Intergranular Volume (IGV) Decline Curves for Evaluating and Predicting Compaction and Porosity Loss in Sandstones
SZABO, J. O., and S. T. PAXTON,* Exxon Production Research Company, Houston, TX
In contrast to conventional empirical porosity prediction schemes based on porosity decline with depth or thermal maturity, development of a conceptually sound porosity prediction algorithm requires quantitative assessment with depth (or thermal maturity) of two independent processes: compaction (as monitored by intergranular volume or IGV) and cementation. IGV is the sum of intergranular porosity, matrix, and cement as measured in thin section. The IGV concept is an underutilized indicator of diagenesis that can yield important information when plotted against depth and compared with porosity decline curves and cement geochemistry.
For this study, in order to evaluate the effects on porosity of compaction alone, a plot of IGV versus depth was constructed using data from greater than 350 relatively uncemented (<5% cement) sandstones from a wide variety of depths, ages, and geographic locations. Because texture and composition affect the initial IGV, the following additional restrictions were applied to the data set: greater than or equal to 0.1 mm grain size, well to moderately well sorted, <5% matrix, <15% feldspar grains, and <5% ductile grains. The resulting IGV decline curve reveals that sands compact mechanically and IGV declines rapidly from 40 to 42% at the surface to about 28% at 5000 ft. Between 5000 and 8000 ft, decline continues slowly until the framework stabilizes at around 26%. No furthe significant IGV decrease is observed to the depth limits of the data set at 22,000 ft.
The most significant implications of this curve for porosity prediction in sandstones are that (1) primary porosity can be preserved to great depths in the absence of cement, suggesting that reported volumes of secondary porosity in sandstones are exaggerated, (2) intergranular pressure solution plays a minor role in localized quartz cementation, and (3) cement prediction is a major key to porosity prediction.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)