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doi:10.2204/iodp.proc.308.207.2009

Introduction

A central objective in structural geology is to interpret the deformation histories of geological bodies and the stress paths responsible for those histories. Such stress and strain histories are important for a proper understanding of mechanical behavior, pore-fluid pressure, and fluid flow in the upper crust. Knowledge of the stress evolution is vitally important in understanding natural phenomena (e.g., slope failures).

The understanding of the nature of stress and stress paths that produce deformation has advanced remarkably over the past decades, both from laboratory experiments and in situ stress measurements (e.g., Jones, 1994; Karig and Morgan, 1994; Dugan and Flemings, 2000; Karig and Ask, 2003).

Laboratory deformation experiments on sediments are inexpensive and easy to perform compared to in situ measurements. Laboratory experiments also enable uniform sediment to be subjected to varying stress and strain conditions to better isolate functional dependencies. However, some limitations are

• A much faster strain rate in the laboratory than in nature,
• Problems with mimicking diagenetic processes in the laboratory, and
• The ability to apply only simple stress paths in laboratory experiments (Karig and Morgan, 1994).

The last limitation can to some extent be mitigated by conducting multiple tests on samples with different orientation and stress paths.

Passive continental margins are examples of tectonically quiescent areas, for which the anticipated stress path is assumed to approximate uniaxial strain, as caused by gravitational loading from deposition. The U.S. Gulf Coast passive margin is suitable for studying properties and processes related to sediment consolidation and fluid flow in sediments with varying pore-fluid pressure (e.g., Flemings et al., 2005).

Based on data collected by the petroleum industry along the U.S. Gulf Coast, Breckels and van Eekelen (1982) investigated the K₀ stress ratio and the variation of pore-fluid pressure. The K₀ stress ratio is the ratio between effective horizontal and vertical stress. It is also referred to as “earth pressure at rest.” Low K₀ values indicate brittle behavior; high K₀ values indicate ductile behavior. Breckels and van Eekelen (1982) proposed that the stress ratio increases from ≤0.3 near the surface to 1.0 near 6 km depth. Karig and Morgan (1994) argued against their interpretation and proposed a higher K₀ stress ratio of ~0.5 for sections with hydrostatic pore-fluid pressure. The more recent Pathfinder Drilling Program consisted of both in situ stress measurements and laboratory deformation tests (e.g., Finkbeiner et al., 2001; Stump and Flemings, 2002). The tests on two lightly cemented mudstone samples by Stump and Flemings (2002) showed that the K₀ stress ratio changes when the samples yield, from preyield values of 0.52–0.63 to postyield values of 0.85–0.86. Further experiments are clearly needed to investigate how the principal stress magnitudes increase with depth in this setting.

The objective for this study is to explore and understand the basic consolidation processes occurring in shallow sedimentary rock formations by

• Measuring how the stress ratio of the effective principal stress magnitudes develop in elastic and plastic-elastic reconsolidation and
• Determining in situ and laboratory relationships between in situ effective stress state and pore volume (e.g., porosity and void ratio).

Among other objectives, these data can be used in pore pressure prediction from drilling and log data.

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