Conference Paper

Intergranular volume (IGV) decline curves for evaluating and predicting compaction and porosity loss in sandstones

Volume: 75:3


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. 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 ({lt}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: {ge}0.1 mm grain size, well to moderately well sorted, {lt}5% matrix, {lt}15% feldspar grains, and {lt}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 further 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.

110 Reads
  • Source
    • "As a result, compaction can be evaluated in sandstones only by using indirect measures and by assuming a depositional porosity based on analogs. Examples of indirect measures of compaction that have been developed include a contact index (Taylor, 1950), a tight packing index (Wilson and McBride, 1988), and an intergranular volume (IGV) (Paxton et al., 1990; Ajdukiewicz et al., 1991; Szabo and Paxton, 1991), which are defined, respectively, as the average number of intergranular contacts per grain; the average number of long, concavo-convex, and sutured contacts per grain; and the sum of intergranular porosity and cements. Although the contact and tight packing indices are useful measures of compaction, McBride et al. (1990) reported that the measures are highly subjective and that they cannot be derived from standard point-count analyses of thin sections. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Presently available techniques for predicting quantitative reservoir quality typically are limited in applicability to specific geographic areas or lithostratigraphic units, or require input data that are poorly constrained or difficult to obtain. We have developed a forward numerical model (Exemplar) of compaction and quartz cementation to provide a general method suited for porosity prediction of quartzose and ductile grain-rich sandstones in mature and frontier basins. The model provides accurate predictions for many quartz-rich sandstones using generally available geologic data as input. Model predictions can be directly compared to routinely available data, and can be used in risk analysis through incorporating parameter optimization and Monte Carlo techniques. The diagenetic history is modeled from the time of deposition to present. Compaction is modeled by an exponential decrease in intergranular volume as a function of effective stress. The model is consistent with compaction arising from grain rearrangement, ductile grain deformation, and brittle failure of grains, and accounts for the effects of fluid overpressures and stable grain packing configurations. Quartz cementation is modeled as a precipitation-rate-controlled process according to the method of Walderhaug (1994, 1996) and Walderhaug et al. (in press). Input data required for a simulation include effective stress and temperature histories, together with the composition and texture of the modeled sandstone upon deposition. Burial history data can be obtained from basin models, whereas sandstone composition and texture are derived from point-count analysis of analog thin sections. Exemplar predictions are consistent with measured porosity, intergranular volume, and quartz cement fractions for modeled examples from the Quaternary and Tertiary of the Gulf of Mexico Basin, the Jurassic of the Norwegian shelf, the Ordovician of the Illinois basin, and the Cambrian of the Baltic region.
    AAPG Bulletin 03/1999; 83(3):433-449. · 2.61 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The reservoir quality of Jurassic and Triassic fluvial and lacustrine-deltaic sandstones of the Yanchang Oil Field in the Ordos Basin is strongly influenced by the burial history and facies-related diagenetic events. The fluvial sandstones have a higher average porosity (14.8%) and a higher permeability (12.7 × 10−3 μm2) than those of the deltaic sandstones (9.8% and 5.8 ×10−3) μm2, respectively). The burial compaction, which resulted in 15% and 20% porosity loss for Jurassic and Triassic sandstones, respectively, is the main factor causing the loss of porosity both for the Jurassic and Triassic sandstones. Among the cements, carbonate is the main one that reduced the reservoir quality of the sandstones. The organic acidic fluid derived from organic matter in the source rocks, the inorganic fluid from rock-water reaction during the late diagenesis, and meteoric waters during the epidiagenesis resulted in the formation of dissolution porosity, which is the main reason for the enhancement of reservoir-quality.
    Science in China Series D Earth Sciences 01/2002; 45(7):616-634. DOI:10.1360/02yd9063 · 1.59 Impact Factor
  • Source
    7th Intern. Conf. Geochem., Alexandria, Egypt; 01/2006
Show more