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Petroleum potential of the Riphean-Vendian Chunya sedimentary basin in the western Siberian Platform
Russian Geology and Geophysics 49 (2008) 176–182 www.elsevier.com/locate/rgg
Petroleum potential of the Riphean-Vendian Chunya sedimentary basin in the western Siberian Platform N.V. Mel’nikov a, *, Yu.A. Filiptsov b, V.I. Valchak c, E.V. Smirnov a, L.V. Borovikova a a
Siberian Research Institute of Geology, Geophysics and Mineral Resources, 67 Krasny prosp., Novosibirsk, 630091, Russia b Krasnoyarsknedra, 62 ul. Marksa, Krasnoyarsk, 660049, Russia c Yeniseigeofizika Ltd., 66 ul. Leningradskaya, Krasnoyarsk, 660074, Russia Received 5 November 2006
Abstract The petroleum potential of the Riphean-Vendian Chunya sedimentary basin has been explored by seismic reflection profiling and drilling in recent years. The results of the study have been used to estimate the initial hydrocarbon resources in the basin and separately in four oil and gas areas distinguished in Riphean, Lower Vendian, and Vendian-Lower Cambrian reservoirs. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: source rock potential; oil and gas potential; basement; Riphean-Vendian deposits; hydrocarbon migration; seismic reflection profiling; reflectors; Batholith profile; oil-bearing area, oil accumulation; initial hydrocarbon resources; Chunya sedimentary basin
Introduction In 2003–2005 seismic exploration discovered a large Riphean-Vendian sedimentary basin of Chunya in the southwestern Siberian Platform, with its most downwarped part in the Chunya River catchment opening northward to the Ilimpeya and Taimura catchments. Reflection profiling in the Chunya basin was run along the W-E Batholith profile and a profile north of Batholith, between the Lebyazhinskaya 2 and Chun’skaya 120 wells (Fig. 1). The two profiles traversed the basin western slope, with a piano-key structure of the Riphean sedimentary complex, and the central part of the basin. The W-E extent of the basin is more than 370 km from the Kuyumbinskoe field in the Kamo arch of the Baikit anteclise to the Nepa-Botuobia anteclise. In the N-S direction, the basin stretches from the W-E Riphean Delanin megaswell in the Podkamennaya Tunguska middle reaches (Taimba field) at least as far as the Srednetaimurinskaya 272 well in the north; the basin continues farther to the north but its northern closure remains so far unknown. The discovered part of the Chunya basin occupies 40,000 km2 and its total area may exceed 150,000 km2. The basin hosts the largest Kuyumbinskoe field of gas, condensate, and oil in Riphean reservoirs in its southwestern slope and the * Corresponding author. E-mail address: [email protected] (N.V. Mel’nikov)
large Sobinskoe and Paiginskoe oil, gas, and condensate fields in Vendian reservoirs in its southeastern slope. The thickness of Riphean deposits along the Batholith profile is over 6 km.
Basin’s architecture Reflection surveys along the two profiles (Batholith and well Lebyazhinskaya 2–well Chun’skaya 120) provided a general image of the basin architecture in its western and eastern slopes and in the deepest central part (Fig. 1). Riphean sedimentary strata rise, in monoclines, to the west and east, where they are discordantly overlain by Vendian formations. The Vendian strata in the basin’s center overlie Riphean formations without sharp unconformities. With this basin structure, hydrocarbon migration must be from the basin center to the west, southwest, and east along low-density zones at the surface of the Riphean-Vendian unconformity. We suggest that the eastern border of the Chunya basin should follow the line where Riphean formations pinch out at the junction between the Katanga saddle and the Nepa-Botuobia anteclise. The border is quite evident around the Khoshonskaya 256 and Ereminskaya 2 and 101 wells in which the Vendian terrigenous sediments (about 15 and 40–50 m, respectively) superpose the crystalline basement. The Batholith profile clearly resolves the pinch-out of the Riphean deposits (Fig. 1); the southward extension of the pinch-out
1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2008.02.001
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Fig. 1. Riphean-Vendian Chunya sedimentary basin. 1 — western slope; 2 — eastern slope; 3 — most deeply subsided part; 4 — piano-key structure; 5 — oil and gas fields; 6 — oil and gas areas; 7 — regional reflection profiles; 8 — deep wells (a), projected (b) and drilled (c) appraisal wells.; 9 — frontier of Evenki Autonomous Area. Abbreviations stand for names of wells, keyed as Bk — Baikitskaya, Vam — Verkhneamnunnakanskaya, Er — Ereminskaya, Km — Kuyumbinskaya, Tts — Teteiskaya, Um — Umotkinskaya, Khsh — Khoshonskaya, Chm — Chambinskaya, Chn — Chun’skaya, Yur — Yurubchenskaya, Schn — South Chun’skaya.
line appears in the Ereminskoe field east of the Sobinskoe and Paiginskoe fields, and its northern extension occurs in the Khoshonskoe field. The basin’s western border has been recognized from the pinch-out of the Riphean strata at the boundary between the Bakhta megasalient and the southwestern side of the Kureika syneclise. The pinch-out line is traceable in the NW direction along the eastern boundary of the basement block stripped by drilling in the northern slope of the Baikit anteclise and in the Bakhta megasalient, from the Kuyumbinskoe field across the Baikit and Upper Amnunnak fields. The occurrence of Riphean deposits east of that line has been proved in the area of Kuyumba field and detected along the well Lebyazhinskaya 2–well Chun’skaya 120 reflection profile. Strong reflectors in the Riphean section separated by more transparent rocks correspond to the surfaces R4 (roof of the Vedreshe Formation), R3 (Kopchera Formation), R2 (Tokur Formation) within the Kuyumbinskoe field. Riphean and Vendian rocks exhibit an evident block structure in the cross section (Fig. 2). In the present framework, there are two Riphean basins in the southwestern Siberian Platform, namely the Chunya and Irkineeva-Chadobets basins separated by the Delanin megaswell. The latter most likely appeared during the pre-Baikalian nondepositional hiatus, but in the Riphean there was a single basin spread continuously over the whole southwestern part of the platform.
Assuming the existence of a single Riphean basin, it appears reasonable to interpret the Riphean formations in the Yurubchen-Tokhomo zone (where the Kuyumbinskoe field was discovered) as the basin’s western part, which experienced an intense tectonic reconstruction during the pre-Vendian hiatus, unlike the eastern part, where reflection profiling shows no sharp Riphean/Vendian unconformities. The difference in the reflection patterns of the two basin’s parts may be due to their different structures and, possibly, basement ages.
