Porosity Variation in Carbonates as a Function of Depth: Mississippian Madison Group, Williston Basin

ABSTRACT Log-determined porosities of argillaceous limestone, limestone, dolomitic limestone, and dolomite of the Mississippian Madison Group in the Williston Basin were analyzed to determine the influence of carbonate mineralogy, shale content, and fabric on porosity loss with depth of burial. Carbonate mineralogy and shale content strongly influence the rate of porosity loss. Argillaceous carbonates lose porosity at the greatest rate with burial, followed by clean limestone, dolomitic limestone, and dolomite. Average porosity of grain-supported limestone is not systematically higher than average porosity of mud-supported limestone in the same depth range, but there is a significant difference in the respective porosity range. Moderately to deeply buried (1.5-3 km) limestones with a grain-supported texture have a small percentage of high-porosity samples, whereas porosity distributions in matrix-supported limestones at equal burial depth cluster around the mean porosity and lack a tail of high-porosity samples. This effectively limits economic porosity in moderately to deeply buried Madison limestones to grain-supported rocks (packstones and grainstones). Results of this study reveal characteristics of basin-scale porosity loss mechanisms. Secondary porosity formed during burial is not evident in the porosity-depth profiles. Porosity loss is strongly influenced by mineralogy; clay content greatly accelerates the rate of porosity loss in limestones. In these rocks, dolomite porosity higher than limestone porosity at a given maximum burial depth is due primarily to selective preservation of dolomite porosity. Porosity decreases with increasing temperature in rocks with otherwise similar burial (effective stress) history. The observed porosity-depth relationships roughly follow an exponential trend; this may indicate that there is some sort of feedback between porosity and the porosity reduction mechanism. End_Page 29------------------------- Data generated in this study can be used to predict porosity distribution at a given depth in the Mississippian strata of the Williston Basin if no other information is available. Average limestone porosity at moderate to deep burial is significantly less than the porosity required for economic development of unfractured petroleum accumulations, so average porosity cannot be used as an estimate of economic porosity in a prospect. However, the distribution of porosity in a depth range can be used to estimate the risk associated with encountering sufficient thickness of economic porosity. The presence or absence of potentially economic porosity is best evaluated as a risk statement. For this reason, the porosity cumulative frequency distribution in a given depth range is a particularly useful tool because it can be interpreted in terms of expected thickness of porosity higher than a given threshold value. If information about vertical spatial correlation of porosity is available, the distribution can be interpreted in terms of risk of finding a minimum net thickness of carbonate exceeding a threshold porosity level. These methods can be used in other wildcat exploration settings where proper calibration data have been collected. The results of this study can be used as a guide to understanding porosity distribution with depth in other Paleozoic carbonates, and perhaps be directly applied to other late Paleozoic carbonates in cratonic settings.