Ordovician Salts and Their Role in the Structure and Oil and Gas Potential of the Northeast of the Timan-pechora Province

In the northeast of the Timan-Pechora province for the last 35 years wells penetrated salt-bearing deposits at depths of 3.0 to 6.0 km of the Upper Ordovician, sometimes containing gas deposits. Analysis of the data allowed us to determine the criteria for allocating salts by means of geological and geophysical methods; to identify areas of their development in Kosyu-Rogovsky and Korotaikhinsky depressions, Varandey-Chernyshevsky aulacogene; to establish a decisive role in the creation of tectonics in the northeast of the province; to outline traps in the pre-salt complex. It is believed that salts are good confining beds, able to hold hydrocarbon deposits. The vast territory of salts in conjunction with the structural plan of Proterozoic sediments top makes it possible to identify areas of possible hydrocarbon accumulations under salts at depths of up to 7.0 km: the central part of Kochmessky stage, Yarvozhsky arch, Saremboy-Lekkeyyaginsky area, Vashutkinsko-Talotinsky autochthonous, Korotaikhinsky depression. The special role of Ordovician salts is shown in the creation of inversion-upthrust-overthrust tectonics of Varandey-Chernyshevsky aulacogene, Chernov ridge. It should contribute to a more focused and reasonable interpretation of geological and geophysical data.

Lower Paleozoic salt rocks in Kosyu-Rogovsky depression and its frame in the Timan-Pechora province were penetrated by wells always unexpectedly (Fig. 1).In 1983, exploration well 3-Kochmes opened salt-containing deposits, which proved to be gas-bearing (Ostrovsky et al., 1987).
Kochmes structure was revealed by seismic exploration in 1958, confirmed by the CDP in 1981 by reflecting horizon IV-V (Silurian-Ordovician), III of (roof of the Lower Devonian), II (bottom of Visean Lower Carboniferous), 1a (roof of carbonate Artinskian Lower Permian), 1u (Ufimian Upper Permian).The gas reservoir in Ordovician sediments was established by well 3-Kochmes.After penetrating sulfate-halogen strata from Shubina 5629 m (bottomhead), a gas fountain was obtained with mineralized water of about 1 million m 3 /day flow rate and high hydrogen sulfide content.The well flowed for several days, and then its trunk was apparently blocked by salt stream.It was assumed that the height of the reservoir is about 450 m, and the promising area is of 160 km 2 .However, the subsequent drilling of wells 5 and 6 in the dome part of the fold northeast to well 3 (bottomheads 6262 and 6276 m) yielded negative results.
According to the logging data depth of salts roof in the well 3-Kochmes is 5360 m, and the well did not come out from salt strata at the bottom 5629 m.
Well 5-Kochmes according to logging data penetrated salt strata at a depth of 5363 m.In the core of the well saltbearing rocks occur since chiseling 6 (int.5524-5529 m, core yield 0.4 m), where they are represented by dolomitesulfate saline rock with single crystals or halite clusters, or salt breccia where coarse gray halite cements fragments of sulfate-dolomite composition.In the chiselings 8, 9 rock salt is gray of various shades from very light to gray-brown.Part of chiseling 13 (int.6009-6012 m) is represented by dark gray, almost black dolomite, unevenly clayish.
Almost vertical cracks are filled with coarse white anhydrite or coarse water-transparent halite.At the very bottom of chiseling 17 (int.6241.2-6245.9 m) salt layer is penetrated with 0.8 m thickness, represented with gray-brown, giant-coarse halite, individual crystals are water-transparent.This layer is underlain by dark gray dolomite, brecciated clearly at the beginning and end of the layer (thickness 1.0 m) (description of N.B. Rasskazova, 1984).In the whole saltbearing interval there are steep angles of stratification 30-45°, brecciated form, fractures of different slopes, which indicate crumple of rocks after sedimentation.
According to logging data in the well 5 within the entire 900-m salt-bearing strata, three interlayers of rocks are allocated with 3-8.5% porosity and thickness 1.6, 2.6 and 2.8 m.Allocated reservoirs are characterized by water saturation.Prospective gas objects on logging data have not been allocated.They were not established neither by expressgeochemical studies.Due to technical reasons salt-bearing strata testing in the well was not carried out.
