Golmshtok-2014-The impact of faulting on the s(7)

As noted above, the temperature, which is increas??ing due to faulting, after a certain while exceeds itsequilibrium value at the base of the gas??hydrate??bear??ing layer, which leads to the decomposition of gashydrates and, thus, shifting of the phase boundary andreconfiguring of the hydrate stability zone. The loca??tion of the phase transition surface n)(z=zph(r,t)) at afixed radial distance r=r( at each moment t=twasdetermined from the calculated values ofjpressure and temperature on the vertical profilespj,n(z)=p(n)w(r,z,tj) and Tn)j,n(z)=T(r(,z,tj), whichwere then used in the numerical solution of the equa??tion

Tj,n(z)–Tph(pj,n(z))=0,

whose root is z=zn)ph(r(,tj).

The evolution of the geometry of the methanehydrate stability zone in the studied part of seismicprofile B92??13 is shown in Fig. 7. Here, the phasetransition surfaces corresponding to differentmoments of time ttion, which makes it possible to correlate the varia??j are shown on the seismic time sec??tions in their locations to the actual pattern of BSRshallowing. For mapping our data onto the seismictime section, we converted the depth values

=?z(n)

n,jph(r,tj) into the two??way travel times(TWTTs, tn, j(s)) using the velocity data provided byprocessing the multichannel seismic records in theCentral Baikal Basin (Golmshtok, 2000).

As follows from this figure, under the action ofwater filtration, the base of the hydrate??bearing layer

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 50

Heat flow, mW/m2

100150

200250

310020

20020

,ht300pe67D400500600

Fig. 5.axis. The numbers on the curves indicate the time after the Vertical conductive heat flow along the channel

onset of faulting (ka).

within the channel rapidly ascends in time within thefirst 15–20 ka, after which this ascent slows down. Forexample, during 20 ka, the base of the layer uplifts onthe axis of the channel from a depth of 430 m to adepth of 53 m, after which the phase boundary addi??tionally shifts upwards by only 27 m and reaches adepth of 26 m at time t = 300 ka. The lateral extensionof the zone where methane hydrate has completelydecomposed changes significantly slower with timethan its vertical size.

The position of BSR in the time section is bestapproximated by the phase transition surface for time

60010005005001002

m/400W30m20

w,300olf ta20010eH100

0200400600800100012001400160018002000

Distance from the channel axis, m

Fig. 6.tom. The numbers on the curves indicate the time after the Vertical conductive heat flow across the lake bot??onset of faulting (ka). No. 4 2014

540

GOLMSHTOKTWTT, msShot pointsFig. 7. The evolution of the geometry of gas hydrate stability zone in the anomalous segment of profile B92??13. The curves markthe position of the lower boundary of the gas hydrate stability zone in the time section. The numbers of these curves in the insetdenote the time after the onset of faulting (ka). The supposed position of BSR before faulting is depicted by the heavy dashed line;the present??day position of BSR is shown by the white dashed line.

t=tmax = 300 ka with va = 5 cm/yr. The solution of the

problem with va = 3 cm/yr and va= 4 cm/yr shows

that in both cases the zone of hydrate decompositioncalculated for this time is much narrower than thezone restrained on its sides by the BSR rising towards

the lake bottom. In particular, if va = 3 cm/yr, qualita??tively reasonable approximation of BSR by the phasetransition surface is achieved at t ≈ 1.5 Ma. This isprobably too long a time period, since it significantlyexceeds the age of the hydrate??bearing sediments out??side the anomalous region, considering the fact thatsedimentation rate in this region of the Central BaikalBasin for the Quaternary is close to 0.5 m/ka (Golm??shtok, 2008a). Evidently, a good approximation ofBSR can also be achieved at the shorter times by

increasing va; however, the values of the heat flow at

the lake bottom should in this case be noticeablyhigher, which is unlikely in the real conditions of

Baikal. It is for these reasons that the value va, being

the main parameter of the model, was specified at5cm/yr.

It is worth noting that, according to the corre??sponding solution of the problem, the impacts of fault??ing would have been quite significant even in the

absence of the fluid filtration flow, i.e., at va=0. In

this case, the phase transition surface at t≥500 kashould have risen to a depth of about 250 m in the axisof the channel and should have occupied the positionclose to the phase transition surface for t=6.2 ka atva=5 cm/yr shown in Fig. 7. At the same time, themaximum heat flow at the lake bottom should havebeen 70 mW/m2, which is by about 70% higher thanthat supposedly existed here before faulting.The mass of the free gas released to time t due tohydrate decomposition and then escaped from thereaction zone (the remaining part of methane gastogether with water fills the pore space of the sedimentin this zone) in our model isRΔmg(t)=2πrI(r,t)dr,0∫whereI(r,t)?zph(r,t)?0[(1–γ)ρhδh–βgρg]φdz,zph(r,t)>zph,?= ??z0ph?0zph(r,t)≤zph,?0,∫and ρg, β

g, and φ are obtained from (11), (15), and

(25), respectively.

No. 4 2014IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 50

THE IMPACT OF FAULTING ON THE STABILITY CONDITIONS OF GAS HYDRATES541To time tmax = 300 ka, the mass of the escaped gas is

Δmg(tmax) ≈ 2 × 1010 kg at δh=0.1. If this entire gas

reaches the lake bottom, the volume of methane which

escapes from the sedimentary layer will be ΔVg(tmax) =

ZgRgTwΔmg(tmax)p0 = 170 million m3 on the lake bot??than 120 kg/m3 with a depth of the lake of 1400 m). If,in addition, the whole of the methane, having passedthrough water, rises up to the lake surface, the meth??ane gas released into the atmosphere should have a

volume of about 23 billion m3, which, according to theclassification accepted in the gas industry, approachesthe upper limit of medium??size gas fields. It should betaken into account, however, that at pressure p0 =

13.8MPa at the lake bottom, the temperature of thissurface (276.45 K) is by 13 K lower than the equilib??rium temperature, which provides extremely favorableconditions here for the crystallization of methanehydrates and the formation of their subbottom accu??mulations. In this case, the volume of methane gasthat escapes to the atmosphere can turn out to benoticeably smaller than mentioned above. For theother values of hydrate saturation, the estimates pre??sented above scale almost proportionally to δh.

The results in Fig. 7 show that the suggested modeldescribing the impacts of faulting on the conditions ofgas hydrate stability quite reasonably explains theobserved anomalous behavior of BSR in the south??western segment of profile B92??13. At the same time,from the above it follows that, along with the data onBSR geometry (multichannel seismic profiling data),also the other experimental data, including thedetailed heat flow measurements along the profile, arerequired for appropriately constraining the selectionof the model parameters.

CONCLUSIONS

Our study has yielded the following results.

(1) The solution is obtained for the problem ofphase transformation of methane hydrate to methanegas in sediments. This solution can be efficiently usedfor analyzing the changes both in the stability condi??tions of gas hydrates in the sedimentary layer and inthe geometry of the stability zone due to various per??turbations in temperature and/or pressure.

(2) The suggested model describing the formationof the anomalous area of hydrate??bearing layerdestruction in the Central Baikal Basin appears to bequite plausible. This model confirms that faulting canlead to changes in the pore pressure and temperaturethat are sufficient for violating the conditions of gashydrate stability, causing their phase transformations,and significantly reconfiguring the domain of hydrate??bearing sediments.