ABSTRACT
Overpressure can be produced by the following process: (1) increa of compressive stress, (2)changes in the volume of the pore fluid or rock matrix, and (3) f luid movement or buoyancy.Loading during burial can generate considerable overpressure due to diquilibrium compaction,particularly during the rapid subsidence of low-permeability diments. Horizontal stress changes can rapidly generate and dissipate large amounts of overpressure in tectonically active areas.Overpressure mechanisms involving change in vol-ume must be well aled to be effective. Fluid vol-ume increas associated with aquathermal expan-sion and clay dehydration are too small to generate significant overpressure unless perfect aling occurs. Hydrocarbon generation and cracking to gas could possibly produce overpressure, depend-ing upon the kerogen type, abundance of organic matter, temperature history, and rock permeability;however, the process may be lf-limiting in a aled system becau buildup of pressure could inhibit further organic metamorphism. The poten-tial for generating overpressure by hydrocarbon generation and cracking must be regarded as unproven at prent. Fluid movement due to a hydraulic head can generate significant overpres-sure in shallowly buried, “well-plumbed” basins.Calculations indicate that hydrocarbon buoyancy and osmosis can generate only small amounts of localized overpressure. The upward movement of gas in an incompressible fluid also could generate
significant overpressure, but requires further investigation. Stress-related mechanisms are the most likely caus of overpressure in many di-mentary basins.INTRODUCTION
Overpressure, sometimes termed “geopres-sure,” is common in subsurface rocks. A pore fluid is overpressured if its pressure exceeds that of the hydrostatic gradient at a specific depth.The hydrostatic pressure gradient is the pressure that would be exerted by a continuous column of static f luid (Figure 1), and will var y slightly depending upon the density of the pore fluid. It is important to understand how overpressure is generated if fluid pressures are to be predicted prior to drilling. In addition, overpressure obrved in rocks today owes its distribution not only to the mechanisms of generation, but also to the redistribution of fluids during an小屁孩日记2
d after the creation of overpressure. The hydrodynamics of dimentary basins are controlled by differentials of fluid pressure created by forces other than hydrostatic differences.
A wide variety of mechanisms have been pro-pod for the generation of overpressure in di-mentary basins. The mechanisms can be divided into three categories: (1) increa in compressive stress (i.e., reduction of the pore volume) caud by diquilibrium compaction and tectonic com-pression; (2) fluid volume change caud by tem-perature increa (aquathermal pressuring), diage-nesis, hydrocarbon generation, and cracking to gas; and (3) fluid movement and
process related to density differences between fluids and gas caud by hydraulic (potentiometric) head, osmo-sis, and buoyancy. In addition, overpressure gener-ated at one site can be redistributed elwhere in the rock succession.
Previous reviews of overpressure mechanisms have concentrated on theoretical or mathematical aspects (Hall, 1993; Neuzil, 1995), or are more focud on overpressure prediction and detection (Mouchet and Mitchell, 1989). Our objective with
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AAPG Bulletin, V. 81, No. 6 (June 1997), P. 1023–1041.
Mechanisms for Generating Overpressure in Sedimentary Basins: A Reevaluation 1
Mark J. Osborne and Richard E. Swarbrick 2
praisal of each mechanism and to deduce which mechanisms are reasonable caus of overpres-sure in the subsurface in geologically realistic sit-uations.
INCREASE IN COMPRESSIVE STRESS
Changes in the stress state of the rock can result from vertical compression (burial) and horizontal compression due to tectonic forces.Diquilibrium Compaction
At a given depth in a dimentary basin, the ver-tical load due to the overlying diments is known as the overburden stress, S, given by
(1)
where Z is the vertical thickness of the overlying diments, b is the average bulk density, and g is the gravitational force. Some of the weight of the overburden is borne by the f luid in the pore spaces; the remainder of the weight is borne by the matrix (effective stress).