Riphean formations The Riphean stratigraphy in the east of the Chunya basin is known from deep drilling in the Katanga saddle (Kraevsky, 2005). The section begins with the 225 m thick Pompot Formation composed of mudstone and siltstone interbedding with thin brecciated dolomite. It underlies the 250 m thick South Chunya Formation of thinly alternating mudstone and siltstone. The overlying Riphean rocks located farther to the southwest between the Paiginskoe and Sobinskoe fields remained underexplored by drilling (hypothetically, the Dzhelindukon and Upper Dzhelindukon Formations). The section farther to the west (Sobinskoe field) is composed of younger Riphean rocks. The drilled section
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Fig. 2. Expected oil-source reservoirs and migration paths of hydrocarbons in Chunya basin (from reflection surveys by Yeniseigeofizika Association, profile Batholith). 1 — Riphean-Vendian unconformity; 2 — reflector B; 3 — expected base of Riphean section; 4 — expected paths of hydrocarbon migration to base of Vendian reservoir; 5 — expected disjunctive tectonic dislocations.
begins with the >170 m thick sulfate dolomite Neryunda Formation composed of alternating dolomite and anhydrite with marl and mudstone interbeds and chert partings. Up the section there follows the 400 m thick Paiga Formation of stromatolite, microphytolite and detrital dolomite with chert lenses overlain by the 140 m thick Ayan Formation of black carbonaceous mudstone with marl and dolomite interbeds. The Ayan Formation underlies the Ushikta Formation 360 m thick, composed of dolomite with mudstone interbeds. At the top of the Riphean section there is a 160 m thick layer of gray or variegated stromatolite dolomite with interbeds of argillaceous dolomite. Generally, the Vendian Vanavara Formation in the territory of the Sobinskoe and Paiginskoe fields apparently lies over different Riphean units which are older in the east and younger in the west of the area. This model of the Riphean structure was validated by reflection profiling along the Batholith profile. Vendian strata overlie the Riphean strata conformably in the basin’s central part but the contact looks ever less
concordant eastward as the underlying Riphean deposits become older. Riphean rocks in the western slope of the basin are alternating argillaceous and carbonate layers stripped by numerous wells in the Yurubchen-Tokhomo zone. At the base of the section, there are quartz or quartz-feldspar sandstone and quartzite of the 280 m thick Delingdeken Formation that correlates with the Ayava Formation of the Katanga saddle. Then there follow argillaceous dolomite, dolomite and mudstone of the 280 m thick Madra Formation that correlates with the Upper Dzhelindukon Formation of the Katanga saddle. The Madra Formation underlies stromatolite, phytolite detrital, detrital, and microphytolite dolomite of the Yurubchen Formation (550 m), a stratigraphic equivalent of the upper Upper Dzhelindukon Formation stripped in the eastern slope of the basin. The Yurubchen Formation is overlain by silty argillaceous dolomite with interbeds of mudstone and glauconite sandstone of the Dolgokta Formation (100 m) which correlates with the Neryunda Formation of the Katanga saddle.
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The Kuyumba Formation of 470 m thick layered stromatolite dolomite correlates with the Paiga Formation in the eastern slope of the basin. The Kopchera Formation consists of 100 m thick silty argillaceous dolomite with mudstone interbeds and is an equivalent of the Ayan Formation in the eastern part of the basin. The Yukten Formation, 400 m thick, is composed of platy-columnar and detrital dolomite and correlates with the Ushikta Formation in the east. The rocks stripped up the section include no stratigraphic equivalents of the units from the Katanga saddle. The Rassolka Formation, 215 m, consists of argillaceous and stromatolite dolomite. The 65 m thick Vingolda Formation is composed of stromatolite dolomite. It underlies the 140 m thick section of red mudstone with dolomite interbeds of the Tokur Formation. At the top of the Riphean section within the YurubchenTokhomo zone there is the Iremeken Formation of columnar stromatolite and detrital dolomite with mudstone interbeds, some of the latter including over 200 m layers of highly carbonaceous rocks.
Vendian formations The Vendian strata in the Chunya basin include the Vanavara, Oskoba, Katanga, Soba, and Tetere Formations traceable throughout the territory. The Vanavara Formation comprises two units: — lower subformation (5–40 m) of two sandstone beds with a mudstone or a siltstone bed between them; — upper subformation (35–95 m) of mainly siltstone and mudstone, with up to three 7–18 m sandstone beds. Sandstone is inequigranular and quartzy, and includes siltstone and mudstone interbeds. General attenuation of the Vanavara Formation is accompanied by pinch-out of sandy layers. The Vanavara Formation is conformably overlain by the Oskoba Formation, which includes three subformations distinguished according to lithology (Mel’nikov, 1994): — lower subformation (31–55 m) of anhydrite dolomite, with argillaceous dolomite at the base; the Vanavara-Oskoba transition is most often gradual; — middle subformation (6–23 m) of mainly terrigenous rocks (mudstones with thin siltstone and sandstone interbeds). The subformation contains a highly radioactive sandstone bed (B-VIII) that gave an economic gas flow in the Omora accumulation; — upper subformation (0–47 m) of dolomite, anhydrite dolomite and argillaceous dolomite. The complete section of the formation was stripped in the western part of the Katanga saddle; the section in the eastern part, at the junction with the Nepa-Botuobia anteclise, misses the upper subformation, possibly as a result of erosion before the Katanga deposition. The Vanavara and Oskoba Formations are erosively overlain by Vendian carbonates of the Katanga, Soba, and Tetere Formations. The latter is of Vendian–Early Cambrian age.
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The Katanga Formation (100–150 m) is composed of dolomite marl and argillaceous dolomite and with a dark-gray dolomite member (Preobrazhenka Horizon) at its base. The Soba Formation (80–120 m) consists of gray dolomite with marl interbeds in the middle and upper sections. The Tetere Formation (30–70 m) is composed of dolomite, which is sulfate-bearing in the middle section. The Tetere Formation is conformably overlain by Lower and Middle Cambrian salt-bearing and dolomite formations (Usolka, Belaya, Bulai, Angara, and Litvintsevo). The latter formation underlies the Middle-Upper Cambrian Evenki Formation of redbed marl. The total thickness of the Cambrian section is 1320 to 1770 m.
Petroleum potential The potential of the Chunya basin is associated with Riphean, Vendian, and Upper Vendian-Lower Cambrian petroleum complexes, as well as with the overlying Lower Cambrian strata. Hydrocarbons in Riphean reservoirs may occur in places where carbonates subject to secondary alteration underlie Vendian deposits along the pinch-out line of Riphean strata at the boundaries of the basin with the Nepa-Botuobia anteclise and the Bakhta megasalient (by analogy with the Yurubchen-Tokhomo zone). The Yurubchenskoe and Kuyumbinskoe fields discovered in the west of the Chunya basin lie in Riphean reservoirs; flows of stratal water at a daily rate of 8–130 m3 were obtained from fractured-cavernous Riphean rocks in the east (Bitner et al., 1990). Porous sandstone beds may exist among Riphean terrigenous sediments in the eastern part of the basin at depths of about 4000 m. This hypothesis is supported by the absence of intense secondary alteration of Riphean sandstone according to drilling evidence from the Sobinskoe and Paiginskoe fields. Especially important is the discovery of highly carbonaceous rocks, possible oil source rocks, in the upper part of the Riphean section, both in the Yurubchen-Tokhomo zone and in the Sobinskoe field (Filiptsov et al., 1998). In the Yurubchen-Tokhomo zone, these rocks exist as 10 m of carbonaceous mudstone in the Iremeken Formation with 8.27% Corg on average. Organic matter (OM) in mudstone from the most highly elevated parts of the Kamo arch (well Yurubchenskaya 104) has a moderate alteration grade (no greater than MK21(G)−MK2(J) according to the Russian scale of catagenesis). Thermal decomposition of OM samples showed a relatively high residual content with a hydrogen index of 500–560 mg/g Corg. Carbonaceous rocks within the Sobinskoe field occur as the 140 m thick Ayan Formation of black mudstone with marl and dolomite interbeds, with a mean carbon content of 1.45%. The Ayan Formation was stripped also in deeper subsided parts of the basin farther to the north, where the alteration grade of organic matter is much higher (MK2(J)−MK13(K)) than that in the highest elevated part of the Soba swell (under MK21(G) grade).
