The Prospects for Petroleum Exploration in the Eastern Sector of Southern Baltic As Revealed by Sea Bottom Geochemical Survey Correlated with Seismic Data

Przegl¹d Geologiczny, vol. 52, no. 8/2, 2004

The prospects for petroleum exploration in the eastern sector of Southern Baltic as revealed by sea bottom geochemical survey correlated with seismic data

Jerzy Dom¿alski*, Wojciech Górecki**, Andrzej Mazurek*, Andrzej Myœko**, Wojciech Strzetelski**, Krzysztof Szama³ek***

A b s t r a c t . In the Polish offshore £eba (B) tectonic block in the southeastern part of the Baltic Sea the oil and gas fields are accumulated in Middle Cambrian quartzose sandstone, often fractured and diagenetically sealed at depth by advanced silification developed in reservoir around the petroleum deposit. Petroleum traps are mainly of structure-tectonic type, i.e., anticlines closed with strike-slip faults. At least four gas-condensate and four oil deposits of total reserves more than 10 Gm3 gas and 30 Mt oil were discovered by the “Petrobaltic” Co. in the Polish Baltic sector. The subsurface petroleum deposits in the Cambrian reservoir are the source of secondary vertical hydrocarbon migration to the surface which produces surface microseepages and hydrocarbon anomalies. Geochemical survey of the sea bottom sediments and waters run along seismic profiles was completed in 1999–2002 within a joint project of “Petrobaltic” Co. Gdañsk and the Fossil Fuels Dept., AGH University of Science and Technology, Kraków, approved by the Ministry of Environmental Protection, Natural Resources and Forestry. It was found that seafloor hydrocarbon anomalies are closely related to subsurface geologic structure and location of petroleum deposits. Particularly the faults as principal venues for vertical hydrocarbon migration are reflected in high-magnitude seafloor anomalies. Above petroleum field there occurs a “halo” effect of high-magnitude anomalies contouring the deposit with damping related to productive zone situated inbetween. Thus, the section of sea bottom anomalies over a petroleum deposit resembles the shape of a volcanic caldera. Positive subsurface structures manifest themselves as neotectonic features in the sea-floor morphology and as petrological variations of the bottom sediments. Along the contours of petroleum field, the sea-floor seeps of gas and submarine springs of subsurface water occur. These are seismically recognizable as gas chimneys, geysers, craters and effusive cones. The sea-floor geysers and springs disturb thermal and density stratification of sea water column. The submarine geochemical studies strictly correlated with seismic profiles may contribute greatly to offshore petroleum exploration and marine environmental protection. Key words: petroleum exploration, Baltic offshore, bottom sea geochemical survey, surface hydrocarbon anomalies

