Golmshtok-2014-The impact of faulting on the s(4)

The sediments composing the uppermost levels ofthe section beneath the seafloor and containing gas

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P400.1

M0.2

,35

0.3pΔ 300.4

tnem250.6

er20

0.7cni150.8

eru10sser5P0

10–1810–1710–1610–1510–1410–1310–1210–11

Permeability κ0, m2

Fig. 1.layer as a function of permeability The maximum excess pressure at the base of theκsolid lines indicate the hydrate saturation 0. The numbers on theδh. The dashedline corresponds to the case described in Section 4.1.

hydrates often consist of unconsolidated sandy clays,silts, and clayey silts with various concentrations ofdiatomite, as is the case, for example, beneath the bot??tom of Lake Baikal (Seminskii et al., 2001). The per??meability of these sediments is typically high. Accord??ing to Bear’s classification (1972), the sub??bottomsediments that are typical for containing gas hydratesare related to semipermeable and characterized by theabsolute water permeability values in the range of 5 ×10–14 to 10–11 m2. According to this classification,highly fractured rocks have water permeability of 10–10to 10–7 m2.

The dependences shown in Fig. 1 indicate that inthe sediments similar to those described above, theexcess pressure caused by the release of free gas on gashydrate dissociation is extremely low at low hydratesaturation δh, and it can be typically neglected in thesolution of the problems of phase transition frommethane hydrate to methane gas in the conditionsdescribed above.

4. MODELING THE STABILITY CONDITIONS OF GAS HYDRATES IN THE ANOMALOUS ZONE OF THE CENTRAL BAIKAL BASINIn order to select the adequate model and itsparameters, we consider some features of both thebasement surface topography and sedimentary sectionalong profile B92??13.

A large deep fault with normal type displacementsis identified in the time section in the southwesternpart of the profile. According to our estimates, the ver??tical amplitude of the displacement of the crystallinebasement is about 4 km (Fig. 2). It is remarkable thatvertical displacements are not observed in the upper??

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TWTT, ms

Shot points

Fig. 2. Seismic profile B92??13 along the axis of the Central Baikal Basin. The boundaries of the main stratigraphic complexes inthe sedimentary sequence are shown by the grey lines; Mio2 + 3 is the supposed age of sediments in the identified strata; the faultsare shown by the subvertical dark lines.

most part of the sedimentary strata and in the bottomtopography in the immediate proximity of the fault.This suggests that normal faulting on this structureonly captured the interval of deposition of the lowerlayers of pre??mid??Miocene age (the interpretation ofthe seismic section and the dating of the reflections arecarried out by the author of this paper and should onlybe considered as optional). The gently dipping appar??ent character of the fault is likely to be due to the acuteangle between the seismic profile and the strike of thefault.

This ancient fault in the seismic section is overlainby quite a wide zone, which is weakened by faulting,where the largest faults are traced from the bottom upto the base of the Pliocene units. The linear extrapola??tion of these faults downwards projects their outlinesinto the upper edge of the scarp (at a depth H ≈ 7500 m),which is formed by the old fault. Despite the presenceof the strong multiple reflections from the lake bot??tom, which mask the deeper configuration of thesefaults, one of them, having a steep dip and no majorvertical displacement along it, is traced to the veryedge of the scarp. The flowerlike structure of this faultnear the lake floor is clearly indicative of its shear char??acter and young age. The weakened zone in the base??ment, which is associated with the older normal fault,is likely to have been reactivated at present; however,the motions on it are dominated by the strike??slip dis??placements approximately along the strike of the olderfault.

The sediments in the upper part of the weakenedzone with a width of up to 20 km are cut by numerousfaults with minor vertical displacements along them(oblique??slip faults?) or even without a vertical com??ponent (strike??slip faults). Within this zone, along itsedges, BSR sharply rises to the surface from the depthBSR=?z0ph = 430 m beneath the bottom and even dis??appears in the central part of this area (inset in Fig. 2).The width of the zone where BSR is absent is 2r

0 =1750 m on the lake floor. Throughout the remainingpart of the seismic profile, BSR is reliably traced con??formable to the lake floor.

In the central part of the seismic profile, betweenthe shot points 900 and 1200, all the layers of the sed??imentary strata are crumpled into anticlinal foldsstacked vertically one above the other throughout allstructural levels, suggesting the presence of predomi??nantly compressive strains. Unlike the central part ofthe profile, its southwestern portion accommodates aseries of normal faults, which are typical of the exten??sional conditions.

Therefore, the strata are subjected to the horizontalpressure gradients, which should cause the southwest??ward flow of the pore water. Having encountered theimpermeable scarp in the basement topography, the

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THE IMPACT OF FAULTING ON THE STABILITY CONDITIONS OF GAS HYDRATES535

pore water in the lower layer should deviate upwardsand, if there is a permeable channel above the scarp, anintense filtration flow should develop here.

As noted above, most of the known seafloor gashydrate occurrences in the world ocean are confinedto the fluid discharge zones, which have an isometricshape, while the highly permeable pathway conduitsare, in our opinion, formed in the intersections of thefaults. It can be assumed that in the case considered inthis study, the similar mechanism has formed the path??way channel in the interval between the shotpoints2390 and 2425 on profile B92??13. Unfortu??nately, it is impossible to determine the true strikes ofboth faults from the seismic data since the seismiclines with the reliably identified faults are absent in theregion of the southwestern end of this profile.4.1. The Model of the Medium with Anomalous BSRBased on the considerations described above, inorder to estimate the impacts of faulting on the stabil??ity conditions of gas hydrates in the considered seg??ment of Lake Baikal, we examine the changes in tem??perature and pressure in the medium in response to theformation of highly permeable channel and allow forthe phase transformation of methane hydrate.

We assume that the sediment material in the faultintersection zones undergoes intense crushing with asubstantial increase in its porosity and permeability.We also consider the significant steepness of the fault(dip angle is above 70°), which extends downwardsfrom the center of the near??bottom area where BSR isabsent up to the edge of the above??mentioned scarp.We approximate the nascent fluid conduit by a highlypermeable vertical cylinder with radius r0.

Let t = 0 correspond to the onset of the formationof the channel that arises at the intersection of twofaults and conveys water and free gas to the dischargezone on the lake floor. Let the formation of the consid??ered faults (from the onset to the decay) be accom??plished during the time interval Θ = 2τ. We assumethat the intensity of faulting initially increases up tothe maximum and then decreases up to ceasing attpierces the porous water??saturated sedimentary layer=Θ. The developing vertical cylindrical channelwith thickness H, which is located beneath the bottomof the lake at depth hw and is confined between theplanes z = –H (the upper surface of the ancient scarp)and z = 0 (bottom of the lake). The origin of the cylin??drical coordinates is located on the surface of the layeron the axis of the channel; the z axis is directedupwards.

The bottom of the lake is maintained at tempera??ture Tw and pressure p0, the water density is ρw, i.e.,p0=patm+ρwghw, where patm = 101325 Pa is theatmospheric pressure. We assume that the vertical