In the well 6-Kochmes salt-bearing rocks are characterized by a core of chiseling 2-5 (int. 5570-5576 m, 5860.6-5866.3 m), where the layers of a thickness exceeding 3.0 m consit of halite prismatic, transparent, clean, almost devoid of impurities.
For samples of halite from wells 3, 5, 6-Kochmes in laboratory studies we determined the velocity of longitudinal waves in the range of 2230-4060 m/s, and density from 2.23 to 2.52 g/cm 3 ; for samples with impurity longitudinal wave speed was up to 4510 m/s at a density of 2.68 g/cm 3 .
The well 6-Kochmes according to logging data penetrated saline strata at a depth of 5500 meters, which is 138 m hypsometrically below the well 3. Based on the description of the core (interval 6121.3-6124.1 and 6184-6185.6 m) and geophysical studies, it came out of the column at a depth of 6154 m, passing it for 654 m.Of the four tested objects two were "dry".Of the other two inflows of highly mineralized reservoir water was obtained by overflow through tubing with density greater than 1.2 g/cm 3 and a weak rate of gas saturation (0.5-0.8 m 3 /day).
These data may indicate the confinement of the gas deposit identified by well No. 3 to the limited carbonate reservoir within salt-bearing strata.
By the position in section, correlation with sections of Khoreyversky depression the salt-bearing stratum is attributed to Mukherskian and Khoreyverskian suites of the Upper Ordovician.Mukherskian and Khoreyverskian suites characterize a major regression in the offshore basin, completing a major sedimentary cyclite.In Khoreyversky depression Mukerskian suite consists mainly of sedimentary dolomites with inclusions of gypsum and anhydrite.In Khoreyverskian suite gypsum and anhydrite prevail, and in addition to dolomite there are layers of carbonate-clay rocks.
Yarvozhsky arch is the second plot of Kosyu-Rogovsky depression where salt-bearing formations are established by core.In the well No. 13-Yarvozh the formation has halite crystals and is composed by alternating dolomite, limestone with sulphate crystals in the boundary of the Silurian-Ordovician sediments at a depth of 5556-5561 m.Anhydrite interlayers up to 25 cm were found in this well in the range of 6000-6004 m.
In 2001, just as suddenly a salt-formation of Ordovician age is penetrated by the well 1-Vorgamusyursky, from which an emergency influx of gas and oil was obtained.The well was drilled by Gazprom within the eponymous structure, prepared by the CDP seismic survey on the Chernyshev ridge, in the range 2700-3000 m after a normal overlying section of Carboniferous, Devonian, Silurian sediments.Salt-bearing deposits are confined to the lower part of allochthonous plate thrust over ridge adjacent from the east with Vorgamusyursky structure in the autochthon.At 5.0 km to the south the well 2-Vorgamusyursky also revealed the Ordovician salt in the base of allochthonous plate higher and under dislocation normally imbedded clastic and carbonate section of the Lower Permian, Carboniferous, Dzhebolskian horizon of Famennian Devonian.A minor oil flow was received when testing during the drilling of the Permian autochthonous deposits.
14 km to the south of Vorgamusyursky area on the Chernyshev ridge Ordovician saline rocks were penetrated at the base of allochthonous block by wells 1,2-Adaksky drilled by Gazprom in a similar tectonic situation.
The well 1-Adaksky Ordovician saline rocks with total thickness of 1076 m are characterized by a core from chiselings 13-30 (int. 1930-3006.4 m), in which they are presented by alternating salt of yellowish-transparent color and anhydrite gray and white with thin undulating cracks and cavities filled with rock salt; dolomite gray, brownish-gray with crystalline coarse breccia areas due to the presence of large crystals of halite, uneven clayish.Texture is inclined, thin-layered and banded, with a few thin stylolite-like clay interlayers, large subvertical fractures and cavities of dissolution.The rock is strong, if split a faint smell of oil is noted.
Similar rocks are found in the core of salt-bearing deposits of well 2-Adaksky.