Petroleum exploration in the Polish economic zone of were also small oil deposits: Dêbki (1971, depth 2,700 the Baltic Sea (totally 26,700 square kilometres) has been metres), ¯arnowiec West (1987) and Bia³ogard East (1990) run since 1975 by the Joint Petroleum Exploration Organi- (Fig. 2). zation (established by the East Germany, Poland and the Oil and gas deposits are accumulated in the Middle Soviet Union) transformed in 1990 into the state-owned Cambrian quartzsose sandstones of the thickness varying company “Petrobaltic”, and again in 2003 into the joint from 120 m in the nearshore zone to 60 m in the northern stock company “Petrobaltic” S.A. Up to date, the company zone extending several tens of kilometres offshore. Gas is completed regional hydromagnetic and gravimetric methane-dominated (70–90 vol.%) and contains higher surveysaswellasseismicsurveys, first regional (4x8 and gaseous hydrocarbons (6–25 vol.%), nitrogen (<5 vol.%), 2x4 km grid), then exploratory (2x4 and 2x2 km grid), and carbon dioxide (up to 2 vol.%) and traces of helium and finally, after location of potential structures, detailed argonium. Condensate concentrations are variable–from survey at 1x1 km grid, in order to localize the wildcat wells. 100 g/m3 in high-methane gas to over 250 g/m3 in other By 1997, offshore seismic lines of total length of gases. Oil from the Baltic deposits is light, low in sulphur 33,000 kilometres were shot. These data led to the identifi- and asphaltenes but rich in the gasoline fraction. Some oils cation of several dozens of structural and structure-tectonic reveal features intermediate between gasolines and the traps out of which 14 were selected as potential accumula- lightest oils, which enables them to be classified as hydro- tions and drilled with 25 wells of cumulative length of carbon condensates. 60,000 metres (Dom¿alski & Mazurek, 1997). Most of the discovered deposits and structures relate to The principal aim for Petrobaltic was to explore the the regional system of fault zones. Gas-condensate depo- offshore £eba (B) — (Figs 1, 2), Kuronian (D) and Gdañsk sits trend along the “£eba Arch” — the 80-kilometre-long (C) tectonic blocks for which the company has been gran- anticlinal belt adjacent to the £eba (Smo³dzino) Fault, ted the exploration and petroleum production concessions. which forms the western boundary of the £eba Block (B). In the offshore part of the £eba Elevation, four gas-conden- Along the £eba (Smo³dzino) fault, the rock formations are sate deposits (B4/1991, B6/1982, B16/1985, B21/1996) thrown at 200–350 metres to the west, i.e., towards the and three oil deposits (B3/1981, B8/1983, B24/1996) were S³upsk Block (A). This particular meridional fault provides discovered of the total reserves reaching 10 Gm3 of gas and closure to the gas-condensate deposits accumulated on its about 30 Mt of oil. In the onshore part of the £eba Eleva- eastern, upthrown wall: B16 and B21 (at 1,700 metres tion first, small oil deposit ¯arnowiec was discovered in depth) and B6 (at 1,410 metres depth) as well as to the oil 1970 at the depth of 2,750 metres. Next onshore discoveries deposit B34 (at 1,410 metres depth). The prospective reserves of natural gas in the Baltic Polish offshore sector are estimated to be of about 100 Gm3 *Petrobaltic Oil and Gas Exploration–Production Company and the prognostic oil reserves amount several hundred Inc., Stary Dwór 9, 80-958 Gdañsk, Poland millions of metric tonnes. **AGH University of Science and Technology Kraków, The offshore part of the £eba Block (B) covers an area Mickiewicza 30, 30-059 Kraków, Poland of some 7,000 square kilometres, which corresponds to ***Faculty of Geology, Warsaw University, ¯wirki i Wigury almost 4% of the entire Baltic Syneclise. The Syneclise is a 93, 02-089 Warszawa, Poland vast, marginal depression of the East European Platform

