OFFSHORE SEDIMENT OVERPRESSURES OF PASSIVE MARGINS: MECHANISMS, MEASUREMENT, AND MODELS B. Dugan1 and T. C. Sheahan2 Received 23 October 2011; revised 10 May 2012; accepted 15 May 2012; published 14 July 2012.
[1] Fluid pressure in excess of hydrostatic equilibrium, or overpressure, in offshore environments is a widespread phenomenon that contributes to the migration and storage of fluids, solutes, and energy and to the potential mechanical instability of these sediments. Overpressure exists in deep and shallow systems and is most likely to be found where low-permeability (mm/yr), tectonic loading, and lateral fluid transfer) and thermal and chemical processes (e.g., aquathermal expansion, hydrocarbon generation, mineral diagenesis, and organic maturation). In systems where nearlithostatic overpressures are generated, potentially unstable sediments are created. Failures of these sediments can create large-scale natural disasters, generate fractures, and
damage seafloor and subseafloor infrastructure. Detailed characterization of overpressured systems has been accomplished through geological and geotechnical analyses, including investigation of physical-mechanical properties (mainly porosity, consolidation state, and shear strength), inversion of geophysical data (e.g., compressional and/or shear velocities), measurement of in situ properties, and postevent analyses. Process-based models have been developed to explain the origin of overpressure in terms of rate of overpressure genesis. This allows identification of potentially unstable zones and assessment of the potential for failure. Future development in measurements and in coupling of models will lead to more accurate analysis and prediction of fluid pressure in offshore sediments, which in turn will facilitate better hazard analyses and will enable safer and more cost-effective offshore drilling practices and other offshore infrastructure development.
Citation: Dugan, B., and T. C. Sheahan (2012), Offshore sediment overpressures of passive margins: Mechanisms, measurement, and models, Rev. Geophys., 50, RG3001, doi:10.1029/2011RG000379.
1.
INTRODUCTION
1.1. Importance of Overpressure [2] Overpressure, defined as fluid pressure in excess of hydrostatic equilibrium (Figure 1), has fundamental control of numerous shallow Earth processes ranging from largescale slope failure to fault localization and mediation of earthquake slip to regional effects on biogeochemical cycles that depend on fluid, solute, and heat migration. (Italicized terms are defined in the glossary, after the main text.) For example, overpressures are believed to have been a major contributing factor to the massive Storegga submarine 1
Department of Earth Science, Rice University, Houston, Texas, USA. Department of Civil and Environmental Engineering, Northeastern University, Boston, Massachusetts, USA. 2
Corresponding author: B. Dugan, Department of Earth Science, Rice University, 6100 Main St., MS 126, Houston, TX 77005, USA. (
[email protected])
landslide [e.g., Solheim et al., 2005]. Many other offshore regions of the world exhibit overpressure caused by rapid sedimentation rates and other mechanisms (e.g., Gulf of Mexico, Caspian Sea, and offshore West Africa). Overpressured zones can cause major technical difficulties during site exploration investigations (e.g., Caspian Sea), particularly during installation of offshore infrastructure, and can place the long-term stability of such structures at risk. Along passive margins, moderate to high overpressures influence regional flow patterns, can create pathways for localized thermal and chemical transport, and may facilitate slip or failure along low-angle normal faults. In tectonically active regions, such as accretionary prisms, overpressures have been cited as the driving force for large-scale fluid flow leading to pore fluid chemical variations, and overpressures may mediate fault localization and earthquake slip. Technological advancements have increased our ability to measure overpressure, which has led to significant advances in
©2012. American Geophysical Union. All Rights Reserved. 8755-1209/12/2011RG000379
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displaced more than 30,000 people. These disasters emphasize the importance of understanding overpressure and sediment stability and have refined the way in which we study and prepare for drilling in overpressured systems.
