Abstract

Production of hydrocarbons from sandstone reservoirs is complicated by their diagenetic history. Although many aspects of sandstone diagenesis are well studied, the character and processes of glacial-associated diagenesis remain unknown. To address this problem, the origin, conditions, and timing of quartz cementation in glacial Ordovician sandstones of the Sbaa Basin, Algeria, were constrained on the basis of fluid-inclusion microthermometry, electron-microprobe data, and quantification by image analysis combined with optical and cathodoluminescence microscopic studies. Samples of the Cambro-Ordovician formations in the Sbaa Basin were investigated from four wells at burial depths between 2.0 and 2.5 km and at ambient temperatures between 110 and 130°C.

Quartz cements range in abundance from 1 to 27% of bulk rock volume in all wells. Three separate phases of quartz cementation are readily distinguished by cathodoluminescence microscopy. The main silica sources are interpreted to be: dissolution of feldspar grains, intergranular quartz dissolution, stylolitization, and illitization of smectite. Fluid-inclusion data depths indicate that quartz overgrowths precipitated between 100 to 160°C. This range of temperature corresponds to the Visean–Namurian burial phase, according to the basin thermal history.

Image analysis demonstrates intergranular quartz dissolution (also called “pressure dissolution”) in the presence of thin illite coatings around detrital quartz grains. These clay coatings were mechanically emplaced below the Ordovician ice sheet by pressurized meltwater circulation and subsequent fluid expulsion within sandy subglacial soft bedrock that was initially porous and permeable. Therefore, the subglacial bedrock lithology directly influenced quartz diagenesis because an increase in the content of clay matrix or the presence of an argillaceous layer can block the circulation of fluids flowing below the ice sheet, which prevents the formation of clay coatings. Quartz cements in this environment are favored in less compacted areas without clay coatings and with high porosity. The results of this study clearly illustrate the relationships among sedimentary facies, ice dynamics, and diagenetic architecture of the formation. These types of processes may control the porosity–permeability distribution in other glacial sandstone successions through quartz cementation and chemical compaction.

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