Energy Procedia 00 (2008) 000–000

Energy Procedia

www.elsevier.com/locate/XXX

GHGT-9

A global assessment of deep-sea basalt sites for carbon

sequestration

David Goldberga* and Angela L. Slagle

a

aLamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA

Elsevier use only: Received date here; revised date here; accepted date here

Abstract

Deep-sea basalt aquifers have been identified for geological CO2 sequestration in worldwide locations and offer vast reservoir

capacities, permanent geochemical trapping potential, and other deep-sea protections that significantly reduce the risk of post-

injection leakage. Using site-specific criteria, we target basalt aquifers along seismic and aseismic oceanic ridges, and calculate

the injection volume potential for each. Results indicate that these aquifers occur in many oceans and could sequester 2.3–11.5

Tt-C along seismic ridges and 5.9–29.6 Tt-C along aseismic ridges. The enormous size of these potential reservoirs warrants

further research and pilot experiments in each geographic setting.

© 2008 Elsevier Ltd. All rights reserved

Keywords: ocean crust; CO2 storage; worldwide reservoirs; climate mitigation; fossil fuel emissions; energy

1. Introduction

The debate over the most effective means to stabilize atmospheric greenhouse gas concentrations has supported

the need for multiple approaches that require a diverse range of technologies [1-4]. The effectiveness of these

methods for CO2 sequestration depends strongly on their storage capacity, retention time, stability, and risk for

leakage [5-7]. Goldberg et al. [8] present a solution for geological CO2 sequestration in deep-sea basalt formations,

an option that offers permanent geochemical trapping of CO2 as stable, benign carbonates in a well-sealed reservoir,

and multiple, low-risk protections against post-injection leakage. Very large reservoir capacities were identified in

deep-sea basalt aquifers along the eastern ridge flank of the Juan de Fuca plate [8]. In this paper, we identify deep-

sea basalt reservoirs with similar characteristics located along other oceanic ridge flanks around the globe and

estimate their storage potential for anthropogenic CO2. We consider locations along the flanks of both seismic and

aseismic oceanic ridges (i.e., arising from seafloor spreading or mid-plate volcanism, respectively) and use site-

specific criteria such as the abundance of scientific data available (e.g., drill sites with basement penetration,

permeability and/or porosity data) to evaluate each potential target region.

* Corresponding author. Tel.: +1-845-365-8674; fax: +1-845-365-3182.

E-mail address: goldberg@ldeo.columbia.edu.

Available online at www.sciencedirect.com

c© 2009 Elsevier Ltd. All rights reserved.

Energy Procedia 1 (2009) 3675–3682www.elsevier.com/locate/procedia

doi:10.1016/j.egypro.2009.02.165

2 Goldberg/ Energy Procedia 00 (2008) 000–000

Deep-sea basalt erupts from volcanic ridges across all the world’s oceans, forming pillow lavas and lava flows on

the seafloor, and consequently, large void spaces in between them. These formations are buried over time, producing

highly permeable aquifers within the oceanic crust [9]. The advantages of these deep-sea basalt aquifers for

geological CO2 sequestration arise from providing multiple physical/chemical trapping mechanisms for CO2 injected

into the subsurface. The primary trapping mechanism involves low-permeability marine sediments that

stratigraphically seal and physically impede the migration of injected CO2 to the surface. Deep marine sediments

consist of fine-grained and impermeable calcareous oozes and clays that effectively hold fluids within underlying

aquifers over broad flanks of an oceanic ridge [e.g., 10-11]. Liquid CO2 injected into aquifers below these sediments

will be physically trapped for long periods of time. Another important trapping mechanism is geochemical or

mineral trapping, which involves reactions of CO2 with basalt to form stable carbonate minerals under in situ

conditions [12-16]. In nature, such mineralization of host rocks occurs in a variety of oceanic environments, such as

hydrothermal alteration at volcanic hot springs [17], deep-ocean vent systems [18]; and through surface weathering

[19]. Indeed, chemical weathering of basalt occurs naturally over time as the oceanic crust ages [20], permanently

depositing carbon and other infilling minerals within open pores and void spaces over millions of years [21]. Thus,

deep-sea basalt acts as a natural, in situ weathering reactor. If sufficiently open and permeable basalt is identified

below well-sealing marine sediments, then CO2 injected into these rocks would similarly be sequestered in the form

of thermodynamically stable and environmentally benign minerals [22].

