REGIONAL PROBLEMS of GEOCRYOLOGY the INFLUENCE of GAS-BEARING STRUCTURES on the CRYOGENIC STRATA THICKNESS in YAMAL AREA Yu.B

SCIENTIFIC JOURNAL EARTH’S CRYOSPHERE

Kriosfera Zemli, 2014, vol. XVIII, No. 3, pp. 11–22 http://www.izdatgeo.ru

REGIONAL PROBLEMS OF GEOCRYOLOGY THE INFLUENCE OF GAS-BEARING STRUCTURES ON THE CRYOGENIC STRATA THICKNESS IN YAMAL AREA Yu.B. Badu Lomonosov Moscow State University, Department of Geography, 1 Leninskie Gory, Moscow, 119991, Russia; [email protected] Cryogenic strata of the north of West Siberia is regarded as a unified cryogenic formation of the Middle and Late Neopleistocene and Holocene formed in the aftermath of the of transgression–regression rhythmic events in the polar basin in the course of cyclic changes in the harsh climate. With reliance on the current data, our research findings have given some insights into the permafrost developing in the subaerial and submarine environments, and into the gas accumulation affecting the cryogenic strata thickness in the section and within the extent of the gas-bearing structures. Gas-bearing structure, cryogenic strata, thickness of cryogenic strata, horizons of frozen and chilled rocks, 0 °C isotherm

INTRODUCTION

A number of famous soviet scientists who pio- horizons of cryogenic strata, ground cover and sedi- neered permafrost studies back in 1950–1960s, sub- mentary rocks, etc.), some conceptions and defini- sequently published their findings, and among them tions are introduced below. data on thickness and vertical structure of the cryo- Gas-bearing structure (GBS) formed at the result genic strata (V.M. Ponomarev, V.F. Zhukov, and of neotectonics development, and is represented by N.I. Saltykov, A.I. Popov, V.V. Baulin, A.A. Zemtsov, unconsolidated Pleistocene and Paleogene sedi ments V.A. Moshanskii, A.A. Ananyan et al. [Trofimov, 1975, in the geological cross-section, overlain by Upper 1980]). Cretaceous compacted rocks and the caprock of the The research carried out in the years to follow Lower Cretaceous gas pay zone (GPZ). During the provided new extensive materials, which made the Pleistocene, Early and Middle Pleistocene the sedi- basis for a series of schematic and small-scale thick- mentation occurred on the top of the eroded deposits ness maps for cryogenic strata (permafrost) in the of Late Cenozoic formations, with Late Pleistocene West Siberian plate, and small-scale maps for many deposits acting as the relief – forming rocks for plains northern areas [Baulin and Chekhovskii, 1972, 1976; and marine terraces in many structures [Badu, 2011b, Trofimov, 1975]. 2012]. Gas-bearing structure covers an area of hun- G.B. Ostryi, V.V. Baulin, B.V. Galaktionov, and dreds of square kilometers in the permafrost zone, V.I. Belousov, V.T. Balobaev analyzed thermal effects with the upper-most part of 300–500 meters in its of the crystalline basement on the base of the cryo- section subsumed into the cryogenic strata (CS). genic strata (zero isotherm position), depending on The CS comprises the upper horizon together the basement age (Hercynian or Caledonian) [Ostryi, with part of the Meso-Cenozoic rocks, with the tha- 1962; Baulin, 1985]. To this end, until present time wed layer (TL) occurring underneath. The caprock there have been extensively used the profiles created can be tectonically disturbed by disjunctive disconti- by V.V. Baulin and B.V. Galaktionova with the Her- nuities or complicated by plicative folding. Gas-satu- cynian and Caledonian basement under the zero de- rated sands occur at temperatures from 5–10 to 20– gree Celsius isotherm. 35 °C beneath the domed caprock overlying gas pay The paper presents explicit data on the thermal zone (GPZ), characterized by high reservoir pressure effect of producing gas reservoir (in Cretaceous de- up to 15–20 MPa. posits) within the gas-bearing structures on the cryo- Cryogenic strata is divided (according to S.M. Fo- genic strata base. tiev [1972]) into horizons of frozen (at natural tem- Due to the fact that the definitions may differ in perature Те of rocks lower than the initial freezing the literature (frozen and cryogenic strata, parts and temperature Тbf) and cooled rocks (at Tbf < Те < 0 °C).