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Fig. 3. A fragment of cross section along regional profile from Lebyazhinskaya 2 well to Chun’skaya 120 well (between wells Bk1–Chn120). For legend see Fig. 2.
Therefore, at least the youngest Riphean strata that sirvived the pre-Vendian erosion within the Chunya basin preserved their potential as source rocks, which was realized in the Phanerozoic. Oil and gas generation in Riphean rocks and the ensuing upward migration of hydrocarbons along faults in the Riphean and then along lateral pathways of low-density zones at the Riphean-Vendian unconformity surface produced several oil and gas areas (see below). The potential of Vendian strata is associated with the Vanavara Formation (regional reservoir) and Oskoba and Katanga Formations (regional screen), especially with producing sandstone beds in the Vanavara Formation. The producers appear where the formation thickness exceeds 30 m, according to drilling data (especially, wells Chun’skaya 120 of >60 m deep, Yuzhno-Chun’skaya 107 of 29 m, and Chambinskaya 114 of >40 m in the eastern basin slope). The Vanavara producers are sealed with a thick sulfate member of the Oskoba lower subformation. In the places where the Vanavara Formation is thinner than 20 m it consists mainly of mudstones which may be a zonal seal for secondary reservoirs located at the head of the Riphean carbonate section. Upper Vendian-Lower Cambrian reservoirs consist of the Soba, Tetere, and Osa carbonates, with the salt-bearing carbonate Upper Usolka subformation as a regional seal. The carbonates have low porosity and permeability. Cambrian reservoirs within the Belaya and Bulai Formations are of low potential. Discovery of hydrocarbons is most probable in the zone of Riphean-Vendian unconformity in the east at the boundary with the Nepa-Botuobia anteclise and in the zone of the piano-key Riphean complex in the west near the Baikit anteclise and the Bakhta megasalient. A lower potential of Riphean and Vendian reservoirs can be expected from anticlines in the basin’s central part.
According to estimates obtained at the Siberian Research Institute of Geology, Geophysics and Mineral Resources (Novosibirsk), the OOIP (original oil in place) reserves in the Chunya basin are approximately 39.8 BBOE in Riphean reservoirs, 24.4 BBOE in Vendian terrigenous reservoirs, and more than 6.4 BBOE in Upper Vendian-Lower Cambrian carbonate reservoirs. Proceeding from the available data, we predict two oil and gas accumulation zones (Ilimpeya and Kochema) confined to the eastern slope of the Riphean-Vendian Chunya sedimentary basin (Fig. 3) and one zone at the western basin slope (Chunku-Uchami), near the discovered large Yurubchen-Tokhomo zone.
Oil and gas accumulation zones The Ilimpeya PAZ is located in the eastern Evenki Autonomous Area west of the N-S course of the Lower Tunguska, in the Ilimpeya upper and middle reaches. It occurs within the Katanga province near its boundary with the Nepa-Botuobia province. The Ilimpeya petroleum accumulation zone (PAZ) occupies a fault terrace in the northeastern slope of the Katanga saddle and extends along the southeastern slope of the Kureika basin as far as the Nakannovsky salient. The discovered portion of the Ilimpeya PAZ covers 17,000 km2. This area remains poorly explored by a scarce network of low-fold (6–12 or occasionally 12–48) reflection profiling; high-fold (80) CMP profiling is currently underway. The production program of East Siberia and Sakha Republic (Yakutia) based on geological and geophysical prospecting and appraisal drilling has projected to check the predicted geological structure and reservoir parameters by drilling a deep well (Verkhneilimpeiskaya 277).
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The potential of the Ilimpeya PAZ has been inferred preliminarily from drilling evidence in the neighbor areas (well Chun’skaya 120, Yuzhno-Chun’skaya 107, Khoshonskaya 256, wells in the Sobinskoe and Paiginskoe fields, etc.) and from new reflection profiling data along the profiles Batholith, well Lebyazhinskaya 2–well Chun’skaya 120, and Altai–Severnaya Zemlya. The stratigraphy of the Ilimpeya PAZ generally correlates with that of the eastern slope of the Chunya basin (see above). A general idea of geology and tectonics of the area follows from its location at the junction of the Katanga saddle, the Kureika basin, and the Nepa-Botuobia anteclise. Of special prospecting importance is the position and geometry of the pinch-out line of Riphean strata and Vendian terrigenous deposits toward the Nepa-Botuobia anteclise superposed on the paleoshoreline and the associated tidal, deltaic, and sand bar facies (in Vendian strata). Most discoveries can be expected from Vendian terrigenous reservoirs, like the Sobinskoe and Paiginskoe fields located south of the Ilimpeya PAZ. Vendian strata in the latter, especially the Vanavara Formation, may bear up to five producing sandstone horizons (Vn-I to Vn-V) with thicknesses of 5 to 35 m separated by mudstone layers and members. The producing sandstone horizons may pinch out eastward to the Nepa-Botuobia anteclise. The rhythmically bedded Vanavara Formation may be a sandwich-type reservoir, with nonanticlinal, depositionally and tectonically truncated and mixed traps. Mudstones of the Upper Vanavara subformation and the sulfate Lower Oskoba subformation may be local screens. More producers may occur within the Oskoba Formation (or its stratigraphic equivalents) in the western Ilimpeya PAZ, where the Vendian section has a greater total thickness. In addition to the Vendian terrigenous deposits, hydrocarbons may be found in the head of Riphean carbonates, which underwent secondary alteration being exposed at the surface during the pre-Vendian deposition gap. Riphean reservoirs are of fractured-cavernous, porous-cavernous-fractured or fractured-karst types, with massive depositionally and tectonically truncated traps. Local screening in this case can be provided by the mudstones of the Vanavara Formation attenuated by pinch-out of sand beds. The OOIP reserves in the Ilimpeya PAZ are 14.6 BBOE (Table 1), as estimated assuming effective thicknesses of 5 to 20 m and 150 to 200 m and percentage porosities of 10–12% and 1.5% for rocks in Vendian terrigenous and Riphean reservoirs, respectively, an oil density of 800 kg/m3, with reference to a density of 795 to 850 kg/m3 in the Katanga saddle (Bitner et al., 1990), a fluid-filled porosity of 0.3
