Chemostratigraphy, tectonic setting and provenance of the Cambro-Ordovician clastic deposits of the subsurface Algerian Sahara

Abstract

The Cambro-Ordovician sandstones are one of the main oil reservoirs in Algeria. They are, in the most part a braided fluvial system and the reservoir is subdivided into four lithozones (R3, R2, Ra, Ri) based on grain size and wireline logs signatures. Geochemical analysis has been applied to these units with no or very poor biostratigraphic control for more detailed and accurate stratigraphy and correlations. Several Wells are selected from the northern part of the Saharan Platform. Geochemical data have been acquired from cores with 20 elements being determined. A new nomenclature of sequences or subunits, based on chemostratigraphy and sedimentology, is established from base to top for the Cambrian and the Lower Ordovician. The majority of sandstones have an average SiO 2 content between 86.4 wt% and 96.8 wt%, i.e. quartz-rich. Cambro-Ordovician sandstones are therefore quartz arenites in their majority. The evolution of the two ratios SiO 2/Al 2O 3 and K 2O/SiO 2, which are significant variables for differentiating between the Cambro-Ordovician units, display an increase in the maturity of sandstones from the base to the top. Ferro-magnesian content and trace elements such as Ni, Co and Cr show that a significant contribution from volcanic rocks is unlikely. Mineralogical maturity of the analysed rocks along with the occurrence of zircon is consistent with a felsic plutonic or reworked sedimentary source rocks. Sediments were derived mainly from deeply weathered cratonic landmasses or recycling sediments. A passive margin origin for the Cambro-Ordovician sandstones is indicated by the discriminant plot of K 2O/Na 2O vs. SiO 2. The maturity of these sandstones is characteristic of cratonic environments, where sedimentary recycling is an important process. The tectonic setting and the provenance of Cambro-Ordovician clastic deposits conform to a passive margin setting. However, sequence isopachs and sedimentary architectures indicate that the Hassi Messaoud area was locally unstable during the Cambrian.