11 Yu.B. BADU

The CS base is determined by the position of 0 °C iso- nomenon appears to be inherited from the ancient therm, while the frozen rocks horizon is separated geomorphological level, which was why the CS with from the cooled rocks horizon by Tbf isotherm, with maximum thickness formed exactly there in the inclusions of segregated ice and ice-cement almost course of the subsequent development of a new level totally missing beneath it [Badu, 2006a]. [Trofimov and Varenyshev, 1974; Trofimov, 1980]. The current knowledge and extent of the cryo- A number of wells penetrated the CS up to 200– lithozone are consistent with the Permafrost (peren- 250 m thick in laida of the Ob and Taz Bays. Well 1 nially frozen rocks, PFR) Thickness Map in the West penetrated the CS base of the South-Tambey gas- Siberian plate with scale 1:1 000 000 [PFR strata..., bearing structure at a depth of 275 m. The CS frozen 1984], which, however, does not characterize the horizon thickness near Antipayuta village and Mon- thermal influence of gas accumulation on the CS base gatalyang trading post was defined by electrical position [Badu, 2011b]. sounding as 230 and 260 m, respectively. This map shows that thicknesses of the vertically Under the floodplains and floodplain terraces of continuous strata range from 150–200 to 300–350 m. small rivers the CS frozen horizon thickness is close The CS maximum thickness was documented within to values typifying the surrounding interfluvial the most ancient marine plains which belong to the zones. Under the large rivers valleys the thickness re- Kasantzevo and Salekhard Formations, and the low- duces at greater rates compared to the contiguous est – within the limits of Late Pleistocene–Holocene interfluval plains. For example, under the central part floodplains and laidas. of the floodplain and Tambey Syador-yakha on Yamal Electric surveys revealed that the CS frozen (72° N), the thickness is only 60–75 m, and in the rocks horizon thickness in the north of the peninsula middle and lower reaches of the Yuribey, Erkutay- (to the north of 72° N) in the Kasantzevo marine akha, Mordyyakha rivers it doesn’t exceed 50 m. In plain varies from 160 to 250 m, occasionally exceed- the Yuribey and Tanama rivers (northern Gydan Pen- ing the limit thereof [Trofimov, 1975, 1980]. insula) floodplains it often averages at 30–45 m, while In this case, a number of the vertical electrical in land areas of these the CS thickness tends to be at sounding points recorded abnormally low thicknesses 100–120 m, occasionally reaching 150–180 m [Trofi- not exceeding 30–50 m. Rock masses with apparent mov, 1980]. electrical resistivity of a few or a few tens of ohm-me- In the Yamal (and Gydan) large lake basins the ters tend to occur lower. Interlayers with similar re- thickness of frozen rocks horizon in CS is markedly sistivity constitute a horizon of cooled rocks in CS, reduced in comparison with the surrounding marine and were identified in the sections of younger geo- plain areas. Electrical resistivity survey method iden- morphological levels at shallow depths. Deep drilling tified the thicknesses of 20–24 m underlying the in the permafrost formations revealed lenses and in- Toungynyanto (Gydan) lake floodplain terrace, which terlayers containing cryopegs [Trofimov and Vareny- does not exceed 85 m beneath its Terrace I, with shev, 1974], which was ascertained by later research thawed rocks occurring lower. The thickness varies [Badu, 2006a,b; Badu and Podbornyi, 2013]. from 70–80 to 120–150 m under the floodplain ter- Up to 70° N (as far as lake Neuto in Yamal and races of Lake Yarota and other large lakes of Yamal the Yuribey headwaters on Gydan Peninsula) the CS area. frozen rocks horizon reaches 300–350 m within the These are, in general, the specific patterns of the Salekhard marine plains, sometimes even greater, but CS distribution in Yamal and the adjacent areas sec- in some areas it thins out to 250–270 m. tions at geomorphological levels. The new data pre- Within the limits of the Kasantzevo coastal ma- sented below, made the basis of the author’s ideas rine plain, the CS frozen rocks horizon thickness var- about the CS developing within gas-bearing struc- ies from 215–220 to 280–300 m or, rarely, greater at tures, where the thicknesses of CS and its constituent these latitudes. The electric survey data are confirmed horizons are featured by quite different patterns. The by the results of field studies in the exploration wells. features of neotectonic structure, the relief forming So, the base of the horizon was penetrated by: well 2 and depositional processes in the Late Pleistocene in Sredneyamalskaya area at a depth of 217 m; five and Holocene, had an appreciable impact on the cur- wells in Arkticheskaya area at depths of 250–272 m; rent state of the entire CS, as well as the constituent well 4 in Yamburgskaya exploration area, at a depth horizons thickness of frozen and cooled rocks, to the of 256 m. depth of the zero isotherm occurrence. The thickness of the CS frozen rocks horizon within the Late Pleistocene marine and lagoon-ma- RESEARCH METHODS rine terraces in Yamal is at an average 50–75 m less than in the Kasantzevo and Salekhard plains. More- The new data were obtained by different me- over, within these (as well as lacustrine and alluvial) thods applied for the CS studies. terraces, its thickness increases from near-berm edges 1. Vertical electrical sounding (VES) survey areas towards its inland areas. At such sites, this phe- was applied to determine the boundary between the

