Parameters Controlling Mudstone Sealing Capacity
Studies in several basins have shown that mudstone sealing capacity, as determined by MICP analysis, is highly variable among superficially similar samples. Detailed consideration of a variety of mudstone parameters, both textural and compositional, suggests that a combination of these parameters determines sealing capacity and that these parameters are in turn controlled by sequence stratigraphic setting. Which parameters are most important may depend on lithologic details of the mudstones. The parameters considered include silt content, carbonate content and timing, organic matter content and orientation, matrix content and fabric, bioturbation, and ash content. Among the parameters we have identified, silt content is generally only important above a threshold abundance, probably about 20%. Above that silt threshold, sealing capacity degrades. Organic matter abundance and preferred orientation correlate with high sealing capacity in carbonate-poor mudstones, but are less important in carbonate-rich ones. High carbonate content can be associated with either excellent seals or poor seals, depending on the nature of the carbonate. Early carbonate, such as in firmgrounds, is generally associated with very high sealing capacity, although there appears to be a maximum carbonate abundance that is optimal. Late carbonate cement, on the other hand, is generally associated with poor seals. In carbonate-poor mudstones, bioturbation is typically found in poor seals, but if bioturbation is extreme and homogenizes the rock, sealing capacity may be high. Integration of microscopic layers of volcanic ash into mudstones degrades sealing capacity, although well-defined bentonite layers may be excellent seals.