Figure 1. Illustration showing overpressure related to hydrostatic pressure, pore pressure, overburden stress, and effective vertical stress (redrawn from Bruce and Bowers [2002]). In this example, the shallow fluid pressure field (u) is hydrostatic (pore pressure equal to hydrostatic pressure and overpressure equal to 0). Overpressure (difference between pore pressure and hydrostatic pressure) increases with depth. The onset and magnitude of overpressure has controls on many geological and geotechnical phenomena. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. our understanding of these systems, their dynamics, and the coupling of fluids and solids related to deformation and fluid flow [e.g., Strout and Tjelta, 2005; Flemings et al., 2008; Stigall and Dugan, 2010]. Based on the importance of overpressure in offshore soft sediments, where strengths are typically very low, accurate characterization of the pore pressure profile is essential. [3] The blowout of the Macondo Well in the northern Gulf of Mexico provided international exposure for overpressurerelated hazards. Overpressures in the subsurface resulted in rapid release of water and hydrocarbons from the wellbore into the water column. The rapid fluid migration related to these overpressures caused local hydraulic fracturing and sediment erosion and created significant technical obstacles for containing the flow from the well [Hickman et al., 2010]. Another phenomenon related to overpressure is the mud volcano (LUSI) of East Java that started erupting in 2006. Original analyses of the mud volcano suggested it could have been induced by fluid pressure transients due to an earthquake or drilling-induced fluid influx. Detailed studies of the fluid pressure response to earthquakes, earthquake history, and drilling data identified that this was a drillingrelated phenomenon where drilling increased fluid pressures and resulted in a critical overpressure that exceeded the strength of the seal [Davies et al., 2008a]. The volumes of mud expelled from the volcano exceed 6 km3 and have
1.2. Scope of This Review [4] This review is motivated by the growing crossdisciplinary nature of overpressure studies to understand geological phenomena driven by long-term evolution and maintenance of overpressure, as well as geotechnical problems that look at short-term impacts and hazards associated with overpressure. Much of the foundation for this review was born from a U.S. National Science Foundation-sponsored workshop on subseafloor overpressure held in Oslo, Norway, in March 2009 [Sheahan and DeGroot, 2009]. Previous reviews [Dutta, 1987; Law et al., 1998] summarized interpretations of overpressure through geological and geophysical analysis and generally emphasized the static fluid pressure regime. More recent reviews focused on different aspects of overpressured systems. Screaton [2010] provides a comprehensive review of advancements in subseafloor hydrogeology. The review provides a background on subseafloor hydrogeology, including the governing equations for mechanics, heat and fluid flow, and solute transport, then expands on what we know about the input parameters to these equations and how they have been employed to represent fluid flow in basement-sediment systems, subduction zones, and passive margins. In a separate review, Saffer and Tobin [2011] describe the current state of knowledge on fluid transport and geomechanical processes in subduction zone settings. They include observations (fluid and geophysical), numerical models, and model constraints to explain how fluid flow and deformation is distributed through subduction settings and how they are evaluated. We build on these previous works by describing the processes that generate overpressure along passive margins, how overpressure is observed and modeled, and how this can be applied to geological and geotechnical problems. Similar to the previous studies, we provide observations and model results. We, however, emphasize and describe how overpressure can begin at very shallow depths and how this is crucial when assessing seafloor stability. In illustrating process-based links between overpressure and mechanics, we review numerous advancements in observing and monitoring overpressure with in situ sensors, long-term observatories, and high-resolution geophysical data. 2.