12 THE INFLUENCE OF GAS-BEARING STRUCTURES ON THE CRYOGENIC STRATA THICKNESS IN YAMAL AREA ice-rich and ice-poor rocks in the formation and es- RESULTS AND DISCUSSIONS tablish occurrence depth of the base of frozen rocks This paper largely relies on other authors’ publi- horizon in CS, short of zero isotherm depth. The cations, materials and data (Yu.F. Andreev [1960], method was used in areal surveys at scale 1:500 000 V.V. Baulin [1985], M.S. Krass and V.V. Lovchuk and provided key data to compile the permafrost [1972], B.V. Monastyrev [Monastyrev et al., 2012], thickness map [Trofimov, 1980; PFR strata..., 1984]. S.I. Rokos [2008], A.A. Samokhin [2011], S.M. Foti- 2. Thermometry is used to determine the zero ev [1972, 2009], reports from A.R. Kurchikov and isotherm position, i.e. CS thickness, in deep explora- V.A. Skorobogatov [2003], etc.). tion and appraisal wells. The graphical combination Based on the research findings a composite sec- of the measured values Те and laboratory determina- tion was compiled and is shown in Fig. 1, 2. The sections of Tbf allowed for determining the position of tion reveals the structure of Quaternary sediments the point (layer/interval) of separation between the [Badu, 2006a, 2011c; Podbornyi and Badu, 2011] up frozen and cooled rocks horizons. The method was through the Paleogene top, not separated from the applied to interpret the “Krios”, OOO NTF survey underlying Lower Cretaceous sediments. The pro- data and discussed in earlier works [Badu, 2006a,b, ducing gas deposits sit in Cretaceous formation com- 2011b; Badu and Podbornyi, 2013]. posed of Cenomanian (K2) and Valanginian (K1) 3. Geophysical survey data allowed for inter- sediments [Kurchikov, 1987; Skorobogatov, 2003]. Re- pretation of the content of ice and unfrozen water in lying on the same data, both the gas accumulation soils of various composition. The method was applied temperature and the position of the gas-water contact in the geocryological studies in the Kharasavey field were determined. [Badu, 2006a]. The entire upper part, 300–450 m high, of the 4. The cryogenic structure description of core section is subsumed into cryogenic strata (CS). The samples recovered from appraisal wells was used to present understanding of the entire permafrost sec- correlate the results of the current research used, and tion of the Yamal area and Taz Peninsula derived from borehole data with the “Krios”, OOO NTF laboratory the analysis of main characteristics of the CS overly- determinations of the composition and physical prop- ing the gas pay zone (temperature gradients, thick- erties of soils, salt content, unfrozen water amount, ness of frozen and cooled rocks horizons, etc.) [Badu, and the freezing point of rocks [Kondakov et al., 2006; 2011a, 2012]. Badu, 2011b; Badu and Podbornyi, 2013]. The resulted Geomorphology of the geological structure sur- thickness values were correlated with the geophy si cal face is not uniform (given the occurrence depth gas- data derived from resistivity and gamma ray logs. bearing deposits is shallow, 700–1000 m). Rusanovs- Since the early seventies of the last century kaya and Leningradskaya structures are located in there have existed schematic maps of the permafrost the subsea coastal slope of the shelf (the sea depth is thickness in gas-bearing structures with scale factors about 100–120 m). Late Pleistocene (III–I) marine 1:50 000 and 1:100 000, both based on standard logs terraces make up the terrain of the Kharasavey and data acquisition after the conductor drilling of explo- Kruzenshtern structures, with their gas accumula- ration wells. The temperature was measured after ce- tions occurring partially in the Kara Sea subseafloor menting the conductor and spacing of the cement area. The Holocene floodplain with the outliers of sheath. The transition point of passing 0 °C was de- Late Pleistocene (III) marine terrace overlies the termined in the standing borehole water column, and Bovanenkovo dome structure. its position in some cases was corrected in the context The marine plains comprising the Middle (Kas- of logging data. A small number of wells drilled and antzevo) and Late Pleistocene (Salekhard) plains, scant log data available for the GBS area allowed for where the duration of subaerial freezing reached its a schematic representation of thickness in isolines not maximum (over 260 and 70 K years, respectively) exceeding 40–50 m. Given the CS thickness was de- [Fotiev, 2009]), compose the surface of the Arktiches- termined by the depth position of the 0 °C point, the kaya, Sredneyamalskaya, Neitinskaya, Nurminskaya, accuracy of measurements proved high only in soil Rostovtsevskaya structures, whereas late Pleistocene horizons with low salinity and weak (no more than lagoon-marine terraces occupy the surface of South 1–2 g/l) mineralization of groundwater. Tumbey, Seyakha, Mys Kamennyi, Parusovoye, Novy The position of the base of the frozen rocks hori- Port, and Yamburg structures. Gas accumulations in zon in CS in wells drilled to a depth of 150 m was de- Mys Kamennyi-sea, Utrennyaya, Geofizicheskaya, termined by the natural temperature Те curve mea- Antipayuta, Nakhodka, Semakov, Yurkharovskaya sured after a long period of borehole water column structures are partially or completely subsumed into standing, prior its intersecting the Tbf curve deter- the Ob and Taz Bays water areas. mined in the laboratory. The position of 0 °C point at This heterogeneity is accounted for the step re- a depth lower than 150 m is determined by the geo- duction of the permafrost strata thickness, from the thermal gradient magnitude. ancient surfaces exposed to prolonged freezing events