In 2007, east to Kosyu-Rogovsky depression salts were again unexpectedly penetrated by Gazprom in the well 1-East-Lemvinsky.The well was drilled in the field of Cretaceous sediments 4-5 km to the northwest of outcrops of Lemvinsky structural-formational zone identified with the Ordovician sediments.In the upper part the well penetrated under the Quaternary and Cretaceous sediments in the interval 170-1970 m strata of the thin raspberry, greenish-gray, brown phyllitic apo-clay shale with interbedded, silt-enriched material and siltstone, similar to grubeinskian suite of the Lower-Middle Ordovician (Gudelman et al., 2009).The apparent thickness of grubeinskian suite is 1800 m.
In the interval 1970-2731 m the well section consists of green sub-arkose silty-sand sediments.Sandstones are predominantly fine-grained, greenish-gray, rarely medium and coarse, gravel of pink tones, massive and indistinctly laminated.Silts are mostly gray and greenish-gray, nonlaminated.Everywhere there is glauconite (3-5 to 10%).The subordinate amount there are layers of encountered limestone, calcareous sandstone and siltstone, and silicate-clay shale.At the core of chiselings 12-19 in the interval 1969-2680 m T.A. Shutova defined spore-pollen complexes confidently attributed to the Lower Permian deposits.
Below salt-bearing strata is penetrated, original by the material composition and texture (Fig. 2).Two packs (int.2730-2759 m and 2896-3067 m) are composed of black coarse-grained salt with a strong smell of hydrogen sulfide, brecciated texture due to the presence of debris, acute and rounded shape up to a few centimeters of siliceous dolomitic rocks and layered black graphitized shale.Between packs of rock salt in the core there are greenish-gray split silty mudstones (int.2766.8-2769.4) and loose tectonic breccia (int.2873.05-2876.05m) of black color and shale texture with a strong smell of hydrogen sulfide.Siliceous-carbonate, silicaclay rocks and quartzites present fragments of various shapes with sharp edges, ranging in size from microscopic to first decimeters.Cement (binder) is black mylonite and rock salt.
According to R&D, the well 1-East-Lemvinsky penetrated part of the section below the halogen strata (int.3067-3550 m).It is represented by interbedded carbonate rocks (limestone and dolomite) of different gray tones, irregularly calcareous, clayey siltstones, locally stylolited, siliceous, bituminous, recrystallized, and black clay, less siliceous shale with sliding mirrors.Layers are with thickness from millimeters to a few meters.Alternating of lithological differences is uneven; intervals are dominated by thin differences.Near the bottomhole shale alternates with both carbonate rocks, and dark gray and gray argillaceous siltstone and silty sandstone of quartz composition.
The section reveals individual conodonts, in int.3178-3262 m of Frasnian Upper Devonian, in int.3401-3403.3m -Eifel Middle Devonian.In the rank of carbonate-terrigenous strata D 2 -C 1 t we allocated genetically single set of layered carbonate-shale rocks with an admixture of siliceous material, syngenetic bitumen and considerably various carbonate, shale and clastic components.Faunistically, the strata age is adopted from Middle Devonian to supposedly Tournasian Lower Carboniferous, completing the Middle Devonian-Tournaisian sedimentation cycle (Gudelman et al, 2009;. Anishchenko et al., 2009).Penetrated visible thickness of carbonate-terrigenous strata is 422 m.
From a depth of 3430 m to the bottomhole of 3550 meters, below the tectonic dislocation the core reveals uneven interbedded black mudstones, dark-gray dense muddy siltstone, sandstone, fine-grained, gray or light gray, and fine-grained secondary dolomite.By definitions of T.I.Shutova, Permian rocks are characterized by spore-pollen complexes.I.e., Permian sediments contain salt-bearing and underlying rocks of carbonate-clastic strata of the D 2 -C 1 t.
In the northern part of the Ural foredeep Permian salts has long been known in Verhnepechorsky depression.According to V.I.Bogatsky (Bogatsky et al, 1977), "salt deposits of the south of Verhnepechorsky depression, composing a huge lens of subformation are the youngest in the Kungurian tier.The greatest thickness of salt-bearing strata (400-470 m) is confined to the synclinal structures to the east of depression.In the arches of anticlines salt-bearing deposits are absent completely (Kuryinsky fold), or significantly reduced in their thickness (Rassokhinsky fold).
