Evidence for reduced quartz-cementation rates in oil-filled sandstones

When quartz-rich sands are buried and heated, pore space is gradually fille d by precipitation of quartz cement from aqueous formation fluids. Here we examine whether the presence of oil in the pore space can retard or halt th is loss of porosity by slowing or stopping quartz cementation. The effect o f oil fill on quartz cementation is examined by using the distribution of q uartz cement in the Brae Formation deep-water sandstone reservoir of the Mi ller oil field (North Sea). Petrographic data demonstrate that sandstones f rom the oil zone have much less quartz cement, and more porosity, than sand stones from the water zone. Sandstones in both oil and water zones are comp ositionally and texturally identical and have been affected by a similar bu rial history. Kinetic modeling of the cementation process suggests that pro gressive oil charging has slowed quartz-cement growth rates by at least two orders of magnitude, halting it completely in the most extreme cases. Our data demonstrate that early oil charging in the crestal part of an anticlin e can preserve porosity in deeply buried sandstones. This knowledge is espe cially relevant to porosity prediction for petroleum exploration in deeply buried sandstones.