Episodic compression-driven fluid venting in layered sedimentary basins Luke M. Kearney1 Christopher W. MacMinn2 Richard F. Katz1 Chris Kirkham1 and

2025-04-29 1 0 1.02MB 28 页 10玖币
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Episodic, compression-driven fluid venting in layered
sedimentary basins
Luke M. Kearney1, Christopher W. MacMinn2, Richard F. Katz1, Chris Kirkham1, and
Joe Cartwright1
1Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, United Kingdom
2Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, United Kingdom
ABSTRACT
Fluid venting phenomena are prevalent in sedimentary basins globally. Offshore, these localised fluid-expulsion
events are archived in the geologic record by the resulting pockmarks at the sea-floor. Venting is widely inter-
preted to occur via hydraulic fracturing, which requires near-lithostatic pore pressures for initiation. One common
driver for these extreme pressures is horizontal tectonic compression, which pressurises the entire sedimentary
column over a wide region. Fluid expulsion leads to a sudden, local relief of this pressure, which then gradually
recharges through continued compression, leading to episodic venting. Pressure recharge will also occur through
pressure diffusion from neighbouring regions that remain pressurised, but the combined role of compression and
pressure diffusion in episodic venting has not previously been considered. Here, we develop a novel poroelastic
model for episodic, compression-driven venting. We show that compression and pressure diffusion together set
the resulting venting period. We derive a simple analytical expression for this venting period, demonstrating
that pressure diffusion can significantly reduce the venting period associated with a given rate of compression.
Our expression allows this rate of compression to be inferred from observations of episodic venting. We conclude
that pressure diffusion is a major contributor to episodic fluid venting in mudstone-dominated basins.
1. INTRODUCTION
Fluid venting phenomena have been frequently observed in sedimentary basins since the advent of 3D seismic
imaging (Løseth et al. 2001,Hovland & Judd 1988). The vents themselves are localised, typically comprising
cylindrical conduits known as fluid-escape pipes that can create pockmarks or feed effusive mud volcanoes (Huuse
et al. 2010,Cartwright & Santamarina 2015). Venting is thought to initiate when the seal of a pressurised
reservoir fails through hydraulic fracturing, creating a high-permeability pathway for the transport of basinal
fluids through kilometres of low-permeability rock (Cartwright et al. 2021). This mode of seal failure poses clear
risks for the subsurface storage of hydrogen and the long-term sequestration of anthropogenic waste such as
carbon dioxide (CO2). Indeed, unexpected vertical fluid migration at the Sleipner CO2storage pilot site is likely
due to exploitation of pre-existing conduits (Arts et al. 2004,Cavanagh & Haszeldine 2014).
Repeated fluid venting from a fixed locus has been documented in a subset of cases (Deville et al. 2010,
Andresen & Huuse 2011,Cartwright et al. 2018,Oppo et al. 2021,Kirkham et al. 2022). In each of these cases,
venting occurs in discrete episodes of fluid expulsion separated by long quiescent periods. In the North Levant
Basin, located in the Eastern Mediterranean, the presence of a flowing salt sheet enables dating of individual
venting episodes (Oppo et al. 2021,Evans et al. 2020,Cartwright et al. 2021), thus providing a robust basis
for investigating episodic fluid venting. More than 300 fluid escape pipes record episodic venting through this
∼1.5 km-thick layer of low-permeability salt. The salt overlies a ∼3 km-thick clastic succession dominated
by mudstone. These fluid-escape pipes are interpreted to form vertically from the crests of folded sandstone
reservoirs, terminating at the seafloor as pockmarks. Viscous flow of the salt layer deforms the relic pipes over
geological time, such that repeated venting leads to a linear trail of pockmarks along the direction of salt flow
(Cartwright et al. 2018). Thirteen pockmark trails have been observed across the North Levant Basin, each
recording up to 45 venting episodes since ∼2 Ma (Oppo et al. 2021). Dating of these venting episodes reveals
1
arXiv:2210.03654v2 [physics.geo-ph] 31 May 2023
a typical time interval between episodes (i.e., venting period) of ∼100 kyr (Oppo et al. 2021,Evans et al. 2020,
Cartwright et al. 2021).