13 Yu.B. BADU

Fig. 1. Gas-bearing structures of the Yamal and Taz Peninsulas. 1 – borehole sections according to P.P. Generalov; 2 – borehole sections according to Yu.B. Badu; 3 – gas-bearing structures areas; 4 – profile line.

during cryochrons to younger, where freezing epi- The temperature gradient of the frozen rocks ho- sodes were much shorter [Trofimov and Varenyshev, rizon varies from –3.0...–3.5 to –1.0...–1.2 °C/m, and 1974; Trofimov, 1975, 1980]. Given the position of the the T gradient from the CS base up to the caprock top permafrost strata base is known, it can be traced in of gas pay zone ranges from 4.2 to 5.0 °C/m, with the the cross-section shown in Fig. 2. occurrence depth of the caprock top from 530–540 to The composite section (Fig. 2) shows that the 1180–1280 m. mean annual temperatures vary from –1.2...–2.5 to The sections of gas-bearing structures of Yamal –7.0...–7.5 °C, and are broadly consistent with the and Taz peninsulas are considered below (Fig. 2). modern landscape and climatic conditions of the sur- As is shown in the cross-section, a small-sized face and subaqueous environment of the seafloor. The frozen rock mass about 30 m thick composed of ice- temperatures in the frozen rocks horizon base corre- rich saline clay loams and clays interbedded with spond to Tbf varying in the range –2.5...–1.0 °C. sand, occurs at a temperature not lower than 1.5 °C in

14 Fig. 2. Cryogenic strata overlying gas accumilations of the Yamal and Taz Peninsulas. A–B, C–D – Profile lines (as in Fig. 1); 1 – marine laida sediments (clay loams, sand loams, ice-rich sands); 2 – alluvial sediments (clay loams, sand loams, ice-rich sands); 3 – deposits of I lagoon-marine terrace (clay loams, sand loams, ice-rich sands); 4 – deposits of I marine terrace (ice-rich clay loams and sand loams); 5 – deposits of II lagoon-marine terrace (clay loams, sand loams, ice-rich sands); 6 – deposits of II marine terrace (clay loams, sand loams, ice-rich sands); 7 – deposits of III lagoon-marine terrace (clay loams, sand loams, ice-rich sands); 8 – deposits of III marine terrace (clay loams, ice-rich sand loams); 9 – deposits of Kazantsevo Fm (ice-rich sands and clay loams); 10 – marine deposits of the Salekhard Fm (clay loams, ice-rich and ice-poor sand clay); 11 – marine deposits of the Kazym Fm (clay loams, ice-poor sand-clay); 12 – glacial-marine deposits of the Poluy Fm (clay loams, ice-poor sand-clay); 13 – Paleogene-Cretaceous marine deposits, unbroken (clay loams, cooled and thawed clays); 14 – boundaries of stratigraphic horizons; 15 – mean annual temperature of CS; 16 – temperature gradient (up to CS base); 17 – temperature at the lower boundary of frozen rocks horizon (Тbf); 18 – temperature gradient from CS base to the top of gas pay; 19 – rocks temperature in gas pay; 20 – gas showings while drilling and testing; 21 – exploration well No.; 22 – base of CS frozen rocks horizon; 23 – base of CS. Yu.B. BADU