Salt-bearing strata is confidently divided into three horizons -the underlying rock salt, potassium and magnesium salts and covering rock salt.Vertical zonation of Verhnepechorsky salt-bearing basin is an analogy of the zoning, which was established in Verkhnekamsky basin.
The horizon of underlying rock salt is represented with uneven-grained halite rock, gray with a yellowish tinge, with an admixture of pelitic material, the content of which in most cases does not exceed 10%.In the lower part of the horizon we marked silty-clay layers, which together with the rock salt pack have thickness of 6-30 m.
Near the horizon roof we traced the second pack of halite-pelite of 2-10 m.The thickness of the underlying rock salt is 120-130 m in the west, and more than 400 m in the east.Horizon of potassium and magnesium salts consists of carnallite and sylvinite rocks with interlayers of clay and rock salt.Sylvinite is presented by spotty and thin-layered varieties.Spotted sylvinite is unevengrained, massive, milky white and bright yellow with red edges, with a considerable admixture of pelitic material.The second type consists of alternating (1-3 cm) layers of red sylvinite and halite.Carnallite rock is usually red (rarely lemon-yellow), laminated and massive with impurities and interlayers of halite and sylvinite.
On the west of the basin the horizon in mineral composition is divided into three zones from east to west: 1) primary development of sylvinite rocks, 2) alternation of sylvinite and carnallite rocks with the presence of mixed carnallitesylvite rocks, and 3) prevalence of carnallite rocks.As part of the horizone we allocated seven layers of thickness to 8.0 m as part of the basin.Weighted average content of potassium chloride in individual low-thickness (0.3-1.0 m) layers rises to 32-44 %, and magnesium chloride -to 26%.For conditional layers of at least 1.5 m, respectively, the content varies between 15-17 and 16-24%.
Two layers of sylvinite can be traced at the base of the horizon.Upper layers are composed of sylvinite and carnallite rocks.The relationships between the latter, according to A.A. Ivanov, have facies character (Ivanov, Voronov, 1968).The total horizon thickness varies from 10 to 52 m.The total thickness of potassiummagnesium salts ranges from 2 to 26 m.The highest values are confined to the western portion of the basin.
The horizon of cover salt is composed of halite salt with the slots and smears of gray silt clay.Horizon thickness does not exceed 25 m.
According to modern concepts, the area of salt-bearing Kungurian stage in Verhnepechorsky basin is limited by south pericline of Vuktylsky fold.
Analysis of the CDP seismic data on areas of Ordovician salts allows us to establish their way on temporary seismic sections, which in turn gives an indication of their development in the north-east of the European platform (Bogdanov, 2004).
By linking the wave pattern to the wells of Kochmes area we found that the roof of salt-including formations is located under the dynamically expressive horizon IV-V (S-D) (Fig. 3).Below there is a chaotic wave pattern, and its bottom is controlled by subhorizontally traced reflection horizon V 2 .Chaotic wave pattern between the two reflectors at the time sections has a lenticular shape of a spindle.We can imagine that Kochmes brachyanticlinal fold, traced above the reflecting horizon IV-V for the deposits of the Silurian, Devonian, Carboniferous and Permian with comparable size and altitude, has a salt dome origin.The time interval of salt-containing strata varies from 50 to 300 ms, which corresponds to 100-600 m.
Lenticular character of the seismic records can be seen in the Ordovician part of the section to the south of Kochmessky structure on profiles 50489-07 (Fig. 4), 17-RS, 13-RS.Bright lenticular pattern is evident in the base of the sedimentary rocks on Nertsetinsky structure pressed against the Chernyshev ridge (Fig. 1.5).
Significant difference in thickness of Ordovician sediments on sectional profile 30-RS2 (Fig. 6) up to 1100 ms (up to 2.0 km) corresponds to Prilemvinsky arch, which may have inversion origin.