GENESIS OF OVERPRESSURE ZONES
2.1. Fundamentals of Overpressure Behavior [5] The role of overpressure on sediment behavior is simply explained through Terzaghi’s effective stress relationship. In three dimensions, the relationship is
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s1′ ¼ s1 u;
ð1aÞ
s2′ ¼ s2 u;
ð1bÞ
s3′ ¼ s3 u;
ð1cÞ
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where si′ is effective stress, si is total stress, and u is the total pore pressure. The subscripts i = 1, 2, and 3 reflect the maximum, intermediate, and minimum principal stresses, respectively. For passive margin settings, it is often assumed that the total vertical stress (sv) defines the maximum principle stress (s1), which allows expansion of the maximum principal effective stress (equation (1a)) Z s1′ ¼ s′v ¼ sv u ¼ rb rf gdz u*:
ð2Þ
Total vertical stress is defined by the depth-integrated bulk density times gravity (g). Here z is depth and rb is bulk density. Total pore pressure is controlled by hydrostatic pressure from the integrated fluid density (rf ) times gravity and overpressure (u*) at the depth of interest (z) (Figure 1). The total vertical stress is often referred to as the lithostatic stress or the overburden stress (Figure 1). This total vertical stress depends primarily on the porosity of the sediment, which itself is dependent on the effective stress of the system [Hart et al., 1995; Osborne and Swarbrick, 1997; Holtz et al., 2011]. The total stress also depends on the density of the solid grains and the density of the fluid(s) in the pore space. Because the total vertical stress can be easily determined from density data, many overpressure models relate porosity to vertical effective stress [e.g., Rubey and Hubbert, 1959; Hart et al., 1995; Dugan and Flemings, 2000]. In natural systems, however, horizontal stresses (s2 and s3 for passive margins) change due to burial (e.g., uniaxial strain conditions). The horizontal stresses are often assumed to be equal, and this may be appropriate near the seafloor and far from faults. Near extensional faults, as exist in passive margins, s2 exceeds s3 and the orientation of these stresses controls the orientation of faults. Changes in horizontal stresses can also cause changes in u*, which suggests the total stress state may have to be defined to estimate in situ overpressure. In the absence of reliable horizontal stress data, many studies rely solely on vertical effective stress-based methods to help interpret the magnitude of overpressure. This approach of relying on vertical stress data has been applied and validated in the shallow section (1012 m2) exceeding that in the low-permeability layer (e. g., clay, 1012 m2) sand body differentially buried by a very low permeability (e.g., 1 km) where temperatures exceed 40 C, aquathermal effects can be more pronounced. However, it is unlikely that aquathermal expansion alone would generate considerable overpressures. For example, when fresh water is heated from 54.4 C to 93.3 C, a volume increase of only 1.65% results [Bethke, 1986; Mello et al., 1994; Osborne and Swarbrick, 1997]. [14] Diagenesis, or mineral transformation, can release large volumes of water. Transformations that are important are montmorillonite (or more commonly, smectite) to illite, gypsum to anhydrite, and opal-A to opal-CT [e.g., Bethke, 1986; Colton-Bradley, 1987; Neuzil, 1995; Osborne and Swarbrick, 1997; Davies et al., 2008b]. The smectite-toillite reaction can release bound water at high effective stresses and temperatures less than 200 C, with the first pulse of water loss occurring at less than 60 C [ColtonBradley, 1987]. These volume changes are generally less than 4% [Osborne and Swarbrick, 1997] at low temperatures and thus may not be a dominant overpressure mechanism, especially at shallow depths. At higher temperatures, this reaction can be a significant source of fluid volume and thus fluid pressure [Bethke, 1986; Neuzil, 1995]. The presence of freshened fluids from this reaction further indicates that it can actively contribute to overpressure and flow fields over geologic scales, most notably in accretionary prisms [e.g.,
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Figure 3. Overpressures for different undrained burial models in two and three dimensions based on equation (4). (a) For a 2-D sandstone that thickens downdip (dashed line) overpressures are elevated relative to when sandstone thickens updip (solid line). (b) Overpressure in a synclinal sand (dashed line) exceeds that for an anticlinal geometry (solid line). (c) A three-dimensional, cone-shaped, anticlinal structure (dashed line) has elevated sandstone overpressure in comparison to a three-dimensional synclinal sandstone. Figure from Flemings et al. [2002]. Reprinted by permission of the American Journal of Science. Moore and Vrolijk, 1992]. By contrast, the gypsum-toanhydrite transformation can result in the loss of bound water at moderate temperatures (40 C–60 C) [Osborne and Swarbrick, 1997]. The water loss corresponds to a loss of volume of the solids (38%) as anhydrite is denser than gypsum but is accompanied by an increase in pore water volume (48%), so there can be a net volume increase of