the Rusanovskaya GBS, which is found in the Kara Sea water areas at a depth of 100–120 m ( true vertical depth interval: minus 114–144 m). The stratum is underlain by thawed sand with gravel and pebbles [Melnikov, 1995]. In the Leningradskaya GBS section, with the sea waters depths less than 50–55 m, the frozen rocks horizon thickness already exceeds 50 m at –2.3 °C and occurs 30–40 m subseafloor (absolute elevations: minus 60–130 m). Saline ice-rich and excessively ice- rich clay loams and clays are underlain by gas-saturated sands. The intense gas release was evidenced while drilling [Melnikov, 1995]. Judging by S.I. Rokos [2008], the upper – most 100 meter interval of the seabed section in some areas of the Kara Sea is composed of the frozen ice-rich clay-loams and clays (up to 20–40 m thick), which is overlaid by the interval of gas-saturated non-frozen sands. S.I. Rokos suggests that gas accumulates in the sand layers and horizons (Zyrjanka age) and is re- tained by the cover of marine clays (Kargino age). As it can be seen on seismograms, the clay deposits in such areas are contorted by plicative folding, which makes provisions for the formation of minor gas traps in their anticlines. There is no gas in the sandy layers, intermittent- ly coming close to the sea floor surface and “...the upper-most layer of continuous gas pay interval coin- cides with the base of the Holocene strata” [Rokos, 2008, p. 26]. This means that the areas of the contorted folds of clay and clay loam were subjected to freezing exactly above the occurrence of gas-saturated sands. Gas saturation proceeds in waterlogged fill in the cold portion of the strata (below 0 °C) still not a part where it is completely frozen through, since natural temperature Te has not yet reached the freezing point Tbf of the saline soil. Given the adiabatic expansion of gas in the me- dium with lower density and rock pressure, ground moisture temperature drops dramatically, which turns the latter into ice-cement, and saturated soil becomes ice soil. With gas steady inflows from the underlying sandy reservoirs, the clayey parts of the section con- Fig. 3. Decrease in thickness of frozen rocks horizon fined to the anticlines joints in the plicative folding of CS above the gas accumulation in Kharazavey area are subjected to freezing. GBS. So, the areas of subaqueous freezing appeared within the areal extent of the cooled seabed rocks and 1–4 – horizon thickness in the onshore area of GBS, m: 1 – less the frozen rocks horizon of CS developed during the than 50, 2 – 50–100, 3 – 100–150, 4 – over 150; 5 – thickness Holocene and the process is ongoing in the present isolines (а – main, spased at 50 m, b – additional, spaced at 10 m); 6 – boundaries of frozen rocks distribution; 7, 8 – thick- time. ness in the subaqueous part of GBS, m (at sea depths of 10– In the Kharasavey GBS section the north-west- 15 m): 7 – less than 50, 8 – over 50; 9 – boundary of frozen ern edge of the caprocks topping GBS is covered by bottom sediments distribution; 10 – boundary of GBS area; the Kara Sea water masses, and the upper-most part 11–15 – geomorphological levels: 11 – laida (mIV), 12 – flood of the gas-bearing doming formation occurs about plain and delta (aIV), 13 – I marine terrace (mIII–IV), 14 – 700 m subsea. The cryogenic strata is located within II marine terrace (mIII3–4), 15 – III marine terrace (mIII2–3); the III–I marine terraces, with its southern ending 16 – boundaries of geomorphological levels. underlying the estuary floodplain of the Kharasavey river. The CS frozen rocks horizon thickness varies

16 THE INFLUENCE OF GAS-BEARING STRUCTURES ON THE CRYOGENIC STRATA THICKNESS IN YAMAL AREA from a few tens of meters of the coast to 150–180 m The cooled rocks horizon 90–110 m thick occurs within the limits of II and III marine terraces beneath the 150–180 m thick base of the frozen rocks (Fig. 3). in Bovanenkovo GBS section. The CS base is found at Its maximum thickness (up to 170–180 m) is depths ranging from 210 to 310 m, but in some parts marked on the peripheral part of the structure, and in of the section the frozen rocks thickness can range its middle part (the GBS caprocks) it decreases to from 107 to 279 m. Minimum occurrence depths of 130–140 m. The CS frozen rocks horizon thickness the CS frozen rocks horizon base range from 110–112 reduces from 80–100 to 45–50 m beneath the Kha- to 107–110 m (Fig. 4). Generally, their position in rasavey Rv. floodplain and laida, while in the sub- the GBS structural plan corresponds to minimum oc- aqueous part at 10–15 m subsea it decreases from currence depths of the CS frozen rocks base. 60–80 to 25–35 m, with the CS receding from the The 180 m isobath delimits absolute minimum coast deeper into the sea. The CS thickness is 260– occurrence depths of the zero isotherm in the CS 270 m along the periphery of the GBS area, and it thickness map (Fig. 5). In the northwestern parts of does not exceed 220–230 m above the caprocks the GBS at least one minimum is linked with the ex- dome. tended reach of the Nadujyaha Rv. floodplain. The

Fig. 4. Thickness of frozen rocks horizon of CS in Fig. 5. CS thickness in the area of Bovanenkovo Bovanenkovo GBS area. GBS. 1–6 – thickness of frozen rocks horizon, m: 1 – 110–140, 2 – 1 – occurrence depth of the CS base from the surface, m; 2 – zero 140–170, 3 – 170–200, 4 – 200–230, 5 – 230–260, 6 – 260–290; isotherm position, m; 3 – isobaths for the gas pay cover 7 – thickness of frozen rocks horizon in core samples, m; 8 – iso- surface, m; 4, 5 – geomorphological levels: 4 – III marine terrace, baths for the top of gas pay, m; 9, 10 – geomorphological levels: 5 – flood plain; 6, 7 – discontinuous taliks: 6 – beneath lakes not 9 – III marine terrace , 10 – flood plain; 11, 12 – discontinuous completely frozen through, 7 – beneath river channels; 8 – taliks: 11 – beneath lakes not completely frozen through, 12 – khasyreys; 9 – disjunctive dislocations in Lower Cretaceous beneath river channels; 13 – khasyreys; 14 – CS cross-section rocks; 10 – boundaries of GBS area. axis (as in Fig. 6); 15 – boundaries of GBS area.

