detailed examination of the posterosion history of the basin.