Analysis of CDP seismic data shows that the lenticular cross-sectional anomaly of the wave field is clearly observed on the Chernyshev ridge of Vorgamusyursky area (Fig. 7).Here, the thickness of the respective salt-containing deposits is up to 2 km.Well 2-Adaksky is located on seismic profile 20993-04.Chaotic record inside the lens is characteristic for salt-bearing rocks; at the time section in well number 2 takes the interval 800-1150 ms, i.e., 350 msec.The thickness of salt rocks is 935 m, then the reservoir speed is 5340 m/s.Although the correct speed is determined according to seismic well logging.
Temporary sections of seismic profiles in wells Vorgamusyursky and Adaksky areas, distant from each other by 15-20 km, show that they belong to allochthonous unit with Triassic-Ordovician rocks, at the base of which plastic salts of the Upper Ordovician lie.At the time sections (Fig. 7, 8) salts have a lenticular cross-section with an apparent thickness up to 400 ms or more, which corresponds to the thickness of up to 1000 m and more.Allochthonous block overlaps with autochthon Vorgamusyursky structure seen in the sediments ranging from the Ordovician to the Permian.The western wing of the structure in the autochthon is interrupted by upthrow-thrust with the west plane dip.The eastern wing of amplitude up to 1000 ms falls in the southeast of Kosyu-Rogovsky depression and complicated by subvertical fault.The amplitude of the wing on the Silurian top exceeds 1000 m.
According to the results of testing, Silurian-Lower Devonian carbonates in the well 2-Adaksky revealed small tectonically screened oil reservoir with oil-water contact at around minus 4065 meters.From our point of view, this result should be considered preliminary, but promising.The fact is that all previous interpreters of seismic materials have chosen a very complex geological model as the basis when allochthonous plate is pulled down to the east on deposits of Kosyu-Rogovsky depression by 15 km.That cannot be true, because this model contradicts the data of gravity and magnetic survey, seismic survey, geological mapping.
Quoted in the article time sections of all seismic profiles of Vorgamusyursky and Adaksky areas allows creating a model where allochthonous block is pushed to the east by 5.0-6.0 km, as shown by the example on the profile 20993-04 (Fig. 8).
This model requires a new interpretation of materials with careful selection of speeds for false structures accounting under high allochthonous block.Making such a model will perform new structural construction for the autochthon.It may well be that an arch of Vorgamusyursky structure by Paleozoic deposits is located further south than wells drilled on Adaksky and Vorgamusyursky structures.It also may well be that the well 2-Adaksky in Silurian-Lower Devonian carbonates penetrated far northern pericline of large reserves.
The highest hypsometrical position of Ordovician-Devonian sediments of the considered autochthon is located in the latitude of Kochmessky and Povarnitsky structures (Fig. 1), as evidenced by inflows of hydrocarbons in these areas.That is why this part of Kochmessky ledge should be regarded as the most likely for the existence of different types of hydrocarbon traps at depths of 3.0 to 6.0 km, including the subsalt strata.
Ordovician sediments at the base of the Gamburtsev swell have lenticular nature, on the far north of pericline within Lekkeyyaginsky area (Fig. 9).
The most impressive wave pattern is present in the southwestern board part of Korotaihinsky trough, where we can see the lens of salt up to 2 km thick under the 'peak' of Vashutkinsko-Talotinsky upthrust-overthrust along its 80-km stretch to Cape Sinkin Nose.Beneath the lens there is fault swell with possible hydrocarbon trap.The swell has a length of over 80 km and amplitude of hundreds of meters away under the Barents Sea.The upper edge of the underlaying salt rocks lies on available for drilling depths of 4-5 km (Fig. 10).
A different picture is observed on time sections of seismic profiles in the area of well 1-East Lemvinsky to the east of Kosyu-Rogovsky depression.Here, according to seismic well logging, salt-containing interval is located, linked to seismic  profiles 50492-14, 50492-05 and penetrated by the well.It is 300 meters thick, and is a chaotic record of dynamically expressive axes in phase coincidence (Fig. 11).
From the standpoint of geological interpretation we can imagine that the salts lens was formed in lagoon separated from the ocean by rising Paleo-Urals.Such a phenomenon could occur in Kungurian during continued closure of the Siberian paleo-ocean.In the well salt-bearing strata are characterized by Permian spore-pollen complexes.Chaotic seismic record with dynamic short reflections is adjacent in the west to Devonian-Carboniferous outcrops in the Urals slopes, and may have a length of 100 km and a width of 40 km, placed to the east of Lemvinsky Barrier Reef.