The rates of these reactions depend on factors such as surface area of the rock, temperature and chemical

composition, and pH of aquifer fluids. The rate of carbonate mineral formation is known to be fast [15], so the

formation of new carbonate will depend on the release rate of ions from the dissolution of host minerals [16]. It is

likely that the CO2-basalt reaction rate will progressively decrease to a finite limit and ultimately change the

porosity and permeability within the basalt aquifer. Estimating these in situ reaction rates and accelerated effects of

carbonate precipitation will require deep-sea injection tests. Regardless, geochemical CO2-basalt processes will

proceed while CO2 is still securely trapped by physical as well as additional, secondary trapping mechanisms.

The secondary mechanisms include both gravitational trapping and hydrate trapping, which occur as a result of

physical changes in the properties of liquid CO2 when injected into the deep-sea environment. Due to the high

compressibility of liquid CO2 relative to water, CO2 becomes denser than water at high pressures and low

temperatures [11, 23-24]. Gravitational trapping occurs at ocean water depths 2700 m, where injected CO 2 is

denser than typical seawater, causing it to sink. Also, the formation of CO2 hydrate, which is a crystalline water-

based solid resembling ice, is denser and less soluble than liquid CO2 in water below 2°C [25] and thus stable below

the seabed in deep-ocean sediments [11]. Figure 1 illustrates all of the concurrent trapping mechanisms affecting

CO2 injection in a deep-sea basalt aquifer. In addition to the primary mechanisms, the secondary ones combine to

overlap in time and enhance the security of the geological storage reservoir. For deep-sea basalt aquifers that meet

Figure 1. Storage security and permanence of CO2 trapping (modified from IPCC [5]). Note uncertainty of time scales for shaded areas.

3676 D. Goldberg, A.L. Slagle / Energy Procedia 1 (2009) 3675–3682

Goldberg/ Energy Procedia 00 (2008) 000–000 3

our assessment criteria, these mechanisms provide multiple independent protective barriers that could safely isolate

the oceans and atmosphere from CO2 leakage for long periods of time.

2. Global assessment of oceanic ridges

We assess the presence of deep-sea basalt aquifers around the global ocean using four criteria: (a) the presence of

an oceanic ridge formed by seafloor spreading (seismic ridges) or by intra-plate volcanism (aseismic ridges); (b)

significant porosity in basalt aquifers (crustal ages <15 Ma for seismic ridges) and within interflow zones (for

aseimsic ridges); (c) sediment thickness of 200 m or more covering basalt crust, and (d) water depth of 2700 m or

greater at the seafloor. These criteria provide broad constraints where physical, geochemical, as well as secondary

trapping mechanisms will operate concurrently. The basalt itself provides an existing aquifer; we use published data

to estimate porosity and available reservoir capacity.

The physical properties of deep-sea basalt at mid-ocean spreading (seismic) ridges, where new ocean crust is

formed, have been well studied through geophysical and bathymetric surveys as well as through scientific drilling

and sampling [e.g., 20, 26]. The flanks of such mid-ocean ridges are known to contain highly fractured and

permeable basalt layers [e.g., 9, 27]. The natural processes of geochemical weathering in the ocean crust as it ages

reduces the initial porosity by carbonate and other infilling mineral precipitation over millions of years [21, 28].

Drilling and seismic evidence indicates that the crust retains roughly half of its initial porosity even after 15 million

years [29]. For this study, we select eleven spreading (seismic) ridge sites along segments of the global mid-ocean

ridge system in the Atlantic, Pacific, and Indian oceans (Fig. 2, green boxes). Spreading rates along these ridges

range from slow to fast spreading [30], and using these, we constrain crustal age to <15 Ma for potential reservoirs.

Deep-sea basalt also forms on the seafloor due to intra-plate volcanism (often called Large Igneous Provinces, or

LIPs) and here referred to as aseismic ridges. These are massive crystal emplacements of intrusive and extrusive

basalt that originate by processes other than seafloor spreading, including continental flood basalts, volcanic passive

margins, ocean plateaus, submarine ridges, and ocean basin flood basalts. Aseismic ridges are the most significant

global accumulations of basalt after those emplaced at seismic ridges, although their formation processes and

Figure 2. World map indicating seismic (green boxes) and aseismic ridges (blue boxes) considered in this study. Note that MAR = Mid-Atlantic

Ridge; EPR = East Pacific Rise; SWIR = Southwest Indian Ridge.