The initiation of a vent via hydraulic fracturing requires fluid pressure in excess of the minimum horizontal
compressive stress (Price & Cosgrove 1990,Scandella et al. 2011). Once initiated, venting continues until this
overpressure is sufficiently relieved that the pathway closes. Subsequently, during quiescence, fractures may
self-heal by solid creep, swelling, and mineral precipitation (Bock et al. 2010,Chen et al. 2013). In the North
Levant Basin, previous pathways are deformed and advected away from their original trajectory by salt flow.
Hence, episodic venting requires the repeated recharge of overpressure to the original point of failure, implying
that the overpressure mechanism remains active across venting episodes. The disparity between the rapid drop
in pressure during venting and the slow growth of pressure during recharge suggests that the time-history of
reservoir pressure across multiple episodes resembles a sawtooth pattern, with the up-slope representing the rate
of pressure recharge and the amplitude representing the pressure drop during venting (Cartwright et al. 2021).
Cartwright et al. (2021) attribute this pressure drop to be the tensile strength of the sealing rock, estimated to
range from 0.6 MPa to 2 MPa. Using the sawtooth concept and the measured period between venting episodes,
Cartwright et al. (2021) inferred a rate of pressure recharge in the North Levant Basin of ∼9 MPa/Myr.
Overpressure can be generated by various mechanisms (Osborne & Swarbrick 1997). For the Levant basin,
Cartwright et al. (2021) ascribe overpressure generation to regional tectonic compression on the basis of qualita-
tive physical arguments. Previous studies have used numerical models to predict overpressures due to tectonic
compression and quantify the role of factors such as duration and rate of shortening (Luo 2004,Obradors-Prats
et al. 2017a,b,Maghous et al. 2014). For example, Obradors-Prats et al. (2017a) showed that an overpressure
of ∼10 MPa can be generated by a shortening of ∼10% over a period of ∼100 kyr. However, overpressure will
typically be heterogeneously distributed throughout the sedimentary column. Ge & Garven (1992) showed that
tectonic compression pressurises stratigraphic layers at different rates due to their different elastic properties.
These pressure differences equilibrate over time through vertical fluid redistribution, which can be described
mathematically as the diffusion of pressure.
Pressure diffusion between sedimentary layers has been investigated in many previous works (e.g., Muggeridge
et al. 2004,2005, and refs. therein). The primary concern of these studies has been to estimate the timescales and
mechanisms of pressure redistribution through low-permeability layers. As a result, these studies typically focus
on the diffusive equilibration of an initially non-hydrostatic pressure distribution while neglecting the origin of
that distribution or any ongoing sources of overpressure generation. This omission may not always be justified,
given that mechanisms such as tectonic compression persist for millions of years and are physically independent
of pressure redistribution. Moreover, these studies generally neglect punctuated effects that modify the pressure,
such as venting. An exception is Luo & Vasseur (2016), who investigated mechanisms of pressure dissipation
including hydraulic fracturing.
Despite the large body of relevant work, most previous studies have neglected at least one of the three key
components of episodic venting: pressure build-up, pressure diffusion, and hydraulic fracturing. The studies
that include all three components predict episodic venting. However, these models incorporate a variety of
additional physics such as reaction and heat transport, necessitating numerical solution (Dewers & Ortoleva
1994,L’Heureux & Fowler 2000). The complexity and computational expense of these models limits them to
generating a small set of results for a specific setting, making it difficult to develop more general insight. Such
insight is facilitated by a simplified theory that incorporates only the physical processes needed to describe the
general, episodic dynamics of fluid venting in sedimentary basins. Moreover, measurements of the venting period
are readily interpreted in this analytical context.
