Golmshtok-2014-The impact of faulting on the s

ISSN 1069??3513, Izvestiya, Physics of the Solid Earth, 2014, Vol. 50, No. 4, pp. 528–542. ? Pleiades Publishing, Ltd., 2014.Original Russian Text ? A.Ya. Golmshtok, 2014, published in Fizika Zemli, 2014, No. 4, pp. 70–85.

The Impact of Faulting on the Stability Conditions

of Gas Hydrates in Lake Baikal Sediments

A. Ya. Golmshtok

St. Petersburg Branch of Shirshov Institute of Oceanology, Russian Academy of Sciences, 1??ya liniya Vasilievskogo Ostrova 30, St. Petersburg, 199053 Russia

e??mail: golmshtok@gmail.com

Received July 04, 2013; in final form, February 10, 2014

Abstract—The phase transition problem of methane hydrate in porous sediments is solved. Based on theobtained solution, the impact of faulting on the stability conditions of gas hydrates is investigated by thenumerical modeling of the filtration and thermal regimes in the sedimentary cover of the Central Basin ofLake Baikal within the segment of the anomalous behavior of the bottom simulating reflector (BSR). It isassumed that such behavior is caused by the tectonic action. The calculations testify to the plausibility of theproposed model of formation of the anomalous area with total decomposition of the contained hydrates. It isshown that dissociation of gas hydrates in sediments due to faulting and the subsequent uplift of the productsof these transformations along the incipient channel toward the bottom of the lake can result in the extensiveaccumulation of gas hydrates on this surface. It is also shown that if the total amount of the free gas, whichleft the hydrate dissociation zone, reached the level of the lake surface at normal pressure and temperature,its volume could be equivalent to the resources of a medium??size gas field. The results of numerical modelingthe violation of the gas??hydrate stability conditions in Lake Baikal sediments can also be valid for the otherregions with hydrate??bearing sediments if the case specific conditions and regional tectonic activity are takeninto account.

Keywords: phase transition problem, gas hydrates, faulting

DOI: 10.1134/S106935131404003X

1. INTRODUCTION

The widespread occurrence of gas hydrates(mainly, methane hydrates) in the marine sedimentsand the vast amounts of the contained in them sim??plest hydrocarbons show them as ecologically cleanenergy sources for the near future (Kvenvolden, 1993).Changes in the thermobaric conditions for the exist??ence of hydrates can destabilize their bearing sedimentand lead to massive submarine landslides on the con??tinental slopes, slumps, and slope failure. These phe??nomena are observed in many regions of the Worldocean. This makes the studies of the stability of gashydrates in the marine sediments indubitably topical.

Gas hydrate accumulations in the near??bottomsediments typically occur in the accretionary wedgeswithin the present??day active continental margins andin other tectonically active, deep??water sedimentarybasins, e.g., in Lake Baikal. In these regions there arenumerous gas hydrate occurrences, which are locateddirectly at the seafloor or close beneath and are alwaysconfined to the discharge zones of the fluids conveyedthrough the faults that are active up to present (Kven??volden, 1993; Ginsburg and Soloviev, 1994;Mazurenko and Soloviev, 2003; Soloviev andMazurenko, 2004). The seabed hydrate accumula??tions may reach hundreds of meters or even a few kilo??meters across (Ginsburg and Soloviev, 1994). All thefluid discharge sources on the seafloor are isometric inplan.At faulting, the sedimentary material undergoesintense fracturing and fragmentation. Although theyoung poorly consolidated sandy clayey sedimentscontaining gas hydrates are ductile, numerous tensilecracks filled with sand, which are formed in the faults(Seminskii et al., 2001), provide high porosity andhydraulic permeability of the sedimentary material inthe fractured zones. The isometric shape of the seabedzones of fluid discharge suggests that the fluid conduc??tive channel are formed by the intersecting faults andtherefore the regions of their intersection are markedwith the highest porosity and permeability (Golmsh??tok, 2008b).Each fractured zone arises as a result of numerousepisodes of faulting activity. The studies of such zonesindicate that, depending on the geodynamic setting,the recurrence period of the displacements along afault is expected to range from a minimum of a fewyears or even months to many hundreds of years dur??ing the active phase of the fractured zone formation(Turcotte and Shubert, 1982). In the sediments whosepore space is partially or fully saturated with water and

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THE IMPACT OF FAULTING ON THE STABILITY CONDITIONS OF GAS HYDRATES529where all pores are hydraulically connected, the for??mation of the fractured zone must rearrange the exist??ing distributions of the pore pressure and temperature.

In the gas??hydrate??bearing sediments, the changesin pressure and temperature can violate the stabilityconditions of gas hydrates, cause their phase transfor??mation, trigger the displacement of the boundaries ofthis stability zone, and significantly reconfigure itsgeometry.