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4 Goldberg/ Energy Procedia 00 (2008) 000–000

physical properties are not as well understood [31]. McGrail et al. [15] suggest that the interbedded layers within

dense low-permeability flood basalts make them a potentially important resource for CO2 sequestration. High

porosity has been measured over thick sequences of such interbedded basalt layers on land [15, 32], although

considerably less information is available from drilling in oceanic flood basalts [31]. For this assessment, we select

seven aseismic ridges in the Atlantic, Pacific, and Indian oceans (Fig. 2, blue boxes).

The same constraints satisfying our assessment criteria are applied to both seismic and aseismic ridges, and thus,

the same concurrent trapping mechanisms apply to all sites. We analyze global sediment thickness and bathymetry

data at each seismic and aseismic ridge site using a Geographic Information System (GIS). The distribution of

sediments in the oceans is controlled by several factors, including age of the underlying oceanic crust, tectonic

history of the crust, structural trends in the basement, the nature and location of sediment sources, and the nature of

sedimentary processes delivering sediments [e.g., 33]. In general, the thickest sediments in the ocean are focused

around continental margins and at the equator. Sediment thickness data was obtained from a digital total sediment

thickness database compiled by the National Geophysical Data Center [NGDC; 34]. This database includes

information from three principle sources: previously published isopach maps, ocean drilling results, and seismic

reflection profiles archived at NGDC. NGDC data are gridded with a grid spacing of 5 arc-minutes by 5 arc-minutes

and provide a minimum value for the thickness of sediment in a particular geographic region. The Marine

Geoscience Data System (MGDS) provided ocean bathymetry, synthesized from publicly available multibeam

bathymetry data (www.marine-geo.org). For oceanic areas, MGDS merges multibeam bathymetry data with regional

lower resolution compilations, including the predicted topography of Smith and Sandwell [35]; land topography data

comes from the NASA Space Shuttle Radar Topography Mission (http://srtm.usgs.gov). We created a grid from the

global multi-resolution digital elevation model, including both low-resolution regional data and multibeam

bathymetry where available. Using the GIS, we query these data to isolate, visualize, and quantify the spatial extent

in each region that meets the necessary sediment thickness and water depth constraints. Two examples of this

analysis are presented below.

3. Seismic ridge example: Juan de Fuca Ridge

The Juan de Fuca Ridge region, a few hundred kilometers off the coast of California, Oregon and Washington in

the Pacific Ocean, is perhaps the most-studied seismic ridge in the world (Fig. 3). The flanks of the ridge are known

to contain highly fractured, channelized, and high porosity (10-15%) basalt that is covered by fine-grained turbidites

Figure 3. Example of aquifer assessment (red shading) along the east and west flanks of the Juan de Fuca Ridge (seismic). Modified from

Goldberg et al. [8]. Drilling locations shown by black dots.

3678 D. Goldberg, A.L. Slagle / Energy Procedia 1 (2009) 3675–3682

Goldberg/ Energy Procedia 00 (2008) 000–000 5

and hemipelagic clay sediments [36]. Geophysical logs and cores from the region confirm the presence of high-

angle fractures and void spaces within pillow lavas and fractured zones, alternating with massive basalt flows [8-9,

37]. Interconnectivity of such porous features yields high bulk permeability values over wide areas in this region

[32], providing considerable capacity for CO2 injection and storage.

Hydrologic studies of the Juan de Fuca region suggest vigorous hydrothermal circulation in the basement rocks

and bulk permeability estimates in the shallow crust range from 10-9

to 10-13

m2, decreasing with depth [37]. Recent

cross-hole flow experiments in basalt basement estimate lateral bulk permeability to be 10-12

m2, an order of

magnitude lower than previous estimates, which has been attributed to basement anisotropy [38]. Coupled thermal

models reproduce the overall temperature and pressure observations, with high-permeability flow channels with

lateral flow rates up to 40 m/year and the requirement that one-sixth of the ocean crust be permeable to 600 m

thickness [39]. Goldberg et al. [8] suggest that >200 Gt-C could potentially be sequestered in these basalt flow

channels on the eastern flank of the Juan de Fuca Ridge alone. Considering both eastern and western flanks

constrained by the same crustal ages, the Juan de Fuca Ridge storage region would be considerably larger (Fig. 3).