Here we develop a poroelastic model of tectonic overpressure generation, diffusive pressure redistribution, and
fluid venting in layered sedimentary basins. We derive analytical solutions that elucidate the associated pressure
dynamics and the parametric controls on venting. We show in particular that the venting period τis given by
τ∝(∆P/ ˙exx)/(1 + ν/γ), where ∆Pis the pressure drop from each venting event, ˙exx is the horizontal strain
rate due to tectonic compression and νand γare dimensionless parameters that are defined below. The quantity
∆P/ ˙exx is proportional to the venting period in the absence of pressure diffusion, as estimated by Cartwright
et al. (2021). We refer to the dimensionless quantity (1 + ν/γ)as the venting frequency multiplier because it
reduces the venting period relative to the compression-only case. We show that this frequency multiplier can
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be estimated using the thickness ratio of the mudstone and sandstone layers. In mudstone-dominated basins
where fluid venting phenomena are commonly observed (Cartwright & Santamarina 2015), pressure recharge and
venting period are controlled by pressure diffusion.
The remainder of the manuscript is organised as follows. In §2.1, we derive and solve the poroelastic equations
governing tectonic compression of, and pressure diffusion between, sedimentary layers in the absence of fluid
venting. In §2.2, we explore the response of the system to fluid venting without compression. In §2.3, we
combine solutions from §2.1 and §2.2 to obtain a full model for episodic venting; we then derive analytical
solutions for periodic venting. In §3, we discuss the wider implications of this work, as well as limitations and
potential generalisations of the model. In §4, we conclude with a summary and suggestions for promising avenues
of future work.
2. MODEL
2.1 Compression
We consider two horizontal layers of rock, a sandstone with thickness hsoverlying a mudstone with thickness hm,
as illustrated in Figure 1a. To focus on large-scale pressurisation from regional tectonic compression, we assume
that these layers have a large lateral extent, such that pressure diffusion occurs exclusively through vertical fluid
migration. The development of large overpressure from tectonic compression thus requires that compression be
rapid relative to pressure diffusion and/or that vertical flow be obstructed.
Sedimentary basins are typically underlain by dense basement rock, so we assume the existence of an im-
permeable layer below the mudstone. Furthermore, motivated by sedimentary basins such as the Levant basin
that are capped with an extensive, thick salt layer, we apply the same assumption above the sandstone. Salt is
considered to be impermeable on geological timescales (though see Ghanbarzadeh et al. 2015). This model con-
figuration prohibits vertical pressure diffusion across the salt, but allows for sudden fluid expulsion via hydraulic
fracturing. The theory below could be generalised to allow for a ‘seal’ with a small but nonzero permeability.
As in previous studies that consider pressure diffusion between sedimentary layers, we assume that flow is
single-phase, isothermal and one-dimensional (Bredehoeft & Hanshaw 1968,Neuzil 1986,Luo & Vasseur 1997,
Muggeridge et al. 2004). Crucially, we deviate from these previous studies by modelling the evolution of pressure
due to ongoing (rather than historical) tectonic compression. Tectonic compression has been conceptualised
as a bulldozer imparting sufficient differential stress to deform weaker sediments (Byrne et al. 1993). It has
been modelled mathematically as an imposed, constant horizontal strain rate; strain rates are routinely used to
quantify tectonic deformation (e.g. Kahle et al. 1998,Kreemer et al. 2014). We denote the imposed strain rate
as −˙exx, such that a positive value of ˙exx indicates shortening. In the absence of venting, tectonic shortening is
accommodated through compression of the pore fluid and/or of the solid grains. We refer to this specific process
as tectonic compression.
˙exx z
x
mudstone, m
sandstone, s
impermeable layer (e.g. salt)
impermeable layer (e.g. basement)
hs
hm
(b)
pressure
hydrostatic
lithostatic
pore
P
z
(c)
overpressure
p=P−Phyd
z
Figure 1: Schematic cross-section of sedimentary-basin model. (a) We consider the tectonic compression of two
permeable sedimentary layers, sandstone (yellow) and mudstone (brown), at a constant horizontal strain rate
−˙exx. The system is sealed by impermeable layers above the sandstone and below the mudstone. (b) Example
pressure–depth plot showing the hydrostatic pressure (blue), pore pressure resulting from compression (cyan)
and lithostatic stress (black). (c) As in (b), but now showing the lithostatic stress in excess of hydrostatic (black)
and the pore overpressure p(cyan), which is the pressure in excess of hydrostatic Phyd.