(according to appraisal data), and probable reserves over about a third of the area. The Kochema PAZ is located east of the Ilimpeya zone, in the Middle Kochema and Teteya catchments. The zone occurs in the territory of the Irkutsk Region in the western Nepa-Botuobia province, within a fault terrace in the western slope of the Nepa-Botuobia anteclise. The Kochema PAZ stretches over 16,500 km2. The available prospecting data are due to deep wells Teteiskaya 208 and Umotkinskaya 217 in the south and southeast of the zone and scarce reflection profiling in their vicinities. These data being insufficient to evaluate the potential of the eastern slope of the Chunya basin, it has been suggested to drill a parametric well (well 251) in the Middle Kochema dome uplift and to extend eastward the regional reflection profile, which ends at the Chun’skaya 120 well. The sedimentary section of the zone misses Riphean strata and Vendian terrigenous formations while Vendian carbonates lie immediately over the basement. The section starts with the Erbogachen dolomites, a reduced stratigraphic volume of the Tira Formation, which are overlain, without gap, by dolomites of the Preobrazhenka Horizon of the Katanga Formation. The Preobrazhenka Horizon stands out in logs by anomalously high apparent resistivity and low gamma-ray values. It is overlapped by argillaceous dolomite and marl of the Katanga Formation, which are apparently zonal seals. Up the section there follow the Ust’-Kut and Osa Horizons of the Upper Vendian–Lower Cambrian carbonate reservoir under a regional seal of the Upper Usolka Formation. The latter fact suggests a high potential of the Osa Horizon. The OOIP reserves in the Kochema PAZ approach 10 BBOE (Table 1), as estimated assuming an effective thickness of 20 m and a percentage porosity of 8% for rocks in Vendian carbonate and Lower Cambrian reservoirs, a fluid-filled porosity of 0.3, and probable reserves over about a third of the PAZ. Two more oil and gas zones (Yurubchen-Tokhomo and Chunka-Uchami) exist in the western side of the basin. The Yurubchen-Tokhomo PAZ occupies more than 15,000 km2 in the central part of the Baikit anteclise in the Baikit district of the Evenki Autonomous Area. Discoveries are mainly expected from Riphean carbonates. The YuTZ has been largely explored by drilling (more than 100 wells) and reflection profiling, which revealed the large Kuyumba and Yurubchen pools of oil, gas, and condensate. Hydrocarbons occur in upper Riphean cavernous-fractured dolomites eroded during the pre-Vendian deposition gap (Kontorovich et al., 1988). The zone contours are defined primarily by the extent
Table 1 OOIP Reserves in Oil and Gas Zones of Chunya Basin Zone
Kochema
Surface area, km2 16.5
Effective thickness of producer, m
Porosity, %
OOIP Reserves, BBOE
R
V
V2 − −C1
R
V
V2 − −C1
R
V
V2 − −C1
—
—
20–40
—
—
8
—
—
10
Ilimpeya
17
100–200
8–20
—
0.5–1.5
8–16
—
9.6
4.9
Chunku-Uchami
18
150–200
8–20
10–20
0.5–1.5
8–16
8
10.4
5.1
7.2
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of strongly eroded and fractured Riphean carbonates and are constrained by the Vanavara Vendian terrigenous carbonates. Riphean carbonate reservoirs are mainly of a cavernous-fractured type, possibly produced by postdepositional processes, in places where the overlying Vendian terrigenous deposits are thin (less than 40 m) or absent. The Yurubchen-Tokhomo PAZ is also bound by faults and by basement exposures where Riphean sediments are absent. The Riphean reservoirs of the zone are depositionally limited by exposure of impermeable or poorly fractured Riphean terrigenous and terrigenous-carbonate rocks at the erosion surface. In addition to Riphean reservoirs, oil and gas occur in Vendian strata, namely B-VIII and B-IX strata of the Oskoba Formation within the YuTZ. Regional, zonal, and local screens are provided by rocks from the Vendian-Cambrian salt-carbonate section from the Katanga Formation through the Usolka Formation. The latter is a regional screen. The stratigraphy of the YuTZ is generally the same throughout the western side of the Chunya sedimentary basin. The Chunku-Uchami PAZ is likewise located in the Evenki Autonomous Area over 18,000 km2 in the northern Baikit anteclise and in the eastern Bakhta megasalient from the Lower Chunku catchment to the middle reaches of the Uchami River. The Baikitskaya 1 and Verkhneamnunnakanskaya 187 wells, the nearest to zone west of it, stripped a complete Cambrian section, the Vendian Tetere, Soba, and Katanga Formations, and basement granites. Within the outlined contours, the Chunku-Uchami PAZ was explored by a scarce network of low- and high-fold (6–24 to 80) CMP profiles which detected a thick Riphean section pinching out westward. Oil and gas accumulations may be discovered in Riphean carbonates under a zonal screen of the Katanga argillaceous dolomite. Terrigenous Vendian deposits are absent from the western part of the zone, where the Vendian section includes the Katanga (with the Preobrazhenka Horizon), Soba, and Tetere (with the Ust’-Kut Horizon) Formations overlain by the Lower Cambrian Usolka Formation with the Osa Horizon at its base. Salts of the Usolka Formation make a regional seal for possible Riphean and Vendian oil and gas accumulations. Therefore, Upper Vendian-Lower Cambrian reservoirs in the western Chunku-Uchami PAZ have a good potential. According to seismic exploration data, the thickness of the Vendian section is greater in the eastern part of the zone, in the Chunku catchment, and terrigenous rocks of the Vanavara Formation may be found there by analogy with the Ilimpeya PAZ. The Vanavara Formation may bear several sand beds alternating with mudstones. The expected types of traps are non-anticlinal, depositional, structural, and mixed.
The Chunku-Uchami PAZ is possibly the eastern extension of the Cambrian southern marginal reef of the Tanachi-Deltula carbonate platform whose western part was explored by reflection profiling in the Bakhta megasalient territory. The potential of Riphean, Vendian, and Upper VendianLower Cambrian reservoirs in the zone can be tested by drilling an appraisal well (Suringdinskaya 274) east of the line where, according to our predictions, the Riphean section may pinch out and the Vendian section may thicken to include the Vanavara Formation. The total estimated OOIP reserves in the Chunku-Uchami PAZ are as large as 22.7 BBOE (Table 1).
Conclusions Thus, the large Riphean-Vendian Chunya sedimentary basin has been discovered and partly mapped in the central Siberian Platform as a result of prospecting through the recent decade. The Chunya basin is examined as the largest HC generator; hydrocarbon migration from the basin center to its periphery produced large PAZs. The large and very large petroleum fields in Riphean and Vendian PCs are discovered in zones explored by drilling.
