Print this pageRelationship Between Deep Diagenetic Quartz Cementation and Sedimentary Facies in a Late Ordovician Glacial Environment (Sbaa Basin, Algeria)
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, quantification by image analyses, combined with optical and cathodoluminescence microscopic studies. Samples of the Cambro-Ordovician Formation in the Sbaa basin were investigated in seven different wells, at a burial depth between 2.0 and 2.5km and at present temperature of 110 to 130°C.
Quartz cements are present in all studied wells, their abundance ranging from 1 to 27%. Three separate stages of quartz cementation are readily distinguished by cathodoluminescence microscopy. Aluminium content data for each phase of quartz cement suggest different silica sources. Two majors internal silica sources are discussed: dissolution of feldspar grains and pressure-solution of detrital quartz grains. Fluid inclusions data from samples presently lying at 2km subsurface (110°C) indicate that global quartz cementation precipitated at 109 to 154°C. This range of temperature corresponds to the Namurian burial phase according to the basin thermal history.
Image analyses quantifications show enhanced intergranular pressure-solution supported by the presence of thin illite coatings (rims) around quartz grains. These clay coatings phenomena can be set up beneath the ice sheet by pressurized water circulation within a sandy subglacial soft bedrock, initially clay free, porous and permeable. The subglacial bedrock lithology is then essential: an increase of clays content can prevent clay coatings formation, or block the circulation of fluids flowing below the ice sheet. All observations confirm the enhancement of pressure-solution between quartz grains by illite coatings. Quartz cements result mainly from pressure-solution, they precipitate largely within low-compacted levels, where clay coatings are not present and porosity is high. Thus, a relationship between the sedimentary facies, the ice dynamic and the diagenetic architecture of the formation is highlighted, controlling porosity-permeability distributions through quartz cementation and compaction by pressure-solution.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil