QUANTITATIVE SEISMIC ANALYSIS OF DOUBLE-BSRs ON THE HIKURANGI MARGIN

1. School of Environment, University of Auckland, 10 Symonds Street, Auckland 1010, NEW ZEALAND 2. GNS Science, 1 Fairway Drive, Avalon, Lower Hu 5011, NEW ZEALAND3. Na onal Ins tute of Water and Atmospheric Research, 301 Evans Bay Parade, Hataitai, Wellington 6021, NEW ZEALAND 4. Ins tute for Geosciences, University of Kiel , O o-Hahn Platz 1, 24118 Kiel, GERMANY* Corresponding author email address: i.pecher@auckland.ac.nz

SUMMARYBo om simula ng reflec ons (BSRs) are thought to be caused by free gas at the base of gas hydrate stability (BGHS). BSRs usually mark pressure-temperature condi ons at the phase boundary for gas hydrates and thus, should occur at a single depth level beneath the seafloor. However, double-BSRs, i.e., BSRs occurring at two depth levels, have been observed at some loca ons. We have recently discovered double-BSRs on the Hikurangi Margin. Two poten al causes for the occurrence of double-BSRs in our study area have been proposed in previous studies: (1) Upfli may have caused the BGHS to shi towards lower temperatures, i.e., upwards, forming a new BSR but gas may s ll be present at the old level of the BSR. ( 2) Addi on of thermogenic higher-order hydrocarbon gases to methane generally increases gas hydrate stability and thus, the lower BSR may mark the phase boundary for thermogenic gas whereas the upper level marks the BGHS for microbial methane hydrate. We present results from high- resolu on velocity analysis and evalua on of reflec vity with focus on the sec on between both BSRs. Results show areas of poten ally elevated veloci es and a decrease of reflec vity, sugges ng co-existence of hydrate and gas. This would favour a thermogenic origin although other observa ons support upli as cause of these double-BSRs.

178˚20' 178˚30' 178˚40' 178˚50'

−38˚50'

−38˚40'

−38˚30'

177˚ 178˚ 179˚

−40˚

−39˚

−38˚

−37˚

−2000−1000

0m

Line 8

RB PACAUS

NorthIsland

Figure 1. Study area. AUS: Australian Plate, PAC: Pacific Plate. Black lines: Seismic tracks. Red lines mark loca ons of double-BSRs. Line 8 is shown in Figure 2.

METHODSWe present results from Line 05CM-04 collected in 2005 with a 12-km long streamer. We conducted a combina on of high-resolu on velocity analysis and pre-stack me migra on as outlined in Crutchley et al. (2015), including true-amplitude recovery. We predicted the seismic response of a hydrate-gas mix in the pore space by calcula ng veloci es and ver cal-incidence reflec on coefficients, simula ng a sand layer with a porosity of 0.3. Rock physics modelling followed the procedure in Navalpakam et al. (2012), using a common model (Helgerud et al., 1999). For the commonly used assump on that gas hydrates support the frame, (absolute) reflec on coefficients are predicted to decrease even at rela vely low hydrate satura on (Figure 3).

OBSERVATIONSResults are presented in Figure 3. The main area of interest is plo ed in the enlargement. Key observa ons include:• Poten ally, high-velocity zone above BSR-2.• Decrease of amplitude (as proxy for reflec vity) from below across BSR-2, followed by an increase near BSR-1.

DISCUSSIONFigures 5 and 6 present both possible forma on models for double-BSRs and expected gas and hydrate distribu on. Individual high-amplitude layers such as the reflector sketched in Figure 5 may mark high-permeability layers with elevated concentra ons of gas and hydrates (Navalpakam et al., 2012). We propose important differences in expected gas and hydrate distribu on between both forma on models in such layers: For gas par oning, hydrate (forming from thermogenic gas) is expected to be present above BSR-2, with decreasing satura on upwards, while gas should be present in the en re sec on from below BSR-2 to BSR-1. Co-exis ng gas and hydrate should lead to a velocity increase and a reflec vity decrease (Figure 3). Gas and hydrate co-existence is generally not expected however, for paleo-BSRs. In high-permeability layers, gas may have migrated upwards toward BSR-1 while it is s ll trapped in low-permeability layers at the level of BSR-2. In that case, high reflec vity should occur along high-permeability layers immediately above BSR-2. Our results are therefore consitent with a thermogenic origin of this double-BSR. We note however, that the smooth topography of BSR-2 would favour a paleo-BSR since small varia ons of gas mixes would lead to significant shi s of the phase boundary and thus, BSR level.

INTRODUCTIONBo om simula ng reflec ons (BSRs) usually mark the base of gas hydrate stability (BGHS). Pressure-temperature profiles in the seafloor are such that a BGHS should only occur at one depth level. Yet, at many loca ons worldwide, double-BSRs have been discovered. Most double BSRs have been interpreted as paleo-BSRs marking the phase boundary for gas hydrates in the geologic past (e.g., Bangs et al., 2005). In fine-grained, low-permeability sediments, free gas is very immobile and thus, gas at a paleo-level of the BGHS may remain in place e.g., a er seafloor upli for a geologically significant me. Alterna vely, double-BSRs may mark two different gas hydrate phase boundaries. The addi on of some higher-order hydrocarbon, such as propane, is known to significantly increase gas hydrate stability by forming Structure-II hydrate (Sloan & Koh, 2007). Since most biogenic gas consists almost exclusively of methane, forming less stable Structure-I hydrate, double-BSRs are in some se ngs considered an indicator for supply of thermogenic gas into the gas hydrate zone (Wu et al., 2005). We analyze a double-BSR in the Tuaheni Basin on the Hikurangi Margin east of New Zealand’s North Island (Figures 1, 2). Double-BSRs have only recently been observed in seismic data from this margin and were ini ally interpreted as paleo-BSRs (Navalpakam et al., 2012).