References Bitner, A.K., Krinin, V.A., Kuznetsov, L.L., Nazimkov, G.D., Nakaryakov, V.D., Neshumaev, V.A., Pravotorov, S.B., Rasputin, S.N., Skrylev, S.A., 1990. The Petroleum Potential of Precambrian Reservoirs in the Western Siberian Platform [in Russian]. Yeniseineftegazgeologiya, SNIIGGiMS, Krasnoyarsk. Filiptsov, Yu.A., Boldushevskaya, L.N., Petrishina, Yu.V., Krinin, V.A., Kontorovich, A.A., 1998. Alteration grade of organic matter and prediction of the phase composition of oil in fields of different ages in the Siberian Platform and the West Siberian Plate, Krasnoyarsk Region, in: Geology and Mineral Resources of the Krasnoyarsk Region [in Russian]. KNIIGGiMS, Krasnoyarsk, pp. 79–94. Kontorovich, A.A., Kontorovich, A.E., Krinin, V.A., Kuznetsov, L.L., Nakaryakov, V.D., Sibgatullin, V.G., Surkov, B.C., Trofimuk, A.A., 1988. The Yurubchen-Tokhomo zone of gas and oil accumulation — an important object of concentration of regional and prospective work in the Upper Proterozoic of the Lena-Tunguska oil and gas province. Geologiya and Geofizika (Soviet Geology and Geophysics) 29 (11), 45–55 (42–50). Kraevsky, B.G., 2005. The Katanga District, in: Stratigraphy of Oil- and Gas-bearing Basins of Siberia. The Riphean and the Vendian of the Siberian Platform and its Folded Periphery [in Russian]. Akadem. Izd. “Geo”, Novosibirsk, pp. 78–86. Mel’nikov, N.V., 1994. Stratigraphic disconformities in a Vendian section of the Katanga Saddle. Geologiya and Geofizika (Russian Geology and Geophysics) 35 (4), 27–35 (22–30).
Editorial responsibility: A.E. Kontorovich 18 July 2007
Petroleum potential of the Riphean-Vendian Chunya sedimentary basin in the western Siberian Platform N.V. Mel’nikov a, *, Yu.A. Filiptsov b, V.I. Valchak c, E.V. Smirnov a, L.V. Borovikova a a
Siberian Research Institute of Geology, Geophysics and Mineral Resources, 67 Krasny prosp., Novosibirsk, 630091, Russia b Krasnoyarsknedra, 62 ul. Marksa, Krasnoyarsk, 660049, Russia c Yeniseigeofizika Ltd., 66 ul. Leningradskaya, Krasnoyarsk, 660074, Russia Received 5 November 2006
Abstract The petroleum potential of the Riphean-Vendian Chunya sedimentary basin has been explored by seismic reflection profiling and drilling in recent years. The results of the study have been used to estimate the initial hydrocarbon resources in the basin and separately in four oil and gas areas distinguished in Riphean, Lower Vendian, and Vendian-Lower Cambrian reservoirs. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: source rock potential; oil and gas potential; basement; Riphean-Vendian deposits; hydrocarbon migration; seismic reflection profiling; reflectors; Batholith profile; oil-bearing area, oil accumulation; initial hydrocarbon resources; Chunya sedimentary basin
Introduction In 2003–2005 seismic exploration discovered a large Riphean-Vendian sedimentary basin of Chunya in the southwestern Siberian Platform, with its most downwarped part in the Chunya River catchment opening northward to the Ilimpeya and Taimura catchments. Reflection profiling in the Chunya basin was run along the W-E Batholith profile and a profile north of Batholith, between the Lebyazhinskaya 2 and Chun’skaya 120 wells (Fig. 1). The two profiles traversed the basin western slope, with a piano-key structure of the Riphean sedimentary complex, and the central part of the basin. The W-E extent of the basin is more than 370 km from the Kuyumbinskoe field in the Kamo arch of the Baikit anteclise to the Nepa-Botuobia anteclise. In the N-S direction, the basin stretches from the W-E Riphean Delanin megaswell in the Podkamennaya Tunguska middle reaches (Taimba field) at least as far as the Srednetaimurinskaya 272 well in the north; the basin continues farther to the north but its northern closure remains so far unknown. The discovered part of the Chunya basin occupies 40,000 km2 and its total area may exceed 150,000 km2. The basin hosts the largest Kuyumbinskoe field of gas, condensate, and oil in Riphean reservoirs in its southwestern slope and the * Corresponding author. E-mail address: [email protected] (N.V. Mel’nikov)
large Sobinskoe and Paiginskoe oil, gas, and condensate fields in Vendian reservoirs in its southeastern slope. The thickness of Riphean deposits along the Batholith profile is over 6 km.
Basin’s architecture Reflection surveys along the two profiles (Batholith and well Lebyazhinskaya 2–well Chun’skaya 120) provided a general image of the basin architecture in its western and eastern slopes and in the deepest central part (Fig. 1). Riphean sedimentary strata rise, in monoclines, to the west and east, where they are discordantly overlain by Vendian formations. The Vendian strata in the basin’s center overlie Riphean formations without sharp unconformities. With this basin structure, hydrocarbon migration must be from the basin center to the west, southwest, and east along low-density zones at the surface of the Riphean-Vendian unconformity. We suggest that the eastern border of the Chunya basin should follow the line where Riphean formations pinch out at the junction between the Katanga saddle and the Nepa-Botuobia anteclise. The border is quite evident around the Khoshonskaya 256 and Ereminskaya 2 and 101 wells in which the Vendian terrigenous sediments (about 15 and 40–50 m, respectively) superpose the crystalline basement. The Batholith profile clearly resolves the pinch-out of the Riphean deposits (Fig. 1); the southward extension of the pinch-out
1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2008.02.001
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Fig. 1. Riphean-Vendian Chunya sedimentary basin. 1 — western slope; 2 — eastern slope; 3 — most deeply subsided part; 4 — piano-key structure; 5 — oil and gas fields; 6 — oil and gas areas; 7 — regional reflection profiles; 8 — deep wells (a), projected (b) and drilled (c) appraisal wells.; 9 — frontier of Evenki Autonomous Area. Abbreviations stand for names of wells, keyed as Bk — Baikitskaya, Vam — Verkhneamnunnakanskaya, Er — Ereminskaya, Km — Kuyumbinskaya, Tts — Teteiskaya, Um — Umotkinskaya, Khsh — Khoshonskaya, Chm — Chambinskaya, Chn — Chun’skaya, Yur — Yurubchenskaya, Schn — South Chun’skaya.
line appears in the Ereminskoe field east of the Sobinskoe and Paiginskoe fields, and its northern extension occurs in the Khoshonskoe field. The basin’s western border has been recognized from the pinch-out of the Riphean strata at the boundary between the Bakhta megasalient and the southwestern side of the Kureika syneclise. The pinch-out line is traceable in the NW direction along the eastern boundary of the basement block stripped by drilling in the northern slope of the Baikit anteclise and in the Bakhta megasalient, from the Kuyumbinskoe field across the Baikit and Upper Amnunnak fields. The occurrence of Riphean deposits east of that line has been proved in the area of Kuyumba field and detected along the well Lebyazhinskaya 2–well Chun’skaya 120 reflection profile. Strong reflectors in the Riphean section separated by more transparent rocks correspond to the surfaces R4 (roof of the Vedreshe Formation), R3 (Kopchera Formation), R2 (Tokur Formation) within the Kuyumbinskoe field. Riphean and Vendian rocks exhibit an evident block structure in the cross section (Fig. 2). In the present framework, there are two Riphean basins in the southwestern Siberian Platform, namely the Chunya and Irkineeva-Chadobets basins separated by the Delanin megaswell. The latter most likely appeared during the pre-Baikalian nondepositional hiatus, but in the Riphean there was a single basin spread continuously over the whole southwestern part of the platform.
Assuming the existence of a single Riphean basin, it appears reasonable to interpret the Riphean formations in the Yurubchen-Tokhomo zone (where the Kuyumbinskoe field was discovered) as the basin’s western part, which experienced an intense tectonic reconstruction during the pre-Vendian hiatus, unlike the eastern part, where reflection profiling shows no sharp Riphean/Vendian unconformities. The difference in the reflection patterns of the two basin’s parts may be due to their different structures and, possibly, basement ages.