792 Przegl¹d Geologiczny, vol. 52, no. 8/2, 2004 developed as a foredeep at the front of meridional range of It is highly probable that similarly as the meridional Pomeranian Caledonides (Po¿aryski & Nawrocki, 2000; Smo³dzino (£eba) Fault turns into latitudinal Bia³ogóra Dadlez, 1993, 1995; Witkowski, 1989a, b). The main explo- dislocation, the meridional KuŸnica (W³adys³awowo) ration target is petroliferous Middle Cambrian sandstone Fault at its southern end (Jastarnia segment) turns to WSW, (Cm2 = Paradoxides paradoxissimus zone, also known as across the Puck Bay towards Wejherowo (Fig. 1). Deymenas Formation in the so-called Latvian, Lithuanian These turns and the transcurrent character of disloca- and Russian “Pribaltica” (Lendzion, 1988). In this particular tions suggest a dextral rotation of the entire £eba Block formation tens of oil deposits were discovered both offshore with the corresponding rotation of the axis of the Baltic and onshore Latvia (Kuldiga–Lipava), in western Lithuania Syneclise by 45o as early as during the Palaeozoic, which (Klaipeda) and in Kaliningrad = Königsberg = Królewiec was suggested in earlier publications. According to Strze- District (Russia) (Geodekian et al., 1976; Suvejzdis et al., telski (1979) and Górecki et al. (1979), the axis of the 1979, Afanasev et al., 1977; Gudelis & Jemelyanov, 1982; syneclise changed its direction from almost meridional Depowski et al., 1979; Witkowski, 1993; Górecki & Strze- (SSW–NNE) in the Cambrian to SW–NE and finally to telski, 1984). WSW–ENE recently. The centre of this rotation was loca- The petroleum source rocks for Cambrian reservoirs ted close to the western edge of the old East-European Plat- were presumably Middle and Upper Cambrian (Cm2+3), form, which suggests that the rotational movement was Ordovician (Or) and Lower Silurian (S1) dark claystones strictly related to the formation of Pomeranian Caledonides (Witkowski, 1988; Jarmo³owicz-Szulc, 2001; Karnkowski, and transcurrent movements along the T-T (Teisseyre-Tor- 2003). The principal hydrocarbon migration and accumula- nquist) fault zone (Brochwicz-Lewiñski et al., 1981; Dad- tion phase might have taken place during the final stage of lez, 1993, 1995; Po¿aryski & Nawrocki, 2000). The £eba Caledonian movements, i.e., in the Late Silurian–Early Uplift, initially located by the axis or in the southeastern Devonian (Siegenian = Erian phase). Subsequent Variscan limb of the syneclise, was finally moved to its northwestern deformations and uplifts caused restructuring and partial or limb, which was crucial for hydrocarbons migration direc- complete destruction of the previously formed petroleum tions and their accumulation in fault-related anticlines. It is deposits (Geodekian et al., 1976; Strzelski, 1979; Górecki et obvious that the faults provided the principal routes of al., 1979; Witkowski, 1993; Karnkowski, 2003). post-entrapment, vertical migration of hydrocarbons Reservoir properties of the Cm2 sandstones are mostly con- (mostly gaseous) which has been reflected by surface (sub- trolled by their secondary silicification and fracturing related to marine) geochemical anomalies observed nowadays. the depth of burial (critical depth of pressure-solution B - B³yskawica KP - Komendant Pi³sudski RY - Ryœ silification is 2,200–2,500 m) as well as to the thickness B³ - Ba³tyk KR - Krakowiak S - Sokó³ and facies development (Strzetelski, 1977a, 1979). BRZ - Burza KRK - Kraków SP-Sêp BT - Batory MW-Mewa ST - Stra¿nik Some deposits are accumulated in structural-lithologic CZ - Czajka MZ - Mazur ŒL - Œl¹zak CZP - Czapla N - Nurek W - Wicher traps sealed diagenetically due to advanced quartzitiza- DR - Dragon OR - Orkan £EBA BLOCK (B) (ROZEWIE II FAULT) WAW - Warszawa DZ - Dzik ORZ-Orze³ WL - Wilk tion of the sandstones (Sikorska, 1998; Jarmo³owi- G - Grom P - Piorun ST ¯ - ¯uraw B GF-Gryf PH - Podhalanin ¯B-¯bik G N cz-Szulc, 2001). However, the decisive entrapment GH-Gen.Haller RB - Rybitwa 3 W£ADYS£AWOWO-KU¯NICA FAULT GL - Garland ORZ factor were structural-tectonic deformation events. The JK - Jaskó³ka B4 GL KJ - Kujawiak 4 DZ B5 main venues of secondary, vertical water and hydrocar- 2 CZP S E´ OR BT PIAŒNICA FAULT D´ bon migration are mostly the micro- and macrofractu- ¯ARNOWIEC-DÊBKI–B3 FAULT KP B4 B3-3/81 B6-2 MZ B3-9/51 BRZ NW C´ . £E res in the fault-confined zones and along limbs of the BA B3-2/81 1 FA D B3-1 8 U B6-1 L T JK B8 SMO£DZINO (£EBA) FAULT TROUGH folds (Strzetelski, 1977a, b, c, 1979). W ROZEWIE 5 ¯B WAW S£UPSK E Generally, all the petroleum traps in the Cm2 b41 B7 BLOCK (A) DR

GH ROZEWIE I FAULT 9 sandstones are fault-related anticlines or cross-fault C RY SP MW 9 b44 KURONIAN A´ B´ BLOCK structures genetically related to parallel or mostly £EB B21 A-S b19 (D)

AM GF T B L IA FA b21b ULT ¯ U b45A PH meridional faults. Most part of the oldest (Baikalian) CZ KJ F JAS. GÓRA FAULT WL

A b21a B³ I KRK

W KR faults were rejuvenated at the end of Caledonian K B16 A b37 R ŒL R A RB L 6 b22 S K H A tectonic epoch and during the Variscan vertical M b1 P F´ A 22a B N 22b - S 7 £EBA-SAMBIA FAULT movements (Balashov et al., 1972; Modlinski, 1976; M O