approximately 10% [Hanshaw and Bredehoeft, 1968; Jowett et al., 1993]. If those excess fluids cannot freely drain from the pores, overpressure will be generated. It is therefore identified as an important mechanism for overpressure generation where temperatures are high enough and gypsum is present. Fluid volume changes could also be caused by hydrocarbon generation due to thermal maturation and hydrocarbon migration from source rocks into adjacent sediments [Luo and Vasseur, 1996; Osborne and Swarbrick, 1997]. Meissner [1978] reported a volume expansion of 25% due to this process. Hydrocarbon generation also requires moderate to high temperatures (60 C–150 C) and a feasible source rock and thus is more important at greater depths where such conditions are more likely to exist. Thus, these thermal and chemical processes can be important at depths where temperatures are 40 C or greater, but they are not significant fluid pressure sources in shallow sediments along passive margins where temperatures are generally lower than 40 C. [15] Another chemically based means that may facilitate u* generation is gas hydrate formation. Grauls [2001] tied gas occurrence in a petroleum system to gas hydrates acting as a seal; overpressure was further enhanced by free gas accumulation beneath the gas hydrate due to buoyancy effects. Flemings et al. [2003] showed the presence of overpressure in the gas hydrate system at Blake Ridge, offshore southeast United States. Daigle and Dugan [2010]
quantitatively described the processes and time frames over which gas hydrate forms in fine-grained sediments, causes overpressures, and ultimately can lead to fracture generation. Such approaches provide a first-order explanation of the occurrence of overpressure in gas hydrate settings, fracturefilled hydrate in clays, and pore-filling hydrate in sands and of the occurrence of free gas venting at the seafloor [Liu and Flemings, 2007; Daigle and Dugan, 2010]. [16] Another potential source of fluid overpressures is the dissociation of gas hydrates arising from lowered sea level and/or an increase in temperature. The overpressures arising from hydrate dissociation has been cited as a contributory cause of submarine slides [Mienert et al., 2005]; as gas is produced from hydrate dissociation, the gas expands, fluid pressures increase, and effective stresses (and associated shear resistance levels) decrease, potentially initiating slope failure. Reagan and Moridis [2007] and Liu and Flemings [2009] have used numerical models of multiphase fluid flow to explore the dynamics of overpressure generation and dissipation in gas hydrate systems in response to climate change (sea level variations and seawater temperature changes). These results emphasize that transient overpressures are critical to overall overpressure assessment as they often feed back into dynamic processes, including fracture genesis and gas venting to the seafloor [Tryon et al., 1999; Hornbach et al., 2004; Liu and Flemings, 2009]. 2.3. Induced, Localized Overpressure [17] Localized overpressures can result from offshore activities such as hydrocarbon exploration and production. High fluid pressures are often induced in drilling operations to maintain borehole stability, but they can cause hydraulic fractures or swelling of the surrounding sediments [Schroeder
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et al., 2008]. High pumping rates and the use of highdensity drilling fluids can induce these overpressures. Overpressures may also develop during routine hydrocarbon production operations such as the injection of water and gas to enhance recovery or the reinjection of produced water or drilling cuttings. Heat from production wells drilled through hydrate-bearing sediments may result in hydrate dissociation and localized formation overpressures. These induced overpressures are local in scale but can be important factors for developing safe and efficient resource production scenarios. While local overpressures are important for resource evaluation and extraction, we do not focus on their details but do note that regional, naturally occurring overpressure is crucial to risk assessment regarding local, induced overpressure. 3. MEASUREMENT AND DETECTION OF OVERPRESSURE ZONES 3.1. Porosity and Consolidation Data [18] Measurements of porosity, on cores or from logging data, and analysis of laboratory consolidation data are often used to estimate in situ overpressure. Some of the first interpretations of fluid overpressure in the deep subsurface were based on measured porosity being higher than expected for hydrostatic conditions [e.g., Athy, 1930]. Later extensions of this model related the porosity to the state of vertical effective stress instead of depth [e.g., Rubey and Hubbert, 1959; Hart et al., 1995] f ¼ fo ebsv ′ ;
ð5Þ