This interpretation does not contradict the data of magnetotelluric sounding performed by JSC CC "Rosgeofizika" on sublatitudinal profile passing through the well 1-East Lemvinsky.Here, salts have a clear picture of 2 anomalies of the electrical resistivity minimums, which should correspond to the salts (Fig. 2.12).In the local component of the gravitational field, the salts may correspond to band minimum of Sub-Urals stretch.
Paleoreconstructions to the beginning of Silurian time show that in place of lenticular swells there could be troughstubs separating modern Khoreyver, Kosyu-Rogovsky and Korotaikhinsky tectonic blocks.Deflections experienced multistage inversion, occurrences of which are set at the Permian-Triassic, Triassic-Jurassic and Post-Jurassic time.
In its present form portionsof ridges Chernyshev, Chernov, Vashutkinsko-Talotinsky upthrust-overthrust have complex tectonic structure, when the earth's surface from a depth of 4-5 km in the form of positive system raised deposits of the Silurian-Devonian-Carboniferous (Fig. 13).Style of the above tectonic dislocation is clearly associated with the presence of plastic salts in the section, contributing to the inversion and folding.
Aulacogens of Pechora-Kolvinsky type, limiting Khoreyver block from the west, are characterized by limited tectonic disturbances, or their systems with magmatism, pressed graben blocks-tubs with increased thickness of Ordovician-Upper Devonian, and total thickness of up to 7.0 km.Swells have inversion nature and consist of adjacent anticlinal structures, reducing the thickness of the overlying Carboniferous-Triassic deposits.Along the external and internal faults of the aulacogene, heterogeneous reef structures are developed, originated in Domanic time.
All these features are inherent to Varandey-Chernyshevsky aulacogene comprising Varandey-Adzvinsky structural zone and Chernyshev ridge, limited by tectonic disturbances and having them inside (Grunis et al., 2001).Inside this aulacogene we can see increasing thickness of the Ordovician-Lower Devonian complex within swells and swell uplifts (Fig. 13), their inversion nature, occurrence of Triassic basalt covers.
Along the western limit of Varandey-Chernyshevsky aulacogene, the following structures were formed (from north to south): Varknavtsky, Passedsky, East-Khoreyver, North-Khosedayusky, Ladotynsky, Pontyvissky, Suborsky and many other Domanik-Famennian carbonate organogenic structures.From north to south along the eastern side of the aulacogene The special role in the creation of Ordovician salt is shown in creation of inversion-reversed overthrust tectonics of Varandey-Chernyshevsky aulacogene, Chernov ridge, which should contribute to a more focused and reasonable interpretation of geological and geophysical data.

Fig. 2 .
Fig. 2. The Lower Permian salts in the section of well 1-East Lemvinsky (using materials of branch LLC VNIIGAZ -Severnipigaz)

Fig. 7 .
Fig. 7.The wave pattern of Chernyshev ridge on the composite time section.

Fig. 10 .
Fig. 10.Ordovician salt-bearing formations and tectonics in the northeast of European platform.

Fig. 13 .
Fig. 13.Seismic geological section of the profile 12-PC, illustrating aulacogen inversion of the Chernyshev ridge.there are Rifovy, Padimeysky, Nertsetinsky, West-Intinsky structures.Inside aulacogene, structures are set according to drilling and seismic survey in Western Lekkeyyage on Vorgamusyursky structure in outcrops along the rivers Sher-Nyadeyta, Sharyu.The collection of drilling data, seismic surveys, geological, geophysical surveys in Kosyu-Rogovsky and Korotaikhinsky depressions, in Varandey-Chernyshevsky aulacogene allowed assuming widely developed salts in Ordovician sediments.It is believed that the salts are good impermeable layers, able to hold the hydrocarbon deposits.The vast territory of salts in conjunction with the structural plan of the roof of Proterozoic sediments allowed identifying areas of salt accumulations at depths of up to 7.0 km: the central part of Kochmessky