£ D Z BIA£OGÓRA F I 4 3 DÊBKI Stolarczyk, 1979). N O 1 W£ADYS£AWOWO 4 123 25 F A 798 4 2 ¯ARN. GDAÑSK BLOCK U 5 MIEROSZ. 8 At present the £eba Elevation forms the northwe- L LUBINY 1 BIA£OGÓRAPIAŒNICA FAULT (C) T £EBA 8 2770,5 2735 DAR¯LUBIE IG 1 HEL IG 1 stern limb of the Gdañsk-Kuronian (Modlinski, 1976; 3046 SMO£DZINO Górecki et al., 1979) central depression of the southern 2761,5 part of Baltic Syneclise (Fig. 1). The top surface of the LÊBORK IG 1 Cambrian formation is identified seismically by the Or 3295 S£UPSK KRYNICA MOR. 2 reflector and dips towards the south and southeast to the 2805,5 depths 2,500–2,900 m along the Baltic Sea coast, then to GDAÑSK IG 1 isolines of Cambrian top (m b.s.l.) NIESTÊPOWA 1 3375 GDAÑSK 3137,5 3,500–3,800 along the S³upsk–Lêbork–Kartuzy line and supposed isolines of Cambrian top (m b.s.l.) N. KOŒCIELNICA finally to 4,000–4,500 m along the axis of the Gdañsk 3202 M3 faults 2765,5 ELBL¥G Depression. The latter rises to northeast, to about 3,000 supposed faults Cambrian hydrocarbon traps 2 m depth at the Hel Peninsula (Fig. 1). DÊBKI major boreholes 3251 PAS£ÊKIG1 KOŒCIERZYNA IG 1 IG-1 2726 AA´ 4283,5 MALBORK 3 Faults interpreted from the seismic data (Piaœnica, section lines 3257 Smo³dzino, £eba–Sambia, ¯arnowiec–Dêbki, Kar- 0 25km wia–Jastrzêbia Góra, Rozewie, KuŸnica) are of stri- Fig. 1. Structure contour map of the Cambrian sediments and petroleum ke-slip character, as revealed by their echelon prospective zones as inferred from sea bottom geochemical studies corre- pattern, alternating position of fault drag structures, lated with subsurface geology from seismic profiles. Perspective zones slightly curved strike and changing throw. were given the names of former warships of the Polish Navy

793 Przegl¹d Geologiczny, vol. 52, no. 8/2, 2004

Onshore surface geochemical survey has been run sin- of Environment Protection, Natural Resources and Fore- ce 1991 by the research team of the Department of Fossil stry. The project included geochemical survey of marine Fuels, AGH University of Science and Technology in Kra- sediments and adjacent coastal zone extending from the ków, headed by one of us (W.G.). The survey confirmed the Gdañsk Bay to the Odra River estuary. In the years applicability of the surface free-gas method for hydrocar- 1999–2002, the geochemical survey of the sea floor was bon exploration in the Polish Lowlands and the Carpathian completed only in the area of the £eba Offshore Block (B). Foredeep, and demonstrated the close relation between the A few reconaissance profiles were made also towards the pattern of surface hydrocarbon anomalies and the subsurfa- Kuronian Offshore Block (D) and Gdañsk Block (C). ce position of petroleum deposits in the Fore-Sudetic area The project relied upon the occurrence of a close linka- and the Lublin Graben, as well as in the Pomerania (Polish ge between the results of geochemical survey of the sea flo- Baltic coast) (Górecki et al., 1995a, b; Strzetelski, 1996, or and the seismic subsurface structure data. Therefore, the Strzetelski et al., 1996). In 1996, the Department of Fossil offshore geochemical traverses were positioned exactly Fuels completed and interpreted seven geochemical profi- along the already completed seismic lines. Such positio- les run along the coast in the onshore part of the £eba ning required precise navigation of the research vessel Elevation and along the Hel Peninsula. along the planned, optimized course. In 1997, one of us (K.S.) has initiated the research pro- The working group from the “Petrobaltic” Company ject aimed at the examination of the Recent sea bottom reprocessed over 1,000 kilometres of seismic sections and sediments in the Southern Baltic, approved by the Ministry compared them with the results of geochemical survey. A' A

NNW SSE [sigma] 2,5 METHANE IN SEA BOTTOM WATERS = MET(W)

2,0

1,5

1,0 F F F 0,5

0 [sigma] 3,0 TOTAL HIGHER HYDROCARBONS IN BOTTOM SEA WATERS = CHC(W) 2,5 CHC(W) 2,0

1,5 "Caldera" "Caldera" 1,0 damping zone damping zone F F F F F 0,5

[mg/l] 27 LIQUID HYDROCARBONS IN WATER = HCP(W) 24 21

204,0

18 HCP(W) do 15 "Caldera" "Caldera" 12 damping zone damping zone 9 6 3 0 THE ZONE OF TURNING OF MERIDIONAL £EBA (SMO£DZINO) £EBA-SAMBIA £EBA (SMO£DZINO) FAULT MERIDIONAL FAULTS ZONE (F) LATITUDINAL FAULT (F) [m] INTO LATITUDINAL BIA£OGÓRA FAULT (F) 0 THE SECTION OF SEA WATER TEMPERATURE AND DENSITY STRATIFICATION 10