3
Assuming that the solid skeleton obeys linear elasticity and adopting the sign convention that tension is
positive, the effective stress tensor σ′is related to the strain tensor evia
σ′=λtr (e)I+ 2µe,(1)
where λand µare the drained Lamé parameters and e≡1
2[∇u+(∇u)T]with udenoting the solid displacement.
The effective stress is related to the total stress σand pore pressure Pby Terzaghi’s principle,
σ=σ′−αP I,(2)
where αis Biot’s coefficient. Mass conservation leads to the storage equation (Verruijt 1969, see Supplementary
Material S1), which is equivalent to the classical continuity equation presented by Biot (1941),
α∂e
∂t +S∂P
∂t =−∇·q,(3)
where e≡tr (e)is the volumetric strain, qis the Darcy flux of fluid through the pore space and S≡ϕcℓ+(α−ϕ)cg
is the storativity, with porosity ϕ, fluid compressibility cℓand grain compressibility cg. The time derivative of
the xx-component of Equation (1) implies that
∂σ′
xx
∂t =λ∂e
∂t −2µ˙exx,(4)
and the trace of Equation (1) implies that (3λ+ 2µ)e=tr (σ′). From these results and Equation (2), we arrive
at ∂P
∂t =λ+µ
α
∂e
∂t +µ
α˙exx −1
2α∂σyy
∂t +∂σzz
∂t .(5)
Equation (5) describes the evolution of pore pressure in response to changes in strain and total stress. The total
vertical stress at a fixed depth can increase (∂tσzz <0) in response to folding and thrust faulting, which is a
common consequence of tectonic shortening. In the North Levant Basin, folding and thrusting leads to localised
salt thinning, resulting in a slowly decreasing total vertical stress (Cartwright et al. 2021). For simplicity, we
neglect this minor effect by assuming that ∂tσzz = 0. The evolution of the total horizontal stress in the orthogonal
direction, ∂tσyy is less clear. Two endmember assumptions are that the orthogonal total stress remains constant
(∂tσyy = 0), or that the associated strain remains constant (∂teyy = 0). The former unconditionally allows for
hydraulic fracturing, whereas the latter may not in some cases (Supplementary Material S2). For simplicity, we
proceed with the former assumption and take σyy to be constant. Combining these assumptions with Eqs. (3)
and (5) gives
∂P
∂t =αµ ˙exx
α2+S(λ+µ)−λ+µ
α2+S(λ+µ)∇·q.(6)
Thus, two processes drive changes in pore pressure: compression and fluid flow. The first term in Eq. (6)
corresponds to compression, which acts to increase pressure everywhere at a rate determined by the compression
rate and the poroelastic properties of the medium. The second term in Eq. (6) demonstrates that pressure
increase at a point is impeded by a net export of fluid (∇·q>0) or enhanced by a net import (∇·q<0).
For a system that is both laterally extensive and laterally homogeneous (i.e., no variations in xor y; Fig. 1),
fluid flow is limited to the vertical direction, q≡qˆ
z. Then, applying Eq. (6) to each layer,
∂Ps
∂t =αsµs˙exx
α2
s+Ss(λs+µs)−λs+µs
α2
s+Ss(λs+µs)
∂q
∂z for z∈[−hs,0],(7a)
∂Pm
∂t =αmµm˙exx
α2
m+Sm(λm+µm)−λm+µm
α2
m+Sm(λm+µm)
∂q
∂z for z∈[0, hm].(7b)
where the subscripts sand mrepresent properties of the sandstone and of the mudstone, respectively. In this
system, the hydrostatic contribution to the pressure remains constant. It has no effect on the dynamics and
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hence we replace total pressure Pwith overpressure p. The overpressure is the pressure in excess of hydrostatic,
p≡P−Phyd (see Fig. 1b,c),
∂ps
∂t =αsµs˙exx
α2
s+Ss(λs+µs)−λs+µs
α2
s+Ss(λs+µs)
∂q
∂z for z∈[−hs,0],(8a)
∂pm
∂t =αmµm˙exx
α2
m+Sm(λm+µm)−λm+µm
α2
m+Sm(λm+µm)
∂q
∂z for z∈[0, hm].(8b)
Sandstones typically have permeabilities that are many orders of magnitude larger than those of mudstones.