The relationship between effective stress and overburden is given by Terzaghi’s equation:
(2)
where is the effective stress and P is the fluid pressure. Becau rocks and soils can be com-presd, their porosity depends on effective stress. If effective s成熟网名
tress is small (high pore pressure), val-ues of porosity will also remain high. If effective stress increas, porosity will decrea and the rock will compact. The overburden stress increas with depth along the lithostatic gradient, which is typi-cally assumed to be 1.0 psi/ft in dimentary basins with dimentary thickness greater then 4.0 km. At any point, the lithostatic pressure, or overbur-den stress, is determined by the density of the over-lying diments:
(3)
wherev′is the vertical stress, s is the density of the rock, f is the density of the fluid, and is the porosity.
As vertical loading of diment increas during burial, rocks generally compact, reducing the pore volume and forcing out the formation f luids. Sandstones compact from about 39–49% porosity at de
position (Lundegard, 1992) to about 15–25% porosity at depths of 2 to 3 km due to rearrange-ment of the sand grains and some chemical dissolu-tion at grain contacts (Sclater and Christie, 1980). At greater depths there is little potential for signifi-cant further reduction in porosity due to mechani-cal compaction involving bed thickness reduction; however, significant further porosity reduction due to diagenetic cementation can occur. By contrast, clays typically have porosity in the range of 65 to 80% at deposition (Sclater and Christie, 1980). Clays will continue to compact by grain rearrange-ment and ductility to great depths (typically 4–6 km), where the porosity can be reduced to only
′==−
()+()
[]
∑
v S f
s
z
1
=−
S p
S Z bg
=
1024Generating Overpressure
1980). Under conditions of slow burial, normal com-paction of diments occurs; that is, the equilibrium between overburden and reducing pore-fluid vol-ume is maintained. Rapid burial, however, requires rapid expulsion of fluids in respon to rapidly increasing overburden stress. Where the fluids can-not be expelled fast enough, the pressure of the pore fluids ris above hydrostatic values. This pro-cess is known as diquilibrium compaction.
Overpressuring due to diquilibrium com-paction is illustrated in Figure 1. As a shale quence subsides, the fluid is initially expelled from the diment and escapes to the surface, so the pore pressure increas by following the hydro-static pressure gradient. However, as subsidence continues, the permeability of the diment declines, and at some point fluid will start to be retained; the depth at which this occurs is the fluid isolation depth (point B, Figure 1). If no fluid escapes below the fluid isolation depth, the pore pressure would then ri along a pressure-depth path (point C, Figure 1) that is parallel to the litho-static gradient. In reality, becau rocks are not entirely impermeable, some fluid will continue to be expelled, so the profile will be subparallel to the lithostatic gradient. Similar pressure gradients have been determined from multiple pore-pressure mea-surements in stacked rervoirs in veral dimen-tary basins worldwide (Mann and Mackenzie, 1990), and the profiles have also been reproduced using one-dimensional mathematical
models of compaction that simultaneously satisfy Darcy’s flow law and Terzaghi’s porosity–effective stress relationship (e.g., the models of Mann and Mackenzie, 1990) (Figure 2). Diquilibrium com-paction is favored as the mechanism to explain overpressure in a number of basins, including the Gulf Coast (Dickinson, 1953), Caspian Sea (Brede-hoeft et al., 1988), and North Sea (Mann and Mackenzie, 1990; Audet and McConnell, 1992).
Note that the pore pressure in Figure 1 never exceeds the lithostatic pressure at any depth. However, the pore pressure can exceed the frac-ture pressure where the fracture gradient is less than the lithostatic gradient. The fracture pressure is the amount of pore pressure a rock can with-stand before its tensile strength is exceeded and hydraulic fracturing occurs. At any specified depth, the fracture pressure of the rock is generally lower than the lithostatic (overburden) pressure, typically about 70–90% of the overburden (du Rouchet, 1981), but may be higher at great depths (for exam-ple, belo劳动实践活动总结
w 5.0 km depth) (Engelder and Fischer, 1994). The measured fracture pressure of the rock at any one depth in a well will also depend upon in-situ stress, the condition of the borehole, and mud characteristics. Shown on Figure 1 is the mini-mum leak-off pressure envelope for central North Sea rocks; this envelope reprents a rough approx-imation of the fracture gradient (Gaarenstroom et al., 1993). Note on Figure 1 that diquilibrium compaction can only produce hydr
aulic fracturing of central North Sea rocks if fluid retention com-mences at approximately 1220 m. If怎么摇骰子
fluid retention occurs at depths greater than 1220 m, hydraulic fracturing is unlikely. Generally, diment perme-abilities will not be low enough to result in fluid retention at such a shallow depth unless the rate of diment deposition is very rapid (>600 ) (Mann and Mackenzie, 1990).