50 0 [cm] 10

852-24 853-25 854-26 851-23

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848-20 840-12 841-13 847-19 837-9 842-14 839A-11A 838-10 843-15 846-18 839-11 844-16 845-17 Profile 539M

60 Seismic line 539B 20

-10

380 180 460 540 260 340 140 220 300

420 500 100

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41 81

361 281 121 401 321 571 441 161 561 201 481 301 551 241 544 B2d GAS B21b PROSPECTIVE B22 PROSPECTIVE POOL B21 STRUCTURE STRUCTURE 571 500 450 400 350 300 250 200 150 100 50 1 150100 150 200 250 300 350 400 450 500 550 0 0

250 250

500 500

750 750

1000 1000

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2250 2250 Fig. 2. Offshore geochemical-seismic profile A-A’ (NNW–SSE) running from B21 gas-condensate deposit accumulated on hanging wall of the Smo³dzino (£eba) meridional fault to the tectonic crossing of the Smo³dzino (£eba) and the latitudinal £eba–Sambia and Bia³ogóra Fault

794 Przegl¹d Geologiczny, vol. 52, no. 8/2, 2004

This enabled a direct correlation of sea-floor geochemical and sediment samples, and their concentrations and fraction anomalies with geological setting of the area. First, the composition were analysed (Tkachenko, 2001). regularities in anomalies distribution over known petro- Totally, 2,600 sediment/bottom water samples and leum deposits were considered. Then, the established regu- 1,500 surface water samples (from the so-called “microzo- larities were applied to the recognition of further ne”) were degassified. The structure of the sea water prospective petroleum structures. column was analysed at 3,000 measurement sites. More- Geochemical samples were collected from the research over, at 1,200 sites the liquid hydrocarbon contents were vessel with the probe equipped with the bathymeter and the analysed in water — HCP (W) and in sediments — HCP instruments recording the physical parameters of sea water. (OS). Finally, the interpretation included changes in con- The “Ocean-0.25” probe ensured the sampling of sediments centration of liquid and gaseous hydrocarbons in the column of undisturbed structure along with the sea-bottom sea-floor sediments and bottom waters, as well as measure- water. Air-tight water and sediment samples were degassi- ments of thermal properties and density of sea water fied and gas was analysed for hydrocarbon concentrations. column (Tkachenko, 2001). For the purposes of the pro- Moreover, liquid hydrocarbons were extracted from water ject, the following patent was applied: “Method of recognition and identification of anomalies of migrating gaseous and C C'

-4 3 10 •cm SSE NNW kg METHANE IN SEA BOTTOM SEDIMENTS AND WATER = MET(OS+W) 19 18 CONTOURING 17 ANOMALIES 16

12 F [sigma] "Caldera" 3 11 damping zone 10 2 9

1 8

7 0 MET(OS+W) 6 5

1 -4 3 10 •cm kg TOTAL HIGHER HYDROCARBONS IN BOTTOM SEA SEDIMENTS AND WATER = CHC(OS+W) 11 10

7 [sigma] 3 6

2 5 CHC(OS+W) "Caldera" 1 4 damping zone 0 3

1 £EBA (SMA£DZINO)

FAULT (F) [mg/l] THE LIQUID HYDROCARBONS IN BOTTOM SEA SEDIMENTS [HCP(OS)] AND WATER [HCP(W)] 12 HCP(W) HCP(OS) [% wt] 9 0,9

6 0,6

3 0,3

0 0

DISTURBED SEA WATER STRATIFICATION [m] THE SECTION OF SEA WATER TEMPERATURE AND DENSITY STRATIFICATION 0 10

[cm] 60 0 70 10

901-1

891-11

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894-8

900-2 884-18 890-12

880-22

896-6 886-16 897-5 887-15 893-9

883-19 876-26 877-25

888-14 898-4 899-3 889-13

879-23 878-24

895-7 885-17

875-27

400 500 540 580 620 660 700 740 780 820 860 900 940 980 1020 B6 GAS-CONDENSATE FIELD SEISMIC LINE 237M F 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 0 0 GAS DIFFUSION CHIMNEY AND BOTTOM SEA CRATER 250 250

