Self‐sealing in sedimentary basins

Abstract

Over the last decade, the picture that has emerged from several geologic disciplines is that the Earth's crust is permeable to great depths and that fluids are more or less constantly moving, transporting both heat and mass and affecting virtually every geologic process. In contrast, workers in the petroleum industry have maintained that sections of the crust contain impermeable pressure seals and hydraulically sealed compartments. We show that these starkly different conceptions can be reconciled by a theory of self‐sealing through gas generation in sedimentary basins. The model especially applies to older sedimentary basins where overpressuring cannot be readily explained as the result of ongoing sedimentation and compaction disequilibrium. In our conceptual model, overpressuring is created by catagenic gas generation and maintained by a combination of vertical and horizontal gas capillary seals. Data from the Anadarko Basin in Oklahoma provide support for the self‐sealing hypothesis. Well logs reveal the presence of 9 or 10 gas‐water interfaces over a 30‐m overpressured interval. Capillary pressure measurements show that the force necessary for gas to displace water from a shale is ∼3 × 106 Pa, a pressure equivalent to that exerted by a column of water 300 m high. A theory of self‐sealing explains the anomalous overpressuring observed in some older sedimentary basins by invoking known mechanisms and forces; it is both parsimonious and falsifiable.

3-D predrill overpressure prediction using prestack depth-migrated seismic velocity in a field of southwestern Malay Basin

“…Other mechanisms such as chemical compaction and fluid expansion can generate overpressure in the deeper part of the sedimentary basins (Deming et al 2002). For pore pressure prediction, it is important to understand the different mechanisms of overpressure generation and their effect on rock properties (Swarbrick 2001).…”

Comment – On maintaining high fluid pressures in older sedimentary basins: a new hypothesis or déjà vu?

“…As co‐author of two articles (Cranganu & Deming 2000; Deming et al. 2002) which have established the self‐sealing model to explain production and maintenance of overpressure in the Anadarko Basin, I believe this claim sounds like déjà vu.…”

“…2002) which have established the self‐sealing model to explain production and maintenance of overpressure in the Anadarko Basin, I believe this claim sounds like déjà vu. While Bredehoeft (2009) cites one of these articles (Deming et al. 2002) twice for other reasons, he never discusses the self‐sealing model, which would have made his hypothesis unnecessary.…”

“…(1989) estimated the ultimate recovery of natural gas from the basin to be 3.1 × 10 12 m 3 . Boyd (2005) states that the total gas production in the basin by the end of 2004 was 77 trillion cubic feet (2.18 × 10 12 m 3 ) Gas is found at interfaces between shales and sandstones (Deming et al. 2002, p. 2932 and fig.…”

“…2002, p. 2932 and fig. 3) Gas is forced out of small‐diameter shale pores by capillary pressures, and accumulates at pore entrances (Deming et al. 2002, p.…”