4. Aseismic ridge example: Walvis Ridge

Walvis Ridge, an aseismic ridge located in the eastern South Atlantic Ocean, has been drilled several times and is

a northeast-southwest trending feature that divides the eastern South Atlantic into two ocean basins, the Angola

Basin to the north and the Cape Basin to the south (Fig. 4). Magnetic and gravity data indicate that the Walvis Ridge

was formed by hotspot volcanism near the spreading ridge as the Atlantic basin gradually widened [26]. Pelagic

sediments drape most of the ridge, increasing toward the continental margin [40]. Geophysical well logs and cores

from the Deep Sea Drilling Project indicate that basement rocks of the Walvis Ridge consist of basalts and

interlayered sediments [41]. Drilling data also suggests that many open fractures and cavities are present in the

basement [41]. Seismic results [41] indicate a basement structure similar to the ocean basins (seismic ridges), which

suggests that they may also have similar permeability and porosity values. In this study, we conservatively estimate

that 10% effective porosity is present over interflow layers in basalts along aseismic ridges, such as the Walvis

Ridge. Considering both flanks of these ridges, and constrained by sediment thickness and water depth, we map

areas of basalt aquifers that could offer potential storage reservoirs (Fig. 4). It is important to note, however, that the

lateral boundaries of these areas are not constrained by crustal age, leading to larger uncertainty in the volume

estimates for these reservoirs than for seismic ridge basalts.

Figure 4. Example of aquifer assessment (red shading) along the Walvis Ridge (aseismic). Drilling locations shown by black dots. Note that

northern and eastern boundaries of aquifer are not well-constrained by bathymetry or sediment cover.

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Table 1. Volume estimates of carbon storage potential for deep-sea basalt aquifers

Regiona Area (km

2)

b 20 m net thickness

c 100 m net thickness

c Carbon (Gt)

Juan de Fuca 244,963 490 2,450 134 – 668

East Pacific Rise 491,561 983 4,916 268 – 1,340

Cocos Ridge 891,510 1,783 8,915 486 – 2,431

Mid-Atlantic Ridge 60N 52,734 105 527 29 – 144

Mid-Atlantic Ridge 40N 139,141 278 1,391 76 – 379

Mid-Atlantic Ridge 0 220,842 442 2,208 120 – 602

Mid-Atlantic Ridge 40S 3,778 8 38 2 – 10

Southwest Indian Ridge 5E 81,329 163 813 44 – 222

Southwest Indian Ridge 45E 15,757 32 158 9 – 43

Gulf of Aden 30,763 62 308 17 – 84

Southeast Indian Ridge 2,068,304 4,137 20,683 1,128 – 5,640

Caribbean flood basalt 541,611 1,083 5,416 295 – 1,477

Rio Grande Rise 831,818* 1,664 8,318 454 – 2,268

Walvis Ridge 1,933,711* 3,867 19,337 1,055 – 5,273

Kerala Basin 952,761 1,906 9,528 520 – 2,598

Ninetyeast Ridge/Broken Ridge 2,198,757 4,398 21,988 1,199 – 5,996

Kerguelen Plateau 1,788,834* 3,578 17,888 976 – 4,878

Ontong Java Plateau 2,616,646* 5,233 26,166 1,427 – 7,136

Total Seismic Sites 2,313 – 11,564 Gt

Total Aseismic Sites 5,925 – 29,627 Gt

a Above the line are seismic ridges; below are aseismic ridges (see Fig. 2). b Asterisks indicate uncertain regional boundaries. c These calculations assume 10% average aquifer porosity. Total for All Sites 8,238 – 41,191 Gt

5. Estimating CO2 storage volume

Using our four geological constraints (a-d, Section 2) at each location identified in Figure 2, we compute the

potential carbon storage capacity for deep-sea basalt reservoirs along both seismic and aseismic ridges. The area

(km2) in each region is calculated to meet the water depth and sediment thickness constraints, as illustrated in

Figures 3 and 4. We additionally restrict our analysis in each region to avoid natural areas of fluid inflow/outflow

between the ocean and underlying basement rocks, avoiding areas within 20 km of all plate boundaries, to further

protect against the possibility of long-term CO2 leakage to the seafloor. We use 10% as a conservative estimate for

average porosity persisting in basalt aquifers, based on observations of in situ density and porosity logs from ocean

drilling sites [e.g., 14]. Then, the net aquifer thickness in ocean basalt crust is estimated to be between 20 and 100 m

over the upper few hundred meters of sediment-covered basement. These ranges define the volume available for

CO2 storage, and hence we compute a minimum and maximum value for net reservoir thicknesses, respectively. The

range of outcomes accommodates site-specific variability in the basalt structure for both seismic and aseismic ridge

types. Assuming that liquefied CO2 (CO2 density ~1 g/cm3, or 0.27 g C/cm

3) is injected to fill the aquifer volume,

and also, that all of the CO2 is eventually fixed as carbonate (CaCO3 density ~2.7 g/cm3, or 0.36 g C/cm

3), we

compute a minimum/maximum range of carbon storage capacity in each potential area. Table 1 summarizes these

computations.