500 500

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2250 2250 Fig. 3. Offshore geochemical-seismic profile C-C’ (SSE–NWW) showing on its northern ending the location of B6 gas-condensate deposit accumulated on the hanging wall of the meridional Smo³dzino (£eba) Fault

795 Przegl¹d Geologiczny, vol. 52, no. 8/2, 2004 liquid hydrocarbons over the sea bottom as indicators of petroleum structures are indicated by high, anomalous haloes leum deposits accumulation and natural contamination of mari- which contour the oil and gas deposits. ne environment (patent No. P346204 Szama³ek et al., 2001). Over 2,700 samples of gases collected from sea-floor Even the preliminary results indicated that liquid and sediments were analysed at the laboratory of the Depart- gaseous hydrocarbon anomalies are mostly of deep origin ment of Fossil Fuels, AGH University of Science and Tech- and correlate well with subsurface petroleum structures, nology in Kraków, namely for methane and higher gaseous thus, providing a credible petroleum exploration indicator. hydrocarbons: alkanes (ethane, propane, i-n butane, i-n The faults are disclosed mostly by increased methane con- pentane) and alkenes (ethylene, propylene, 1-butene). centrations, whereas both the offshore and onshore petro- N-hexane was so rare that its analyses were cancelled.

D D'

WSW ENE TOTAL HIGHER HYDROCARBONS IN BOTTOM SEA SEDIMENTS AND WATER = CHC(OS+W)

"Caldera" damping zone

METHANE IN BOTTOM SEA SEDIMENTS = MET(OS) MET(OS) F

SATURATED HYDROCARBONS IN BOTTOM SEA SEDIMENTS = NHCH(OS) NHCH(OS) "Caldera" damping zone

THE LIQUID HYDROCARBONS IN BOTTOM SEA SEDIMENTS [HCP(OS)] AND WATER [HCP(W)] HCP(OS) HCP(W)

THE SECTION OF SEA WATER TEMPERATURE AND DENSITY STRATIFICATION ¯ARNOWIEC -DÊBKI FAULT ZONE

SUBMARINE SPRING

B3 PROSPECTIVE PETROLEUM ZONE No1”BURZA” FIELD

1475-17

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40 80

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1396-39 1397-38 1398-37 1399-36 1400-35 1401-34 1402-33 1403-32 1404-31 1407-28 1408-27 1409-26 1410-25 1411-24 1412-23 1413-22 1414-21 1415-20 1416-19 1417-18 1418-17 1419-16 1420-15 1421-14 1422-13

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100 200 500 400 300

1501

1200 1400 1300

550 500 450 400 350 300 250 200 150 100 425 400350 300 250 200 150 100 50 1 1400 1350 1300 1250 1200

GAS DIFFUSION CHIMNEY AND BOTTOM SEA CRATER

0 500 1000m

Fig. 4. Offshore geochemical-seismic profile D-D’(WSW–ENE) showing the location of B3 oilfield accumulated in a fault-confined anticline on the northern extension of the ¯arnowiec–Dêbki Fault zone (western ending of the profile) and the prospective zone “Burza” on the hanging wall of the meridional Rozewie Fault