Consequently, pressure diffuses much faster in sandstone than in mudstone. Hence over timescales of pressure
diffusion in the mudstone, the overpressure in the sandstone is approximately vertically uniform. Considering
this, we integrate Equation (8a) over the thickness of the sandstone,
dps
dt=αsµs˙exx
α2
s+Ss(λs+µs)−λs+µs
α2
s+Ss(λs+µs)
q(0, t)−q(−hs, t)
hs
,(9)
where psis the depth-averaged overpressure in the sandstone. Therefore, the rate of change of the average
overpressure in the sandstone is given by a term from compression and a term from the difference in flux across
the boundaries. The sandstone is overlain by an impermeable layer so q(−hs, t)=0. Since the overpressure in
the sandstone is approximately uniform, we assume ps=psand obtain
dps
dt=αsµs˙exx
α2
s+Ss(λs+µs)−λs+µs
α2
s+Ss(λs+µs)
q(0, t)
hs
,(10)
where the flux out of the sandstone q(0, t)must be equal to the flux into the top of the mudstone (in the absence
of venting). Fluid transport in the mudstone is governed by Darcy’s law,
q=−km
η
∂pm
∂z ,(11)
where ηis the viscosity of the fluid and kmis the permeability of the mudstone, both assumed to be constant.
Equations (8b), (10) and (11) combine to form a coupled system for the overpressure of each layer,
dps
dt=αsµs˙exx
α2
s+Ss(λs+µs)+λs+µs
α2
s+Ss(λs+µs)
1
hs
km
η
∂pm
∂z
z=0
at z= 0,(12a)
∂pm
∂t =αmµm˙exx
α2
m+Sm(λm+µm)+λm+µm
α2
m+Sm(λm+µm)
km
η
∂2pm
∂z2for z∈[0, hm].(12b)
Equation (12a) is an ordinary differential equation for the time-evolution of the overpressure in the sandstone.
Equation (12b) is a partial differential equation for the overpressure in the mudstone in depth and time. The
latter requires two boundary conditions. The first boundary condition is that the sandstone and mudstone
overpressures must match at the contact, pm(0, t) = ps. The second boundary condition is that there is no
fluid flux through the impermeable layer at the bottom of the mudstone ∂pm/∂z|hm= 0. Equations (12) are
simplified by introducing the following parameters,
Dm=km
η
λm+µm
α2
m+Sm(λm+µm), Ds=km
η
λs+µs
α2
s+Ss(λs+µs),(13)
Γm=αmµm˙exx
α2
m+Sm(λm+µm),Γs=αsµs˙exx
α2
s+Ss(λs+µs),(14)
The parameters Γmand Γsrepresent the rates of pressure build-up in the mudstone and sandstone layers,
respectively, due to compression. The parameter Dmrepresents the diffusivity of pressure across the mudstone
whereas the parameter Dsrepresents the diffusivity of pressure across the sandstone–mudstone boundary. The
5
摘要：

Episodic,compression-drivenfluidventinginlayeredsedimentarybasinsLukeM.Kearney1,ChristopherW.MacMinn2,RichardF.Katz1,ChrisKirkham1,andJoeCartwright11DepartmentofEarthSciences,UniversityofOxford,OxfordOX13AN,UnitedKingdom2DepartmentofEngineeringScience,UniversityofOxford,OxfordOX13PJ,UnitedKingdomABS...

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