Riphean formations The Riphean stratigraphy in the east of the Chunya basin is known from deep drilling in the Katanga saddle (Kraevsky, 2005). The section begins with the 225 m thick Pompot Formation composed of mudstone and siltstone interbedding with thin brecciated dolomite. It underlies the 250 m thick South Chunya Formation of thinly alternating mudstone and siltstone. The overlying Riphean rocks located farther to the southwest between the Paiginskoe and Sobinskoe fields remained underexplored by drilling (hypothetically, the Dzhelindukon and Upper Dzhelindukon Formations). The section farther to the west (Sobinskoe field) is composed of younger Riphean rocks. The drilled section
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Fig. 2. Expected oil-source reservoirs and migration paths of hydrocarbons in Chunya basin (from reflection surveys by Yeniseigeofizika Association, profile Batholith). 1 — Riphean-Vendian unconformity; 2 — reflector B; 3 — expected base of Riphean section; 4 — expected paths of hydrocarbon migration to base of Vendian reservoir; 5 — expected disjunctive tectonic dislocations.
begins with the >170 m thick sulfate dolomite Neryunda Formation composed of alternating dolomite and anhydrite with marl and mudstone interbeds and chert partings. Up the section there follows the 400 m thick Paiga Formation of stromatolite, microphytolite and detrital dolomite with chert lenses overlain by the 140 m thick Ayan Formation of black carbonaceous mudstone with marl and dolomite interbeds. The Ayan Formation underlies the Ushikta Formation 360 m thick, composed of dolomite with mudstone interbeds. At the top of the Riphean section there is a 160 m thick layer of gray or variegated stromatolite dolomite with interbeds of argillaceous dolomite. Generally, the Vendian Vanavara Formation in the territory of the Sobinskoe and Paiginskoe fields apparently lies over different Riphean units which are older in the east and younger in the west of the area. This model of the Riphean structure was validated by reflection profiling along the Batholith profile. Vendian strata overlie the Riphean strata conformably in the basin’s central part but the contact looks ever less
concordant eastward as the underlying Riphean deposits become older. Riphean rocks in the western slope of the basin are alternating argillaceous and carbonate layers stripped by numerous wells in the Yurubchen-Tokhomo zone. At the base of the section, there are quartz or quartz-feldspar sandstone and quartzite of the 280 m thick Delingdeken Formation that correlates with the Ayava Formation of the Katanga saddle. Then there follow argillaceous dolomite, dolomite and mudstone of the 280 m thick Madra Formation that correlates with the Upper Dzhelindukon Formation of the Katanga saddle. The Madra Formation underlies stromatolite, phytolite detrital, detrital, and microphytolite dolomite of the Yurubchen Formation (550 m), a stratigraphic equivalent of the upper Upper Dzhelindukon Formation stripped in the eastern slope of the basin. The Yurubchen Formation is overlain by silty argillaceous dolomite with interbeds of mudstone and glauconite sandstone of the Dolgokta Formation (100 m) which correlates with the Neryunda Formation of the Katanga saddle.
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The Kuyumba Formation of 470 m thick layered stromatolite dolomite correlates with the Paiga Formation in the eastern slope of the basin. The Kopchera Formation consists of 100 m thick silty argillaceous dolomite with mudstone interbeds and is an equivalent of the Ayan Formation in the eastern part of the basin. The Yukten Formation, 400 m thick, is composed of platy-columnar and detrital dolomite and correlates with the Ushikta Formation in the east. The rocks stripped up the section include no stratigraphic equivalents of the units from the Katanga saddle. The Rassolka Formation, 215 m, consists of argillaceous and stromatolite dolomite. The 65 m thick Vingolda Formation is composed of stromatolite dolomite. It underlies the 140 m thick section of red mudstone with dolomite interbeds of the Tokur Formation. At the top of the Riphean section within the YurubchenTokhomo zone there is the Iremeken Formation of columnar stromatolite and detrital dolomite with mudstone interbeds, some of the latter including over 200 m layers of highly carbonaceous rocks.
Vendian formations The Vendian strata in the Chunya basin include the Vanavara, Oskoba, Katanga, Soba, and Tetere Formations traceable throughout the territory. The Vanavara Formation comprises two units: — lower subformation (5–40 m) of two sandstone beds with a mudstone or a siltstone bed between them; — upper subformation (35–95 m) of mainly siltstone and mudstone, with up to three 7–18 m sandstone beds. Sandstone is inequigranular and quartzy, and includes siltstone and mudstone interbeds. General attenuation of the Vanavara Formation is accompanied by pinch-out of sandy layers. The Vanavara Formation is conformably overlain by the Oskoba Formation, which includes three subformations distinguished according to lithology (Mel’nikov, 1994): — lower subformation (31–55 m) of anhydrite dolomite, with argillaceous dolomite at the base; the Vanavara-Oskoba transition is most often gradual; — middle subformation (6–23 m) of mainly terrigenous rocks (mudstones with thin siltstone and sandstone interbeds). The subformation contains a highly radioactive sandstone bed (B-VIII) that gave an economic gas flow in the Omora accumulation; — upper subformation (0–47 m) of dolomite, anhydrite dolomite and argillaceous dolomite. The complete section of the formation was stripped in the western part of the Katanga saddle; the section in the eastern part, at the junction with the Nepa-Botuobia anteclise, misses the upper subformation, possibly as a result of erosion before the Katanga deposition. The Vanavara and Oskoba Formations are erosively overlain by Vendian carbonates of the Katanga, Soba, and Tetere Formations. The latter is of Vendian–Early Cambrian age.
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The Katanga Formation (100–150 m) is composed of dolomite marl and argillaceous dolomite and with a dark-gray dolomite member (Preobrazhenka Horizon) at its base. The Soba Formation (80–120 m) consists of gray dolomite with marl interbeds in the middle and upper sections. The Tetere Formation (30–70 m) is composed of dolomite, which is sulfate-bearing in the middle section. The Tetere Formation is conformably overlain by Lower and Middle Cambrian salt-bearing and dolomite formations (Usolka, Belaya, Bulai, Angara, and Litvintsevo). The latter formation underlies the Middle-Upper Cambrian Evenki Formation of redbed marl. The total thickness of the Cambrian section is 1320 to 1770 m.