The specificity of gas hydrate decomposition inporous media lies in the fact that a considerable massof the released gas turns out to be confined withinquite a limited volume. Based on the solution of theself??similar problem of phase transformation due tothe thermal impact on the boundary of a uniform half??space, which is filled with low??porous material signif??icantly saturated with methane hydrate, R.I. Nigmat??ulin et al. showed that this medium (with immobilepore water) admits the emergence of anomalouslyhigh pressure (Nigmatulin et al., 1998; 1999). Clearly,such pressure should considerably slow or block thevery process of phase transformations.

Although the near??surface gas hydrate accumula??tions are confined to the faults that are exposed on theseafloor, the mechanism of influence of the fault tec??tonics on the stability of gas hydrates is still poorlyunderstood. We study this problem by the example ofgas hydrate occurrences in the sediments of LakeBaikal, where the multichannel seismic studies carriedout by the authors in 1989 and 1992 (Golmshtok,2000) revealed a “bottom simulating reflector” (BSR)corresponding, to all attributes, to the base of the sta??bility zone of methane hydrates. It was established thatBSR is present in the Southern and Central Basins ofLake Baikal in all areas where the water depth exceeds500–700 m. The BSR depth from the lake bottomranges from 35–40 to 450 m.

The presence of gas hydrates predicted by seismicstudies was later confirmed by the deep drilling in theSouthern Basin in 1997 (Kuzmin et al., 1998), gravitycoring (Matveeva, 2003), and recent observations bythe deep manned submersibles MIR.

In the seismic sections, there are segments onmany profiles where BSR demonstrates anomalousbehavior, which we attribute to the influence of faulttectonics (Golmshtok, 2000). The most prominentanomaly is observed on profile B92??13 along the axisof the Central Baikal Basin (inset in Fig. 2). In ouropinion, this anomaly is a suitable object for exploringthe mentioned tectonic controls of the stability condi??tions of methane hydrates and assessing their role inthe formation of the subbottom gas hydrate accumula??tions by numerical modeling. Since significant pertur??bations in pressure and temperature are anticipated inthe vicinity of the dislocations, the numerical simula??tions should be conducted by solving generally thethree??dimensional phase transition problem consider??ing the filtration of the pore water and gas. With this inview, we formulate the constitutive relationships for

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 50 the problem of the methane hydrate??to??methane gasphase transformation in sediments, which will be fur??ther used in the numerical simulations.2. PHASE TRANSFORMATION OF GAS HYDRATES IN SEDIMENTS2.1. The Properties of the MediumWe assume that active porosity φ of the sedimentsvaries both in space and time, i.e., φ = φ(x,y,z,t).An increase in temperature and/or decrease inpressure of the medium can cause dissociation of gashydrates and shift the phase boundary (the base of thegas??hydrate??bearing sediments). The decompositionof gas hydrates is accompanied by the release of addi??tional water and free methane gas into the sediment.The fraction of gas hydrate in the pore space ??decreases from δδhzero after its completion. It depends on temperatureh in the beginning of the dissociation toTand pressure p in the pore water, which vary duringhydrate dissociation. Here, the mass fraction of waterreleased due to methane hydrate dissociation is γ =(0.87–0.88), and the mass fraction of the released freemethane gas is 1– γ = (0.12–0.13) (Dyadin, 1998).If we assume within the limits of our problem (justas was done in (Nigmatulin et al., 1999) that themechanical properties of methane hydrate are thesame as of the solid skeleton, the solid phase occupiesthe relative volume δs=1???hof the volume that is free of (1?δthe )φ, and the fractionsolid phase (or theeffective porosity φe) isφe=1?δs=(1?δ??h)φ.(1)Let sw be water saturation of the free pore space andsboth these saturations be allowed to vary in space andg=1?sw be the gas saturation of this space, and lettime. The fractions of water and gas per unit volume ofsediment are δw=φesw, and , δg=φesgrespectively.Since the gas hydrate dissociation can be accompa??nied by a significant increase in the gas pressure, waterwill be partly displaced from the pore channels, andgas will partly dissolve in the pore water according toHenry’s law. The latter should lead to the increase inthe water saturation of the pores and to the corre??sponding decrease in gas saturation. It is typicallyassumed that the gas dissolved in the pore water negli??gibly changes the density of the pore fluid; therefore,we set the density ρw to be constant everywhere.The permeability of porous medium for water andgas is calculated by the Kozeny–Carman formula(Kozeny, 1927; Carman, 1937). When doing this, wereplace the total porosity φ in that relationship by theeffective porosity φe. For water we have3κ(e)w=κ0????(????φ??????e??s????w????)????????,(2)(1–φ)2e No. 4 2014