17 Yu.B. BADU – glacial- 13 – flood plains 7 – marine, Kazym Fm, 12 – ice-rich sands with interlayers of clay 3 – top of gas pay caprocks with disjunctive faulting d – well No. with absolute marks of wellhead, bottomhole, iden- 16 – glacial-marine, Salekhard Fm, 11 cooled horizon of CS; – origin, age of the relief-forming and underlying sediments: c – 7–15 – clay loams with interlayers of sands, ice-rich, 2 – isotherms; 6 – CS base with TVD, m; b – coastal-marine, Kazanstevo Fm; – marine, Lower Cretaceous Tanopcha Fm; – marine, Lower Cretaceous Tanopcha 10 15 – ice-rich clays; – ice-rich clay loams, 5 1 – rocks composition: 1–5 – marine III of terrace, 9 br); – base of frozen rocks horizon CS, m; 1 а – ice-rich sand clays and clay loams, 4 – Cretaceous-Paleogene marine, unbroken, – Cretaceous-Paleogene 14 – marine II of terrace, 8 alluvium, loams and plant remains, in deposits of Tanopcha Fm (K in deposits of Tanopcha Fig. 6. CS section above gas pay cover of Bovanenkovo GBS along line I–I. of Bovanenkovo cover gas pay Fig. 6. CS section above Cross-section axis as in Fig. 4; marine, Poluy Fm, tified layers.

18 THE INFLUENCE OF GAS-BEARING STRUCTURES ON THE CRYOGENIC STRATA THICKNESS IN YAMAL AREA second minimum (up to 175 m) is confined to the 2008]. Paleogene and Late Cretaceous clay deposits, Seyakha (Mutnaya) Rv. floodplain in the east of the overlying the GBS caprocks were subject to faulting area. The third one is located in the rear part of the and dislocations during Late Neogene – Early Qua- high floodplain of the river near the bench of III ma- ternary neotectonics activity [Skorobogatov, 2003; rine terrace, with its position in the structural plan Samokhin, 2011]. It is obvious that the gas saturation corresponding to minimum (–530 m) depth mark the of the Middle Pleistocene Salekhard Formation and of the GBS caprocks surface. its regional analogues [Badu, 2011c] was taking place In the meridional section (Fig. 6), the zero iso- during more than 260 K years [Fotiev, 2009] only in therm follows the top of gas accumulation caprocks the GBS sections overlying the caprocks. This is evi- position, gradually thinning from the dome top north- denced by the gas shows maximums documented wards, however, it drops down sharply to the south while drilling in this part of Pleistocene strata [Badu, from the dome top, which is consistent with its steep 2011a; Badu and Podbornyi, 2013]. dip. The depth of the frozen rocks base also follows The upper part of the sections of the Yamal east- the zero isotherm position, but decreases both in the ern coast and Taz peninsula is composed of lagoon- section parts composed of strongly saline soils, and marine and alluvial deposits (Yuzhny Tambey, Mys- beneath the hasyrey systems, where large taliks had Kamenny, Nakhodka, etc.), where Middle, Late Pleis- developed (Fig. 4), and it appears intermittent verti- tocene and Holocene sediments are also gas-prone. cally in the sand, saturated with cryopegs. However, The reasons for this are attributed to the neotectonic the depth increases in the section parts with iso- development features in the Ob and Taz Bays, the ar- therms –4 and –5 °C, which accounts for the most eas of long-term subsidence with the sloping low-am- heavily frozen parts of III marine terrace. plitude traps in the cover of Cenomanian deposits When evaluating the consistencies in the struc- [Skorobogatov, 2003; Samokhin, 2011]. It is highlight- tural plan of disjunctive dislocations in the GBS cap- ed by the authors that gas is transferred through the rocks top with abnormal depths of the CS base [Badu, faults system by means of convection (and it is not 2012; Badu and Podbornyi, 2013] it was established heat, which deals with conductive transfer – Yu.B.). that the grabens are linked with areas of the zero iso- The CS frozen rocks horizon interbedded with therm reducing depth, which evidences (in the sec- cooled rocks is clearly distinguishable within the tions) the heat release from the dome, and the fact Novy Port GBS section. Lower in the section of the that horsts strike in the structural plan is most com- CS cooled rocks horizon, rare interlayers of frozen monly associated with the isotherm drop down in- sands are encountered with thicknesses not exceed- tervals. ing 1–2 m. The GBS thickness map relying on the It is obvious that the disjunctive faults in the data from B.V. Galaktionova [Trofimov and Vare- doming uplifts definitely contradict the established nyshev, 1974; Trofimov, 1980] shows that the depth earlier patterns of changes in depth of the rocks cool- range of the CS base changes greatly peaking at 250– ing within the various geomorphological levels [Tro- 260 m within the Kasantzevo marine coastal plain in fimov, 1980; Badu and Podbornyi, 2013]. the northern parts of the GBS area. The sections of Neitin, Arkticheskaya, Rostov- Along the periphery of the area in the west tsevskaya, Nurmin, Sredneyamalskaya, Novy Port, (wells 63, P-71 in Fig. 7) and north (wells 62, 66 in and Yamburg areas are located in the Middle-Late Fig. 7), the CS base occurs at depths of 220–230 m, Pleistocene marine plain in Yamal and Taz peninsulas going deeper to 240–260 m in the south in the middle (Fig. 1, 2). The maximum duration of subaerial freez- reaches of the Ngoyaha river (wells 49, 79 in Fig. 7). ing episodes within these areas was estimated by To the north the base occurrence rises sharply to a S.M. Fotiev [2009] as 283–267 K years, beginning depth of 170–155 m (wells 59, 64) from the surface, from the Samarovsky Chron which corresponds to and even up to 140 m (well 77). Further northwards MIS-8, when the watershed area was subjected to (in the upper reaches of the Setnaya river) the depth draining within the marine plains during the late contour of less than 150 m delineates the cryogenic Middle Pleistocene. The CS thickness reaches 360– strata (CS) base. 380 m there, peaking at 400–420 m in Yamburg and Minimum depths coincide with structural noses Pestsovaya GBS. of the GBS caprocks dome at depths of 440–450 m, The subaqueous depositional episode should also i.e. the minimum depth point of the zero isotherm po- be mentioned, which lasted through the Early and sition is located exactly above the doming of the gas Middle Pleistocene, when the sediments accumulated accumulation. The minimum value is marked on the in Poluy, Kazym, and Salekhard Formations, as is periphery of the southern part of the GBS area within shown in the GBS sections in Fig. 2. the bounds of II and III lagoon-marine terraces in the The accumulated sediments were saturated with lower reaches of the Ngoyaha river. gas charged from beneath the gas reservoir clayey The freezing depth of the floodplains rocks in the caprocks in the process of deposition and diagenesis upper reaches of rivers usually differs little from that during the Cenozoic [Skorobogatov, 2003; Rokos, of the adjacent areas and tends to exceed 150 m.

