ABSTRACT: Quartz Cementation in Sandstone Reservoirs

EGEBERG, PER KHRISTIAN, Agder Distriktshoyskole, Kristiansand, Norway, and KNUT BJORLYKKE, University of Oslo, Oslo, Norway

In continually subsiding sedimentary basins like the North Sea and the Gulf Coast, sandstones buried to less than about 3 km have rather little quartz cement. Fluid inclusion data from North Sea reservoirs indicate that most of the quartz cement forms at temperatures exceeding 90-100 degrees C, often approaching bottom-hole temperatures. This suggests that quartz cementation may continue, probably at a reduced rate, after oil emplacement. Modeling shows that diffusion of silica is insignificant on a large scale (hundreds of meters) but is important on a smaller scale, particularly when amorphous silica and opal CT are present.

A pore water flux of about 108 cubic centimeters/sq. centimeters is required to precipitate significant volumes of quartz (1%) from external sources by pore water flow. Such fluxes could only be obtained on a local scale assuming an extreme degree of focusing of compactional water or by thermal convection. Modeling of advective flow suggests that for cooling (rising) pore water the rate of calcite dissolution exceeds the rate of quartz cementation by a factor of 30 to 300 depending on the pH of the pore water, which is then assumed to be buffered by the silicate minerals. This modeling predicts that if significant volumes of quartz cement have been introduced by advective flow, all calcite should have been dissolved. If the source of quartz cement in sandstones is dominantly internal ithin the sandstones, the porosity loss in reservoirs must be predicted from textural and mineralogical properties of the sandstones and the temperature and pressure history.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)