The total volume potential for the eastern and western flanks of the Juan de Fuca Ridge (see Fig. 3) ranges from

134 to 668 Gt-C, representing minimum and maximum net reservoir thicknesses (20 m and 100 m, respectively).

Observations and modeling of basalt aquifers along the Juan de Fuca Ridge flanks support the higher volume

estimate [8, 39]. The total volume potential along the flanks of the Walvis Ridge (see Fig. 4) ranges from 1,055 to

5,273 Gt-C, for the same thickness estimates. Because data and observations along aseismic ridges are sparse, the

minimum thickness yields a more conservative, and potentially realistic, estimate for this location. Along seismic

ridges, the total volume potential for deep-sea basalt aquifers ranges from 2.3–11.5 Tt-C; along aseismic ridges, the

total ranges from 5.9–29.6 Tt-C. Note, however, that although the volume range for aseismic ridges is more than

twice that for seismic ridges, the aseismic area boundaries are not nearly as well-constrained and drilling data is

more sparse, which leads to greater uncertainty inherent in their volume calculations. Given these uncertainties, the

worldwide total volume potential ranges from 8.2 to 41.2 Tt-C – an enormous volume, even assuming the minimum

reservoir thickness estimates.

3680 D. Goldberg, A.L. Slagle / Energy Procedia 1 (2009) 3675–3682

Goldberg/ Energy Procedia 00 (2008) 000–000 7

6. Discussion

The technical challenges of utilizing any of these vast reservoirs for sequestration, such as pipeline transport, sub-

sea maintenance, and post-injection monitoring systems, must be quantified and evaluated. Certainly the proximity

of possible locations to accessible ports, pipelines, and potential sources of CO2 will affect their relative costs and

benefits. Our plans for future analyses will consider the practicalities of specific deep-sea reservoirs, such as those

closer to the continents and those nearer to regional CO2 sources. To begin that analysis, regional technical and

economic assessments are required, in addition to more detailed and site-specific geological research. For some

deep-sea locations, these assessments may conclude that it is acceptable to relax one or more of the geological

constraints used in this study, such as the minimum marine sediment thickness or minimum ocean water depth, as a

trade-off for a greater technical or economic advantage. For example, basalt aquifers on ridge flanks in the Gulf of

Aden could provide a storage location in close proximity to Persian Gulf CO2 sources. Table 1 indicates that the

Gulf of Aden area could support sequestration of 17-84 Gt-C under all the proposed geological constraints. If

thinner sediment cover (as little as 100 m) were considered, however, the volume of these basalt aquifers would

increase by a factor of two. Or, if shallower ocean depths (< 2700 m) were considered, the volume of these basalt

aquifers and their sequestration potential would increase by a factor of about nine. In such cases, additional risks

would result from fewer active trapping mechanisms (e.g., no gravitational trapping in shallower waters) or from

less sediment cover, however the value of avoiding such risks remains to be established. The long-term benefits of

the enormous worldwide volumes and low-risk stability of CO2 sequestered in deep-sea basalt aquifers should also

be valued in broader economic assessments.

7. Conclusions

Table 1 summarizes our assessment of deep-sea basalt aquifers as potential reservoirs for safe and secure

sequestration of anthropogenic CO2. These computations suggest that an enormous volume is available in these

reservoirs, which have been identified worldwide. The total volume potential for these aquifers ranges from 2.3–

11.5 Tt-C along seismic ridges to 5.9–29.6 Tt-C along aseismic ridges, and total between 8.2 and 41.2 Tt-C.

Although the volumes presented are broad estimations, they are based on global databases and scientific drilling

data. For greater certainty in the estimates for any particular target location, site-specific geological research and

pilot studies must be conducted locally. In particular, the Juan de Fuca Ridge, the Cocos Ridge, and the Sheba Ridge

in the Gulf of Aden, as well as the Walvis Ridge and the northern section of the Ninetyeast Ridge, offer large-

capacity storage volumes within reasonable proximity to continental margins and to potential industrial CO2

sources. The huge size of even one of these deep-sea basalt aquifers, in addition to the critical advantages for

sequestration that deep-sea basalts offer, warrants further research and investigation. Research should aim towards

pilot injection studies to address questions such as site-specific hydrological testing, the in situ reaction rates for

dissolution of injected CO2, in situ carbonate precipitation rates and the resulting rates of change in permeability.

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