Predictive Models for Sandstone Diagenesis and Reservoir Heterogeneity

Ronald C. Surdam, Donald B. MacGowan

At temperatures less than 80°C, the diagenetic reactions producing carbonate cements in sandstones can be explained nicely by the model proposed in 1986 by C. D. Curtis and M. L. Coleman. Briefly, the distribution of early carbonate cements is controlled by dissolved sulfate concentration and is a function of the processes which affect sulfate concentration (i.e., depositional water composition, microbial sulfate reduction, and water mixing). In order to use this model in a predictive sense, a knowledge of the original depositional environment's hydrology and hydrochemistry is necessary.

Predictive models for sandstone diagenesis in the 80° to 130°C thermal interval can be developed based on carboxylic acid/CO₂ distributions and ratios. The model assumes that over this thermal interval the alkalinity in the reservoir facies is dominated by carboxylic acids and that a significant portion of CO₂ present is the product of decarboxylation of the acids (assuming there has been no significant mixing of water bodies). Furthermore, it is assumed that the stability of carbonates is a function of the carboxylic acid/CO₂ ratio, and the stability of framework grains is a function of the distribution and concentration of carboxylic acids.

At temperatures greater than 130°C, diagenetic reactions controlling the distribution of cements and the stability of framework grains in sandstones generally can be explained by thermocatalytic sulfate reduction. The determinative aspects of this process are the type of organics present in the system, the sulfate/organic ratio, and the presence or absence of iron. In addition to this information, if a time-temperature profile and kinetics for the redox reaction of interest are available, the process and resultant mineral reactions can be modeled.

Using the techniques described above, it is possible to model, from the surface to depths exceeding 20,000 ft, the cementation patterns and framework-grain stabilities in sandstones in sand-shale systems.

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