796 Przegl¹d Geologiczny, vol. 52, no. 8/2, 2004

Totally, 28,000 single component analysis were comple- bottom waters (up to 30 centimetres over the sea floor). ted with the use of GC 8160 Fission Instruments chromato- The deep structures, e.g., B4 and B5, are manifested by graph and flame-ionisation selection. Accuracy of analyses neotectonic morphological heights and sandy sea-floor was 0.005 ppm. Obtained chromatographs were subject to sediments. Also the faults reaching the sea-floor surface computer processing and interpretation using WINNER are marked with neotectonic forms: scarps and breaks. Spectra Physics software. Results including methane, total The patterns of faults and subsurface petroleum struc- higher alkanes and total alkenes concentrations were proces- tures are undoubtedly reflected by general changes in sed statistically and normalized to geochemical background hydrocarbon concentrations in sediments and bottom waters. level determined with an original method developed at the From the point of view of petroleum exploration the most Department of Fossil Fuels, AGH University of Science and important are zones in which anomalies of various compo- Technology in Kraków (Dzieniewicz & Moœcicki, 1983; nents overlap. Additional shows confirming the zones of Dzieniewicz & Sechman, 2001). increased vertical hydrocarbon migration are sea-floor Methane was detected in all analysed samples and rea- springs of subsurface waters, floor gas seepages and related ched magnitudes as high as 22,710 ppm. Methane anoma- disturbances in water column stratification. Studies on the lies were found in 1/3 of all samples. Anomalous structure of sea-water column enabled the location of inten- concentrations of total higher gaseous alkanes up to 104 sive hydrogeothermal fluxes over the sea-floor groundwater ppm and those of total alkenes up to 11 ppm were indicated springs. Such flows, in turn, correspond in most cases to in 45% of all samples. increased concentrations of liquid and gaseous hydrocarbons Statistical distributions of methane, total alkanes, i-bu- in sediments and bottom waters. Flows of warm, highly mine- tane and i-pentane concentrations point to the contribution ralized groundwaters disturb and break through the layers of of several sources of hydrocarbon emanations. This may cold sea water thus causing the thermal inversion. result from diversified mechanisms of vertical hydrocar- Directly over the petroleum deposits the zones of bon migration (diffusion, effusion, filtration, microfracture suppressed hydrocarbon anomalies are visible. These are flow of gas bubbles) from petroleum deposits of various contoured by high-magnitude anomalies, which in a sec- composition (oil, gas condensate) or from the sea-floor tion resembles as a whole the shape of a volcanic caldera. biochemical processes and sea-floor contamination. An example of such a “caldera” is the seismic-geoche- However, the statistical approach applied enables to distin- mical traverse A-A’ (NNW–SSE) (Fig. 2) running obliqu- guish the effects of recent biochemical processes and ely to the B21 gas-condensate deposit which is sea-floor contamination from the anomalies that really ori- accumulated in fault-confined anticline of an amplitude ginate from the deep sources, i.e., petroleum deposits or 10–35 metres located at 1,700 metres depth on the eastern, source rocks. Nevertheless, statistical distribution analysis upthrown wall of the Smo³dzino (£eba) Fault. Over the of gaseous hydrocarbon concentrations in bottom sedi- deposit the relative suppression of liquid hydrocarbon HCP ments clearly indicates the deep origin of most of the ano- (W) and total higher gaseous hydrocarbon CHC (W) anomalies observed. Moreover, the character of some malies is observed (“caldera effect”). The caldera effect is anomalies advocates for the occurrence of oil deposits in visible further to SSE (Fig. 2) where the traverse crosses the subsurface. the B21b petroleum prospective zone (Fig. 1), which forms Correlation of surface geochemical anomalies with the the structural nose plunging to the south. Therefore, the seismic data included the comparison of the seismic sections, pattern of anomalies shows the additional petroleum pro- the structural map of the Cambrian top surface (or seismic spects confined with that secondary structure thus proving reflector, Fig. 1) and the results of geochemical survey (Figs the B21 structure to be more extended and diversified. 2–4) which presented the changes in concentrations of: Here appears the problem of possible petroleum prospects methane (MET) in bottom sediments MET (OS) and of seismically poorly reflected structural terraces and noses bottom waters MET (W), total higher gaseous hydrocarbons (CHC) in bottom that occur in the central parts of tectonic blocks, apart from sediments CHC (OS) and bottom waters CHC (W), readily visible framing faults which directly close and seal total saturated hydrocarbons (NHC) in bottom sedi- the already discovered tectonic traps. ments NHC (OS) and bottom waters NHC (W), and in More to the north, along the same fault-related £eba waters with the application of smoothing procedure anticlinal trend, the seismic-geochemical traverse C-C` (“Heming filter”) NHCH (W), (SSE–NNW) was run (Fig. 3). The traverse is running total liquid hydrocarbons (HCP) in bottom sedi- obliquely to the fault-confined anticline of an amplitude 45 ments HCP (OS) and bottom waters HCP (W). metres which reservoirs the B6 gas-condensate deposit at The clayey bottom sediments reveal high sorption capa- the depth of 1,450 metres. The zone of damped concentra- city, thus entrapping a significant part of migrating hydro- tions of methane (MET), higher gaseous hydrocarbons carbons. Particularly, the content of liquid hydrocarbons in (CHC) and liquid hydrocarbons (HCP) is visible both in unconsolidated black clays covering the sea floor at depths sea-floor sediments (OS) and the bottom waters (W). The below 90 metres is a perfect indicator of deep-sourced ano- “caldera” shape of surface anomalies determined by “met- malies. Of course, the black, deep-sea clays with characteri- hane shadow” is 1.4 km wide and its position corresponds stic, greenish hue contain high amounts of decaying organic to the location of the B6 gas-condensate deposit. From the matter, hydrogen sulphide and biogenic methane. NW the contouring anomalies, MET (OS+W), CHC Lithology of bottom sediments is closely related to the (OS+W) and HCP (W), are particularly high as they occur morphology of sea floor and wave-base. In the central part over the Smo³dzino (£eba) Fault, which provides the trap of Baltic Sea the zones shallower than 60–70 metres are closure. It should be noted that over the faults which do not eroded by storm waves of wavelength over 100 metres. disturb the Silurian strata, gas diffusion chimneys and sea Morphological sea-floor heights of neotectonic origin are bottom effusion craters are also visible. The B6 anticline is covered with sand/gravel sediments in which gas sampling also indicated by the present morphology of sea-floor was useless. In such areas gas samples were taken from height and sandy character of Recent bottom deposits,

