Analysis of Overpressure on the Gulf of Mexico Shelf

Abstract

Abstract Shale velocity and density vary with overpressure in different ways across the Gulf of Mexico shelf. Overpressure in younger, colder sediments is often consistent with disequilibrium compaction as a pressure mechanism. Older, hotter sediments often appear to have been compacted before overpressuring. In some cases, thick compacted overpressured shale beds are bounded above and below by permeable beds at lower pressure, indicating an internal pressure source such as the smectite-illite transformation. A modification of Eaton's method is proposed which deals approximately with the different modes of overpressure, and which also obeys physical constraints at low effective stress. The method uses a model for density instead of using measured density as an independent variable for unloading siturations. It is thus suited for predrill prediction of pressure from seismic velocities. Introduction Although overpressure in the Gulf of Mexico (GoM) has been studied for a long time (e.g., Dickinson1), some questions remain about the nature and causes of overpressure, as well as how to predict it. Most would agree that disequilibrium compaction or undercompaction is a primary overpressure mechanism in much of the GoM as it is worldwide (e.g., Osborne and Swarbrick2). On the other hand, the smectite-illite transformation has been thought to be a significant cause of overpressure by some, and a minor cause by others (see Dutta3, Osborne and Swarbrick2, and references therein). One purpose of this paper is to present evidence from the GoM shelf that some shales appear to be overpressured due to an internal mechanism, such as the smectite-illite transformation. These shales have physical properties that are consistent with reduction of effective stress from a previous maximum, Such reductions in effective stress will be called unloading here. Bowers4,5 has described how to analyze overpressure due to unloading, rather than undercompaction. Two key indicators of unloading are that (1) relatively small variations in sediment velocity correspond to large changes in effective stress, and (2) the sediment density is insensitive to unloading. This behavior is illustrated in Figure 1. Unloading causes points to fall above and to the left of the compaction trend on stress-velocity crossplots, and to right and above the compaction trend on sonic-density crossplots. When overpressure is due only to undercompaction, the shale properties remain on the compaction trend, but at lower velocity and density than normally pressured shales at the same depth. Figure 1. (a) Variation of velocity with effective stress for compaction and unloading. (b) compaction and unloading on a sonic-density crossplot. (AVAILABLE IN FULL PAPER) Undercompaction is relatively straightforward to analyze using velocities because effective stress is a unique function of velocity. Unloading poses problems for pressure estimation from velocities because the stress is no longer a single-valued function of velocity. Because it is insensitive to unloading, the density can be used to resolve the ambiguity5. Unfortunately measured densities are often unavailable, as when stress is estimated predrill from seismic velocities. A modification to Eaton's6 method described in this paper uses a locally-calibrated model to estimate density in unloaded sediments.