Petroleum potential The potential of the Chunya basin is associated with Riphean, Vendian, and Upper Vendian-Lower Cambrian petroleum complexes, as well as with the overlying Lower Cambrian strata. Hydrocarbons in Riphean reservoirs may occur in places where carbonates subject to secondary alteration underlie Vendian deposits along the pinch-out line of Riphean strata at the boundaries of the basin with the Nepa-Botuobia anteclise and the Bakhta megasalient (by analogy with the Yurubchen-Tokhomo zone). The Yurubchenskoe and Kuyumbinskoe fields discovered in the west of the Chunya basin lie in Riphean reservoirs; flows of stratal water at a daily rate of 8–130 m3 were obtained from fractured-cavernous Riphean rocks in the east (Bitner et al., 1990). Porous sandstone beds may exist among Riphean terrigenous sediments in the eastern part of the basin at depths of about 4000 m. This hypothesis is supported by the absence of intense secondary alteration of Riphean sandstone according to drilling evidence from the Sobinskoe and Paiginskoe fields. Especially important is the discovery of highly carbonaceous rocks, possible oil source rocks, in the upper part of the Riphean section, both in the Yurubchen-Tokhomo zone and in the Sobinskoe field (Filiptsov et al., 1998). In the Yurubchen-Tokhomo zone, these rocks exist as 10 m of carbonaceous mudstone in the Iremeken Formation with 8.27% Corg on average. Organic matter (OM) in mudstone from the most highly elevated parts of the Kamo arch (well Yurubchenskaya 104) has a moderate alteration grade (no greater than MK21(G)−MK2(J) according to the Russian scale of catagenesis). Thermal decomposition of OM samples showed a relatively high residual content with a hydrogen index of 500–560 mg/g Corg. Carbonaceous rocks within the Sobinskoe field occur as the 140 m thick Ayan Formation of black mudstone with marl and dolomite interbeds, with a mean carbon content of 1.45%. The Ayan Formation was stripped also in deeper subsided parts of the basin farther to the north, where the alteration grade of organic matter is much higher (MK2(J)−MK13(K)) than that in the highest elevated part of the Soba swell (under MK21(G) grade).
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Fig. 3. A fragment of cross section along regional profile from Lebyazhinskaya 2 well to Chun’skaya 120 well (between wells Bk1–Chn120). For legend see Fig. 2.
Therefore, at least the youngest Riphean strata that sirvived the pre-Vendian erosion within the Chunya basin preserved their potential as source rocks, which was realized in the Phanerozoic. Oil and gas generation in Riphean rocks and the ensuing upward migration of hydrocarbons along faults in the Riphean and then along lateral pathways of low-density zones at the Riphean-Vendian unconformity surface produced several oil and gas areas (see below). The potential of Vendian strata is associated with the Vanavara Formation (regional reservoir) and Oskoba and Katanga Formations (regional screen), especially with producing sandstone beds in the Vanavara Formation. The producers appear where the formation thickness exceeds 30 m, according to drilling data (especially, wells Chun’skaya 120 of >60 m deep, Yuzhno-Chun’skaya 107 of 29 m, and Chambinskaya 114 of >40 m in the eastern basin slope). The Vanavara producers are sealed with a thick sulfate member of the Oskoba lower subformation. In the places where the Vanavara Formation is thinner than 20 m it consists mainly of mudstones which may be a zonal seal for secondary reservoirs located at the head of the Riphean carbonate section. Upper Vendian-Lower Cambrian reservoirs consist of the Soba, Tetere, and Osa carbonates, with the salt-bearing carbonate Upper Usolka subformation as a regional seal. The carbonates have low porosity and permeability. Cambrian reservoirs within the Belaya and Bulai Formations are of low potential. Discovery of hydrocarbons is most probable in the zone of Riphean-Vendian unconformity in the east at the boundary with the Nepa-Botuobia anteclise and in the zone of the piano-key Riphean complex in the west near the Baikit anteclise and the Bakhta megasalient. A lower potential of Riphean and Vendian reservoirs can be expected from anticlines in the basin’s central part.
According to estimates obtained at the Siberian Research Institute of Geology, Geophysics and Mineral Resources (Novosibirsk), the OOIP (original oil in place) reserves in the Chunya basin are approximately 39.8 BBOE in Riphean reservoirs, 24.4 BBOE in Vendian terrigenous reservoirs, and more than 6.4 BBOE in Upper Vendian-Lower Cambrian carbonate reservoirs. Proceeding from the available data, we predict two oil and gas accumulation zones (Ilimpeya and Kochema) confined to the eastern slope of the Riphean-Vendian Chunya sedimentary basin (Fig. 3) and one zone at the western basin slope (Chunku-Uchami), near the discovered large Yurubchen-Tokhomo zone.
Oil and gas accumulation zones The Ilimpeya PAZ is located in the eastern Evenki Autonomous Area west of the N-S course of the Lower Tunguska, in the Ilimpeya upper and middle reaches. It occurs within the Katanga province near its boundary with the Nepa-Botuobia province. The Ilimpeya petroleum accumulation zone (PAZ) occupies a fault terrace in the northeastern slope of the Katanga saddle and extends along the southeastern slope of the Kureika basin as far as the Nakannovsky salient. The discovered portion of the Ilimpeya PAZ covers 17,000 km2. This area remains poorly explored by a scarce network of low-fold (6–12 or occasionally 12–48) reflection profiling; high-fold (80) CMP profiling is currently underway. The production program of East Siberia and Sakha Republic (Yakutia) based on geological and geophysical prospecting and appraisal drilling has projected to check the predicted geological structure and reservoir parameters by drilling a deep well (Verkhneilimpeiskaya 277).
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The potential of the Ilimpeya PAZ has been inferred preliminarily from drilling evidence in the neighbor areas (well Chun’skaya 120, Yuzhno-Chun’skaya 107, Khoshonskaya 256, wells in the Sobinskoe and Paiginskoe fields, etc.) and from new reflection profiling data along the profiles Batholith, well Lebyazhinskaya 2–well Chun’skaya 120, and Altai–Severnaya Zemlya. The stratigraphy of the Ilimpeya PAZ generally correlates with that of the eastern slope of the Chunya basin (see above). A general idea of geology and tectonics of the area follows from its location at the junction of the Katanga saddle, the Kureika basin, and the Nepa-Botuobia anteclise. Of special prospecting importance is the position and geometry of the pinch-out line of Riphean strata and Vendian terrigenous deposits toward the Nepa-Botuobia anteclise superposed on the paleoshoreline and the associated tidal, deltaic, and sand bar facies (in Vendian strata). Most discoveries can be expected from Vendian terrigenous reservoirs, like the Sobinskoe and Paiginskoe fields located south of the Ilimpeya PAZ. Vendian strata in the latter, especially the Vanavara Formation, may bear up to five producing sandstone horizons (Vn-I to Vn-V) with thicknesses of 5 to 35 m separated by mudstone layers and members. The producing sandstone horizons may pinch out eastward to the Nepa-Botuobia anteclise. The rhythmically bedded Vanavara Formation may be a sandwich-type reservoir, with nonanticlinal, depositionally and tectonically truncated and mixed traps. Mudstones of the Upper Vanavara subformation and the sulfate Lower Oskoba subformation may be local screens. More producers may occur within the Oskoba Formation (or its stratigraphic equivalents) in the western Ilimpeya PAZ, where the Vendian section has a greater total thickness. In addition to the Vendian terrigenous deposits, hydrocarbons may be found in the head of Riphean carbonates, which underwent secondary alteration being exposed at the surface during the pre-Vendian deposition gap. Riphean reservoirs are of fractured-cavernous, porous-cavernous-fractured or fractured-karst types, with massive depositionally and tectonically truncated traps. Local screening in this case can be provided by the mudstones of the Vanavara Formation attenuated by pinch-out of sand beds. The OOIP reserves in the Ilimpeya PAZ are 14.6 BBOE (Table 1), as estimated assuming effective thicknesses of 5 to 20 m and 150 to 200 m and percentage porosities of 10–12% and 1.5% for rocks in Vendian terrigenous and Riphean reservoirs, respectively, an oil density of 800 kg/m3, with reference to a density of 795 to 850 kg/m3 in the Katanga saddle (Bitner et al., 1990), a fluid-filled porosity of 0.3