where f is the observed porosity, fo is a reference porosity at zero vertical effective stress, and b describes the bulk compressibility of the sediment. Rearranging equation (5) and employing the definition of vertical effective stress (sv′, equation (2)), the observed porosity can be used to evaluate the in situ overpressure if the total stress, initial porosity, and bulk compressibility are known. Hart et al. [1995] documented that shallow subsurface data could be assumed as hydrostatically pressured to constrain the bulk sediment compressibility and initial porosity for sediments buried at depth in the shelf of the Gulf of Mexico. Other studies have used this approach (i.e., establishing a hydrostatic reference to constrain the model) to determine overpressure at a range of depths (shallow to deep) and in passive and active margin settings [e.g., Dugan and Flemings, 2000; Screaton et al., 2002]. In some environments, such as the deepwater sediments of Gulf of Mexico, overpressure begins within 10 m of the seafloor [Dugan and Germaine, 2008], no hydrostatic section exists, and assuming a shallow hydrostatic interval would lead to incorrect constraints on b and fo and underprediction of fluid pressure. Therefore, it is important to have a secondary means to validate any hydrostatic reference assumption. [19] A similar approach to the porosity-effective stress method is a traditional soil mechanics approach relating the void ratio (e = f/(1 f)) to the vertical effective stress e ¼ eo Cc logsv ′;
ð6Þ
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where Cc is the compression index which defines the change in void ratio per logarithmic cycle of vertical effective stress and eo is a reference void ratio. After constraining the compression index and the reference void ratio, the observed void ratio can then be used to interpret the in situ overpressure based on the relation between total stress, effective stress, and overpressure [e.g., Dugan et al., 2003a; Saffer, 2003]. Long et al. [2011] also used this approach and showed the importance of understanding how the compression index varies with void ratio when predicting overpressure in the upper 100 mbsf. [20] Preconsolidation stress (spc ′ ) interpreted from compression behavior is another approach to evaluate in situ effective vertical stress and overpressure [Casagrande, 1936; Becker et al., 1987; Ladd and DeGroot, 2003]. Data from load-increment or constant-rate-of-strain consolidation tests provide a means to estimate the spc ′ . The preconsolidation stress represents the maximum sv′ sediment has experienced and also indicates the yield stress in 1-D compressional loading. To determine spc ′ with the Casagrande method [Casagrande, 1936], the void ratio-consolidation data during the consolidation experiment are tracked, and a geometric analysis of the compression behavior yields the preconsolidation stress. In the work stress method [Becker et al., 1987], linear extrapolations of the preyield and postyield behavior during the consolidation experiment are constructed. The intersection of the extrapolations defines spc ′. Comparison of the vertical effective stress for hydrostatic conditions (svh ′ ) and the preconsolidation stress from either method provide an estimate of the in situ overpressure (u* = svh ′ spc ′ ) (Figure 4). Sample disturbance can produce consolidation data with poorly defined spc ′ ; however, various approaches exist to put error bounds on the preconsolidation stress and thus on the overpressure [Santagata and Germaine, 2002; Saffer, 2003; Dugan and Germaine, 2008]. Overpressure refinements can also be made if the average stress condition is known, i.e., there are adequate constraints on the vertical stress and the horizontal stresses, and if the stress history is known. [21] When using porosity, void ratio, or consolidation data to interpret overpressure, the geologic history (including the existence of cementation agents and the time of their formation), stress path, and the stress state must be adequately understood. For example, if significant unloading has occurred via erosion of overburden or fluid pressure increase after burial, then these methods will not provide accurate estimates of the overpressure. Also, if the principal stress orientations have changed significantly for a sediment package during time, e.g., sediment entering an accretionary prism where the maximum stress becomes horizontal, these methods can create inaccurate fluid pressure estimates. Therefore, these methods are most effective when employed in settings where burial can be assumed to control the major principal stress, where erosion is minimal, and where there is little evidence for any late stage fluid pressure genesis (e.g., hydrocarbon generation and aquathermal expansion). The precipitation of cements after deposition will alter the bulk compressibility and compression index of the sediments,
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using porosity, void ratio, or compression data work well in shallow sediments along passive margins where loading defines the maximum principal stress, temperatures are low, hydrocarbon generation is minimal, and cements are generally absent. Migration of hydrocarbons from depth, however, can cause late stage fluid pressure increases. Slope failures driven by overpressure can also cause erosional unloading of the underlying material. Analysis of geophysical data can help determine if hydrocarbons are present or if erosion has occurred. Analysis of core samples or sediment cuttings can be used to determine if cements are present.
