Porosity Prediction in Sandstones: Casual vs. Causal Relationships

S. Bloch, R. K. Suchecki, J. H. McGowen

Porosity in sandstones of a given initial composition and texture decreases with increasing time, increasing effective stress, and increasing temperature. Present-day porosity and permeability in many reservoirs appear to be related to burial histories because of the influence of these parameters. We were able to successfully predict reservoir quality from burial history, depositional facies (a measure of texture), and sandstone composition prior to drilling in a number of reservoirs, including the Kekiktuk quartzarenites (Mississippian, North Slope of Alaska) and subarkoses of the Tomma Formation (Middle Jurassic, offshore Norway). In each case, a calibration data base was available for the studied basin. The question that now needs to be addressed is the nature of the p edictions: are they causal (the variables used to predict porosity are the cause of porosity reduction) or are they casual (the variables used to predict porosity are not the the cause of porosity reduction)?

Petrographic evidence documents the importance of pressure dissolution and quartz cementation for porosity reduction in the quartz-rich sands analyzed. This finding suggests that the relationship between burial history and porosity is causal because pressure dissolution and quartz cementation are kinetic processes that respond to effective stress, temperature, and time. A microprobe cathodoluminescence study, however, indicates that pressure dissolution between detrital grains is not a major source of silica cement. Pressure dissolution at overgrowth contacts can produce significant amounts of silica, but the source of initial silica for the overgrowths that are subsequently dissolved is uncertain. The initial hypothesis on the mechanism of porosity reduction is not substantiated by c oser inspection. Until the processes responsible for porosity reduction in sandstones are understood, even successful prediction attempts based on the relationship of reservoir quality with various measures of thermal maturity will represent a casual rather than a genetic causal approach to the problem of predrill porosity prediction.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.