19 Yu.B. BADU

Fig. 7. Decrease in occurrence depth of CS base abo- Fig. 8. Thickness of CS in the area of Yamburg GBS ve gas accumulation in the area of Novy Port GBS. (according to data by B.V. Galaktionov, Z.B. Cher- 1–5 – occurrence depth of CS base, m: 1 – less than 100, 2 – emnykh, amended and updated). 100–150, 3 – 150–200, 4 – 200–250, 5 – over 250; 6–9 – geo- 1–6 – TVD for zero isotherm position, m (in brackets – CS morphological levels: 6 – floodplain with terrace above flo- thickness, m): 1 – 275–300 (315–340), 2 – 300–325 (340–365), odplain , 7 – II lagoon-marine terrace, 8 – III lagoon-marine 3 – 325–350 (365–390), 4 – 350–375 (390–415), 5 – 375–400 terrace, 9 – coastal- marine plain; 10 – boundaries of geomor- (415–440), 6 – 400–425 (440–455); 7 – well No. (a numerator), phological levels; 11 – well No. (a numerator), thickness absolute mark for zero isotherm position, m (a denominator); (a denominator); 12 – boundary of GBS area. 8 – boundary of GBS area.

In the middle reaches of small rivers the freezing Minimum absolute marks of the CS base occur- depth decreases to 140–100 m, and down to 50–40 m rence depths (290–303 m) are were documented in or less in the lower reaches. well 17 in the northern part of the GBS area and in In some areas of the Salekhard marine plain, oc- wells 33 in the south (Fig. 8). This marks correspond cupied by large thermokarst lakes, the occurrence to the CS thickness of 315–340 m (Fig. 2). Maximum depth of the CS base shrinks down to 80–90 m. occurrence depths (386–396 m and lower than The section of Yamburg GBS is distinguished 400 m) were documented in wells 16, 12 within the from Novy Port GBS by the clearly distinguished Salekhard marine plain extending as far as the cliffs of frozen and chilled rocks horizons, which is attributed the Ob Bay shores, and in wells 25, 8 in the upper to a significantly lower salinity of Quaternary sedi- reaches of the Poilovoyaha river, closest to the Taz ments and to the lack of cryopegs therein [Badu, Bay. These marks account for the SC thicknesses of 2011c]. 430–440 m (Fig. 2).