Figure 4. Overpressure interpreted from penetrometer data (solid circles) [Flemings et al., 2008] and void ratio data from intact, nonfailed sediment (thin black lines) [Long et al., 2011] at IODP Sites (a) U1324 and (b) U1322 in the Ursa basin of the northern, deepwater Gulf of Mexico. Measured and interpreted overpressures are compared to 2-D, basinscale hydrogeologic model results for the Ursa basin (gray lines). Model details provided in Stigall and Dugan [2010]. Measurements and models show overpressure beginning near the seafloor and increasing to values exceeding 1 MPa. The flow-focusing model explains the origin of this shallow overpressure distribution. Figure modified from Stigall and Dugan [2010] to include data from Long et al. [2011]. which will change the compression behavior of the materials and ultimately result in unreliable estimates of fluid pressure from porosity or void ratio. Therefore, these approaches
3.2. Penetrometers [22] Penetrometers have been used for rapid (cm/yr), even with the moderate diffusivity (order of 108 m2/s) in this finegrained system [Stigall and Dugan, 2010]. Slope stability calculations for the Ursa region during pore pressure genesis suggest that during periods when sedimentation rates exceeded 30 mm/yr, high overpressures and flow focusing initiated slope failure in the upper 10 mbsf. When sedimentation rates were 15 mm/yr, overpressures were generated but were insufficient to drive the slope failures that have been observed. Thus, while the overpressure provided low effective stress, an external trigger was required to initiate failure. Sawyer et al. [2009] developed a mechanistic model for the evolution of these types of slope failures at Ursa to explain how fluid migration and low effective stresses promote retrogressive failures on low-angle slopes after failure has been initiated. [45] Geochemical and thermal data near mud volcanoes in the deepwater Gulf of Mexico have also been used to interpret overpressure-driven flow in shallow sediments of passive margins. These studies invoke but do not quantify the degree of overpressure. Sampling transects across mud mounds in the deepwater Gulf of Mexico provide type examples of these localized, high fluid pressure, high-flux sites. Data collected from outside the mounds moving toward the mound centers document increased salinity and heat flux near the center of the mounds, which can be explained by vertically upward focused fluid flow near the mound centers [Ruppel et al., 2005]. Overpressure is required to drive these fluxes. 6.3. Offshore Norway [46] Shallow and deep sediments offshore Norway have been rigorously studied for overpressure related to drilling and infrastructure hazards vis-à-vis hydrocarbon exploration and as related to large-scale submarine slope failure. [47] The Storegga Slide is one of the most extensively studied submarine landslides and has been related to subsurface overpressure through geotechnical and geophysical analysis and numerical modeling studies. Geophysical studies that incorporate geomorphic and sediment age data of the Norwegian slope have documented the volume (2500–3500 km3) and age (main slide 8100 years B.P.) of the Storegga Slide [e.g., Bugge et al., 1987; Haflidason et al., 2005]. These geophysical surveys also document a bottom-simulating reflection that was indicative of the presence of gas hydrate and free gas in the slide region [e.g., Bouriak et al., 2000; Bünz and Mienert, 2004]. This led to conceptual models of gas hydrate and free gas in generating overpressure and slope failure in shallow slope sediments; these failures ultimately created the Storegga Slide [Bouriak et al., 2000].
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[48] While overpressure is required to create such a large slope failure on a very low angle slope (