Conditions that favor diquilibrium compaction are rapid burial and low permeability. Diquilibrium compaction therefore is likely to be found commonly in thick clay, mud, marl, and shale successions during continuous rapid burial. Overpressure in adjacent, higher permeability rervoir rocks may also be gen-erated either through stratigraphic isolation of the rervoir within a finer grained, low-permeability
Osborne and Swarbrick1025
ction or by lateral permeability reduction such as faulting. However, where sandstones are laterally well connected, overpressure may be dissipated due to expulsion of fluid through the sands. In the N交通事故案例分析
orth Sea, the Tertiary mudrock ction is inferred from modeling and log characteristics to be under-compacted and overpressured (Ward et al., 1994), although direct measurements of mudrock porosity are not available. By contrast, the permeable rer-voirs with good lateral connectivity are typically hydrostatically pressured, implying that fluid can equilibrate with surface conditions through the sandstones (Cayley, 1987).
In addition to Terzaghi’s laboratory experiments, indirect evidence for diquilibrium compaction includes anomalously high porosity estimates for low-permeability ctions, as recorded on borehole porosity tools such as sonic and density logs. Overpressuring is thought to inhibit compaction, and hence abnormal pressures could theoretically prerve diment porosities that are higher than expected for the burial depth if the effect of cemen-tation on porosity is negligible. Esntial to interpre-tation is the validity of a “normal” trend line for porosity (or sonic traveltime) and the inference that higher sonic values equate to higher porosity. Critical asssment of the rocks is required to verify whether departure from the normal line relates to changing lithologies or mineralogy, or is a real man-ifestation of overpressure and undercompaction (Japn, 1993, 1994).
In the central North Sea, the Tertiary mudrock ction commonly has anomalously high sonic tran-sit times, implying that compaction has been inhib-ited and fluid retained (Figure 3). Both overpres-sured and hydrostatically pressured Tertiary rocks in the North Sea exhibit a linear relationship betwe酸菜汤的家常做法
en effective stress and sonic velocity (Figure 4). As a rock subsides and undergoes normal com-paction, porosity is steadily reduced and the inter-val velocity increas. Hence, for normal diment compaction, if the rock remains hydrostatically pressured, both the effective stress and velocity should increa as subsidence progress. The effective stress–velocity relationship defines a line known as the virgin curve (Bowers, 1994), and all rocks undergoing normal compaction should move along this virgin curve during subsidence. However, if diquilibrium compaction occurs, porosity loss is reduced and velocity increa slows. If all fluids are retained, porosity and veloci-ty will remain constant. If compaction ceas, effective stress will remain constant with depth. Hence the rock is “frozen” at a point on the virgin curve, and the pore fluid will be overpressured. Both hydrostatically pressured and overpressured Tertiary mudrocks in the central North Sea lie on the same virgin curve (Figure 4); this is consistent with the hypothesis that diquilibrium com-paction is the cau of the overpressure in the Tertiary ction.
From the numerical modeling of veral different rearchers, it is clear that the principal control on f
low is the diment permeability, which usually is poorly known in mudrocks and other low-permeability rock types becau it is difficult to mea-sure directly (Ungerer et al., 1990; Audet and McConnell, 1992; Luo and Vasur, 1992). Porosity can readily be estimated from well logs. A relation-ship between porosity and permeability is assumed. Permeability is likely to vary with mineralogy and rock fabric. In addition, the compaction coefficients for mudrock lithologies are poorly constrained. The compaction coefficient governs the rate at which porosity and permeability will decline with effective stress. The effect of cementation on mudrock poros-ity and permeability is poorly understood. Conquently, basin modeling can show the poten-tial for building up overpressures in the subsurface, but the actual values are not well constrained (e.g., Luo and Vasur, 1992). What we do know is that rapid rates of burial (high dimentation rates) will cau overpressure in low-permeability diments. Rapid dimentation rates will mean that more fluid must be expelled in a shorter space of time to avoid the buildup of fluid pressures. Slow rates of burial
1026Generating Overpressure
Figure 3—Interval travel time vs. depth for UK well 21/20a-1, central North Sea. Shale values from the top Miocene to ba Eocene are interpreted as overpres-sured due to the high traveltimes relativ
e to the “normal trend.” Shale values plot clo to the normal trend adja-cent to the normally pressured Paleocene sands. The cau of the overpressure in the Tertiary ction is dis-equilibrium compaction.