20 THE INFLUENCE OF GAS-BEARING STRUCTURES ON THE CRYOGENIC STRATA THICKNESS IN YAMAL AREA

CONCLUSIONS section). The outgassing deposits, generally, cool the The results of many years research into rocks overlying sediments down while they accumulate and structure and conditions in the sections of gas-bear- are diagenetically transformed in the subaqueous en- ing structures in Yamal [Trofimov, 1975, 1980; Badu, vironment. 2006a,b, 2011a–c; 2012; Badu and Podbornyi, 2013] 6. It has been established that massive gas accu- suggested the following conclusions. mulations (with no faulting dislocations in the cap- 1. The CS thickness developing within the study rocks) are heat-generating. After the GBS area tran- area depended mostly on the cryochrons duration, sition into the subaerial regime the CS continues to when deposits of the plains and terraces were sub- grow and develops into a hard screen, preventing gas jected to freezing. At that, maximum thicknesses ac- migration. However, gas accumulation can proceed in cumulated within the bounds of Middle Pleistocene the cooled rocks horizon, forming gas hydrate depos- plains, whereas the thickness minimums are related its in the context of the ongoing cooling and sufficient to modern laidas, and the freezing process never in- rock pressure. Gas migration slows down or stops, terrupted in the harsh climatic conditions of the ther- and conductive heat flow from beneath the GBS cap- mochrons of the Late Pleistocene and Holocene. rocks, with the velocity orders of magnitude higher Within the area of gas-bearing structures the CS than the rate of mass transfer, “warming” the CS base, development process is complicated by a number of reducing its thickness. natural factors. 7. Thee zero isotherm position in the section of 2. Given that the CS thickness depends on the the GBS has proven to markedly affect the conduc- geological setting in the Pleistocene, the gas-bearing tive heat flow from the bottom of the gas accumula- structures within the area formed in various sedi- tion. mentation facies and freezing environments, in- 8. Grabens (i.e. disjunctive dislocations in the cluding: caprocks of gas reservoir) are associated with areas – subaqueous environment of cooling and subse- where the zero isotherm depth decreases, showing (in quent freezing of the Late Pleistocene deep-sea and the sections) heat released from the caprocks dome, coastal sediments in the open sea basin or shoal semi- and sites linked with the zero isotherm subsidence are closed lagoon (gulfs, bays), respectively, with differ- related to horsts (fault ridges ) striking in the struc- ent salinity of sea water, which stipulated various tural plan. degrees of salinity of seabed sediments; References – the subaerial environments of epichronous freezing of marine plains deposits that accumulated in Andreev, Yu.F., 1960. Permafrost zone and its challenges in the the Middle and Late Pleistocene. searches for structures in the Northern West Siberia. Essays 3. It has been inferred that the CS forms in the on Geology of the North of West Siberian Lowland (in Rus- subaqueous environment in the course of gas-satu- sian). Gostoptekhizdat, L., p. 191–218. rated saline sediments cooling below 0 °C and their Badu, Yu.B., 2006a. Rocks conditions in the section. Cryosphere of Oil and Gas Fields on the Yamal Peninsula. V. 1. Cryo- “preservation” in the course of their simultaneous ac- sphere of Kharasaveyskoye Gas-condensate Field (in Rus- cumulation and freezing. As they outcrop to the day sian). Nedra, SPb., p. 101–104. surface the CS develops in the subaerial environment Badu, Yu.B., 2006b. The permafrost strata thickness (in Rus- at lower mean annual air temperatures. At the same sian). Ibid, p. 105–111. time, the frozen rocks horizons retain all the evidenc- Badu, Yu.B., 2011a. Cryoanomalies in the permafrost strata of es of the subaqueous freezing: ice soil deposits with a gas-bearing structures in the north of Western Siberia: Sci- complex structure with ground ice presence in the entific works collection (in Russian). “TyumenNIIgiprogaz”, joints of anticlinal folds, cryogenic textures, which Flat, Tyumen, p. 27–30. highlights the uncharacteristic patterns of subaerial Badu, Yu.B., 2011b. The cryogenic strata of the gas-bearing freezing, and other forms of periglacial landforms. structures in the north of Western Siberia. Proceedings of the Fourth Conference of Russian geocryologists (in Rus- 4. The SC frozen rocks horizon forms at adiabat- sian). Univ. kn., M., Vol. 2, part 5, p. 9–15. ic expansion of gases entering into the overlying se- Badu, Yu.B., 2011c. Geological structure of the cryogenic diments and channeled by disjunctive faults in the strata in the north of Western Siberia (in Russian). Inzh. caprocks of producing gas reservoir. The sediments Geologia, No. 1, p. 40–56. overlying the caprocks dome saturated with gas Badu, Yu.B., 2012. The cryogenic strata of gas bearing structures throughout the Pleistocene deposition. in the north of Western Siberia – view from the future. 10th 5. As is known, tectonically disturbed deposits International Conference on Permafrost (TICOP): Re- release gases, so it stands to reason that the CS began sources and Risks of permafrost regions in a changing world (in Russian). Pechatnik, Tyumen, Vol. 3, p. 25–30. to form and its thickness aggraded in the episodes of Badu, Yu.B., Podbornyi, Ye.Ye., 2013. The cryogenic strata subaqueous regime development in the Early and thickness. Cryosphere of Oil and Gas fields on the Yamal Middle Pleistocene above the GBS caprocks with Peninsula. Vol. 2. Bovanenkovskoye oil and gas field (in disjunctive faulting (in the Cretaceous parts of the Russian). Gazprom expo, M., p. 391–414.

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