(according to appraisal data), and probable reserves over about a third of the area. The Kochema PAZ is located east of the Ilimpeya zone, in the Middle Kochema and Teteya catchments. The zone occurs in the territory of the Irkutsk Region in the western Nepa-Botuobia province, within a fault terrace in the western slope of the Nepa-Botuobia anteclise. The Kochema PAZ stretches over 16,500 km2. The available prospecting data are due to deep wells Teteiskaya 208 and Umotkinskaya 217 in the south and southeast of the zone and scarce reflection profiling in their vicinities. These data being insufficient to evaluate the potential of the eastern slope of the Chunya basin, it has been suggested to drill a parametric well (well 251) in the Middle Kochema dome uplift and to extend eastward the regional reflection profile, which ends at the Chun’skaya 120 well. The sedimentary section of the zone misses Riphean strata and Vendian terrigenous formations while Vendian carbonates lie immediately over the basement. The section starts with the Erbogachen dolomites, a reduced stratigraphic volume of the Tira Formation, which are overlain, without gap, by dolomites of the Preobrazhenka Horizon of the Katanga Formation. The Preobrazhenka Horizon stands out in logs by anomalously high apparent resistivity and low gamma-ray values. It is overlapped by argillaceous dolomite and marl of the Katanga Formation, which are apparently zonal seals. Up the section there follow the Ust’-Kut and Osa Horizons of the Upper Vendian–Lower Cambrian carbonate reservoir under a regional seal of the Upper Usolka Formation. The latter fact suggests a high potential of the Osa Horizon. The OOIP reserves in the Kochema PAZ approach 10 BBOE (Table 1), as estimated assuming an effective thickness of 20 m and a percentage porosity of 8% for rocks in Vendian carbonate and Lower Cambrian reservoirs, a fluid-filled porosity of 0.3, and probable reserves over about a third of the PAZ. Two more oil and gas zones (Yurubchen-Tokhomo and Chunka-Uchami) exist in the western side of the basin. The Yurubchen-Tokhomo PAZ occupies more than 15,000 km2 in the central part of the Baikit anteclise in the Baikit district of the Evenki Autonomous Area. Discoveries are mainly expected from Riphean carbonates. The YuTZ has been largely explored by drilling (more than 100 wells) and reflection profiling, which revealed the large Kuyumba and Yurubchen pools of oil, gas, and condensate. Hydrocarbons occur in upper Riphean cavernous-fractured dolomites eroded during the pre-Vendian deposition gap (Kontorovich et al., 1988). The zone contours are defined primarily by the extent
Table 1 OOIP Reserves in Oil and Gas Zones of Chunya Basin Zone
Kochema
Surface area, km2 16.5
Effective thickness of producer, m
Porosity, %
OOIP Reserves, BBOE
R
V
V2 − −C1
R
V
V2 − −C1
R
V
V2 − −C1
—
—
20–40
—
—
8
—
—
10
Ilimpeya
17
100–200
8–20
—
0.5–1.5
8–16
—
9.6
4.9
Chunku-Uchami
18
150–200
8–20
10–20
0.5–1.5
8–16
8
10.4
5.1
7.2
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of strongly eroded and fractured Riphean carbonates and are constrained by the Vanavara Vendian terrigenous carbonates. Riphean carbonate reservoirs are mainly of a cavernous-fractured type, possibly produced by postdepositional processes, in places where the overlying Vendian terrigenous deposits are thin (less than 40 m) or absent. The Yurubchen-Tokhomo PAZ is also bound by faults and by basement exposures where Riphean sediments are absent. The Riphean reservoirs of the zone are depositionally limited by exposure of impermeable or poorly fractured Riphean terrigenous and terrigenous-carbonate rocks at the erosion surface. In addition to Riphean reservoirs, oil and gas occur in Vendian strata, namely B-VIII and B-IX strata of the Oskoba Formation within the YuTZ. Regional, zonal, and local screens are provided by rocks from the Vendian-Cambrian salt-carbonate section from the Katanga Formation through the Usolka Formation. The latter is a regional screen. The stratigraphy of the YuTZ is generally the same throughout the western side of the Chunya sedimentary basin. The Chunku-Uchami PAZ is likewise located in the Evenki Autonomous Area over 18,000 km2 in the northern Baikit anteclise and in the eastern Bakhta megasalient from the Lower Chunku catchment to the middle reaches of the Uchami River. The Baikitskaya 1 and Verkhneamnunnakanskaya 187 wells, the nearest to zone west of it, stripped a complete Cambrian section, the Vendian Tetere, Soba, and Katanga Formations, and basement granites. Within the outlined contours, the Chunku-Uchami PAZ was explored by a scarce network of low- and high-fold (6–24 to 80) CMP profiles which detected a thick Riphean section pinching out westward. Oil and gas accumulations may be discovered in Riphean carbonates under a zonal screen of the Katanga argillaceous dolomite. Terrigenous Vendian deposits are absent from the western part of the zone, where the Vendian section includes the Katanga (with the Preobrazhenka Horizon), Soba, and Tetere (with the Ust’-Kut Horizon) Formations overlain by the Lower Cambrian Usolka Formation with the Osa Horizon at its base. Salts of the Usolka Formation make a regional seal for possible Riphean and Vendian oil and gas accumulations. Therefore, Upper Vendian-Lower Cambrian reservoirs in the western Chunku-Uchami PAZ have a good potential. According to seismic exploration data, the thickness of the Vendian section is greater in the eastern part of the zone, in the Chunku catchment, and terrigenous rocks of the Vanavara Formation may be found there by analogy with the Ilimpeya PAZ. The Vanavara Formation may bear several sand beds alternating with mudstones. The expected types of traps are non-anticlinal, depositional, structural, and mixed.
The Chunku-Uchami PAZ is possibly the eastern extension of the Cambrian southern marginal reef of the Tanachi-Deltula carbonate platform whose western part was explored by reflection profiling in the Bakhta megasalient territory. The potential of Riphean, Vendian, and Upper VendianLower Cambrian reservoirs in the zone can be tested by drilling an appraisal well (Suringdinskaya 274) east of the line where, according to our predictions, the Riphean section may pinch out and the Vendian section may thicken to include the Vanavara Formation. The total estimated OOIP reserves in the Chunku-Uchami PAZ are as large as 22.7 BBOE (Table 1).
Conclusions Thus, the large Riphean-Vendian Chunya sedimentary basin has been discovered and partly mapped in the central Siberian Platform as a result of prospecting through the recent decade. The Chunya basin is examined as the largest HC generator; hydrocarbon migration from the basin center to its periphery produced large PAZs. The large and very large petroleum fields in Riphean and Vendian PCs are discovered in zones explored by drilling.
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Editorial responsibility: A.E. Kontorovich 18 July 2007