Sedimentological and diagenetic controls on Cambro-Ordovician reservoir quality in the southern Hassi Messaoud area (Saharan Platform, Algeria)

Abstract

The Cambro-Ordovician reservoirs of the Hassi Messaoud area comprise quartzitic sandstones, which rest unconformably on granitic basement and are capped by the Hercynian unconformity. Two sequence stratigraphic cycles are identified: a lower cycle of lowstand, transgressive and highstand deposits, and an upper cycle in which only lowstand deposits are preserved below the Hercynian unconformity. Sandstone of the greatest thickness and highest porosity and permeability occur within the lowstand deposits of the two sequences. Petrographic and scanning electron microscope studies were conducted in two wells in the southern Hassi Messaoud area on five sandstone units, to which the following chronostratigraphic terms are applied, from base to top: Ra + Ri lithozones (which contain the bulk of the reserves), Alternance Zone, E1 Atchane Sandstones and Hamra Quartzite. Reservoir quality is found to bear a strong relationship to clay content and mineralogy. All reservoirs are characterized by an extensive quartz cementation, which reduces porosities to generally below 10%. Permeability is related to clay content and type, with effective permeabilities generally limited to the quartz-and kaolinite-rich Ra lithozone. Permeability is lowest in reservoirs rich in fibrous illite and/or chlorite, such as the Hamra Quartzite. As porosity shows a less direct relationship to matrix mineralogy, there is a poor statistical relationship observed between porosity and permeability. Comparative diagenetic studies carried out within both the oil-bearing and the water-bearing parts of the reservoirs have determined that all the secondary processes occurred under freely operating diagenesis, pre-dating oil emplacement in the structure.

Regional geology and petroleum systems of the Illizi–Berkine area of the Algerian Saharan Platform: An overview

“…the Hassi Messaoud Field, due to the abundant Illite cement (Claret and Tempere, 1967;L'Homer, 1967;Djarnia and Fekirine, 1998); however, they may show secondary mouldic porosity associated with dissolution of feldspars that greatly improve reservoir quality.…”

“…In this field, the main reservoir corresponds to fluvial and shallow marine facies of the Hassi Leila Fm. The producing sandstones are less argillaceous than the underlying fluvial series, and cement is mostly associated with kaolinite instead of illite (L'Homer, 1967;Djarnia and Fekirine, 1998). Sandstones exhibit porosities below 10%, and permeabilities in the order of 10 mD (Claret and Tempere, 1967).…”

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

“…Quartz and siderite cements are present in varying amounts. (Djarnia and Fekirine, 1998). Sedimentary structures consist of planar-tabular cross bedding and trough cross-beddings with minor ripple cross-laminations.…”

“…It consists of poorly bedded, massive and structureless quartzites at the base, passing upwards into quartz arenites interbedded with shales towards the top. The sandstones are well sorted and the quartz grains well rounded, silica (quartz and chalcedony) being the main cement with minor illite and kaolinite (Djarnia and Fekirine, 1998;Bessa, 2004). The abundance of Skolithos burrows and the occurrence of invertebrates such as lingula in these rocks suggest a shallow marine depositional environment.…”