ACKNOWLEDGMENTSAcquisi on of 05CM-04 was commissioned by Crown Minerals (now New Zealand Petroelum & Minerals). Funding for data analysis was provided by the Ministry of Business, Innova on, and Employment (contract C05X1204). CSMHYD (Colorado School of Mines) was used for gas hydrate stability calcula ons, OpendTect® (dGB Earth Sciences) for seismic displays.

REFERENCESBangs, N. L., R. J. Musgrave, and A. M. Tréhu (2005), Upward shi s in the southern Hydrate Ridge gas hydrate stability zone following postglacial warming, offshore Oregon, J. Geophys. Res., 38, B03102. Crutchley, G. J., D. R. A. Fraser, I. A. Pecher , A. R. Gorman, G. Maslen, and S. A. Henrys (2015), Gas migra on into gas hydrate-bearing sediments on the southern Hikurangi margin of New Zealand, J. Geophys. Res., 120.Helgerud, M. B., J. Dvorkin, A. Nur, A. Sakai, and T. Colle (1999), Elas c-wave velocity in marine sediments with gas hydrates: effec ve medium modeling, Geophys. Res. Le ., 26, 2021-2024.Navalpakam, R. S., I. A. Pecher, and T. Stern (2012), Weak and segmented bo om simula ng reflec ons on the Hikurangi Margin, New Zealand — Implica ons for gas hydrate reservoir rocks, J. Pet. Sci. Eng., 88-89, 29-40.Pecher, I. A., G. Crutchley, J. Mountjoy, A. R. Gorman, D. Fraser, and S. A. Henrys (2014), Double BSRs on the Hikurangi Margin, New Zealand -- possible implica ons for gas hydrate stability and composi on, in Proc. 8th Interna onal Conference on Gas Hydrates, edited, p. 6 pp., Beijing.Sloan, E. D., and C. A. Koh (2007), Clathrate hydrates of natural gases, 3 ed., 721 pp., Marcel Bekker, New York.Wu, S., G. Zhang, Y. Huang, J. Liang, and H. K. Wong (2005), Gas hydrate occurrence on the con nental slope of the northern South China Sea, Mar. Petr. Geol., 22, 403-412.

Fig. 5. a. Forma on of double-BSRs from admixing of thermogenic gas from a leaky hydrocarbon reservoir (gas par oning). The frac on of thermogenic gas decreases away from the fluid conduits (Pecher et al., 2014). b. Expected gas and hydrate distribu on and resul ng reflec vity pa ern in higher-permeability layer containing elevated concentra ons of hydrates and gas.

Figure 6: a. Forma on of localized double-BSRs following upli . Depressuriza-

on causes the BGHS to move upward from its original depth (lower panel) to a shallower level with respect to the seafloor (upper panel). b. Expected gas and hydrate distribu on and resul ng reflec vity pa ern.

Joel Macmahon1, Ingo Pecher1,2,*, Gareth Crutchley2，Joshu Mountjoy3, Sebas an Krastel4, and Stuart Henrys2

Figure 4: Seismic line 05CM-04. Bo om: pre-stack me-migrated seismic image. Centre: instantaneous amplitudes (roughly normalized such that seafloor is maximum amplitude=6). Top: velocity profile on top of seismic reflec ons. Right: enlargements . Note indica on of high-veloci es and decrease of reflec vity above deeper BSR.

A A

BB

Figure 2. Example of double-BSR (Pecher et al., 2014)

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

TWT

(s)

4200 4400 4600 4800 5000 5200 5400

CDP

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

TWT

(s)

0 2 4 6

amp

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

TWT

(s)

1300 1600 1900 2200 2500

m/s

1.5

1.6

1.7

1.8

1.9

2.0

2.1

4400 4500 4600 4700 4800 4900 5000

CDP

1.5

1.6

1.7

1.8

1.9

2.0

2.1

1.5

1.6

1.7

1.8

1.9

2.0

2.1

−0.2

−0.1

0.0

0.1

0.2

refle

ctio

n co

effic

ient

0.0 0.2 0.4 0.6 0.8 1.0hydrate saturation

Figure 3: Preliminary rock physics models for ver cal-incidence reflec on coefficient of layer containing hydrate-gas mix, using common model (Helgerud et al., 1999). Input parameters from Navalpakam et al. (2012). Absolute reflec on coefficient decreases strongly even at low hydrate satura ons for commonly used frame model.

hydrates suppor ng frame

hydrates floa ng in pore space

2 km

BSR-1

BSR-2

EE W1 km

1 km

High-velocity zoneabove BSR-2?

Decrease ofamplitudeabove BSR-2

high-amplitude eventBSR-2

BSR-1