(low dimentation rates) are likely to equate with normal pore-fluid pressures becau the fluid has more time to escape (Mann and Mackenzie, 1990).
Deming (1994) showed that the minimum per-meability needed for a geological unit to act as a pressure al for more than is 10–21to 10–23 m2(10–6to 10–8md). This range is lower than most measurements of shale permeability. Becau no natural shales can act as perfect als, diquilibri-um compaction must be a transient phenomenon that dissipates through time. Pressure will leak off at a rate that is proportional to the vertical perme-ability and inverly proportional to the shale thick-ness. In support of this, note that overpressuring is more common in Tertiary quences than in Paleozoic successions, suggesting perhaps that the amount of overpressure has diminished through time. In addition, most pressure transition zones are not sharp, but gradual (Swarbrick and Osborne, 1996), implying vertical flow of pore fluid across the leaky al. Eventually such fluid flow allows rervoir pressures to return to hydrostatic.
In summary, diquilibrium compaction is a feasible overpressuring mechanism in thick low-permeability quences that are being rapidly buried. Sand bodies interbedded and isolated with-in the shales also can be overpressured due to expulsion of fluid from the adjacent mudrocks (Magara, 1978), hence maintaining equilibrium of pressures between the mudrocks and the shales. The pressure produced by diquilibrium com-paction will dissipate gradually through time either due to slow vertical fluid movement through the al or by lateral migration of fluid through aquifers interbedded with the shales.
Tectonic Compression
In a basin where no lateral compression occurs, horizontal stress would be equal to or less than vertical stress. Lateral compression can increa pore pressures in the same way as vertical stress can cau overpressuring through diquilibrium compaction. Overpressured zones 650–800 km long and 40–130 km wide are associated with the San Andreas fault in California (Sleep and Blanpied, 1992). Overpressured diments also occur beneath accretionary prisms at destructive plate margins (Davis et al., 1983; Fisher and Zwart, 1996). Long-range migration of brine across the North American craton is thought to have been driven by overpressure that was generated during periods of tectonic compression (Ge and Garven, 1989; Bethke and Marshak, 1990).
Overpressure buildup due to tectonic process can be very rapid, and decrea of pressure can be similarly rapid if large volumes of fluid driven by ismic valving or pumping escape up fault planes (Sibson, 1990). Fault zones such as the San Andreas are particularly susceptible to failure becau duc-tile creep in the fault zone leads to compaction that increas fluid pressure and makes the fault weak (Sleep and Blanpied, 1992). Salt diapirism also could produce overpressures400米跑步技巧
, although doming of diments above the salt also might produce ten-sion fractures that allow overpressure reduction becau fractures have an extremely high perme-ability and the fluid flow rates through them can be extremely high (Giles, 1987). Hence, tectonic pro-cess can create transitory, rapidly fluctuating overpressures unless the compressive stress are so small that the rock neither buckles nor fractures. The latter situation (i.e., where no fracturing is involved) is similar to diquilibrium compaction, except the stress is acting horizontally instead of vertically.
Measurement of in-situ stress and breakout data from wells provides information on the stress regime today, although changes in intraplate stress through time are not well documented. Price (1974) showed that in large basins the radius of the curvature of the Earth produces horizontal com-pression of the diments during shallow burial. Grauls and Baleix (1994) described a Tertiary basin in southeastern Asia where the upper part of the
Osborne and Swarbrick1027
Figure 4—Both overpressured and hydrostatically pres-sured mudrocks in the Quaternary and Tertiary of the central North Sea have velocity and effective stress val-ues that lie on a loading curve, which suggests that the abnormal pressures in the rocks were produced by diquilibrium compaction. Data from Bowers (1994) plus our unpublished data.