797 Przegl¹d Geologiczny, vol. 52, no. 8/2, 2004 which resulted from a neotectonic uplift and erosion of accompanying fracture zones manifested by sea bottom Recent sediments over the crest of the structure. anomalies (mostly methane) occurring directly over the Over the crest part of the B6 structure, as well as over its faults or aside the fault-plane, commonly with two peaks eastern limb and southern pericline the sea-floor springs of separated with a mute point over the fault, which suggests warm, mineralized waters were found together with adequ- the sealing character of the fault fracture itself. ate thermal inversion of the sea water column. Disturbances 3. Directly over or aside the faults the zones of seismi- and thermal/density inversions of sea waters correlate with cally recognizable diffusion chimneys (gas clouds) and effu- geochemical anomalies and their intensity reaches its maxi- sive sea-floor craters and cones appear, marking the gas mum values over the contour of petroleum field (Fig. 4). seepages and warm subsurface waters submarine springs About 25 kilometres ENE from the B6 structure the seis- disturbing the thermal and density stratification of sea water. mic/geochemical profile D-D` (WSW–ENE) was run (Fig. 4. Methane and total higher gaseous hydrocarbons ano- 4). The traverse cuts the fault-related B3 structure (amplitu- malies correlate mostly with tectonic zones and limbs of de: 50 metres, depth: 1,280 metres) closed by the offshore structures. Methane anomalies appear also over the crests of extension of the ¯arnowiec–Dêbki fault zone (Fig. 1). The hydrocarbon traps. Both the methane and the higher gaseous traverse revealed a wide (about 5 kilometres) zone of rela- hydrocarbons concentrations in the “microlayer” of surface tive decrease in methane MET (OS), higher gaseous hydro- sea water correspond solely to the strike of faults. The total carbons CHC (OS), saturated hydrocarbons NHCH (OS) higher gaseous hydrocarbons concentrations in sea water and liquid hydrocarbons HCP (W) anomalies, bordered increase down the limbs and over local crestal heights of from both sides by contouring, high-magnitude anomalies. structures. Above the petroleum deposits the overall incre- The zone clearly corresponds to the position of the B3 oil ase of liquid hydrocarbons concentrations in sediments and field. Over the ¯arnowiec–Dêbki fault zone, the methane sea bottom waters is visible. Such anomalies rise up over the anomaly MET (OS) was found. Moreover, close to the con- limbs of structures and over cross-cutting faults. It is infer- tour of this petroleum deposit, the submarine springs of red that such anomalies are the most adequate indicators of warm, mineralised waters were discovered. These also pro- vertical hydrocarbon migration from the depth. duce the temperature inversion, thickness variations and 5. The petroleum accumulation zones in the subsurface disruptions of cold sea water transitional layer with its war- are reflected by a general increase in hydrocarbon concen- ped top surface (Tkachenko, 2001). trations in the sea bottom zone. On the other hand, the halo- It is to be mentioned that the “caldera” of low-magnitu- es of anomalies over petroleum-bearing structural traps de concentrations of liquid hydrocarbons over the B3 oil commonly occur. As a consequence, the zones of petro- deposit which has been in operation for 12 years (since leum accumulation are reflected as relative decreases in 1991) demonstrates its environmentally friendly explora- anomalous hydrocarbon concentration values contoured tion and production run by the “Petrobaltic”. by “haloes” or “calderas” of high-magnitude anomalies Basing upon the results of the above geochemical project (Figs 2–4). Such a pattern results presumably from most “Geochemical indicators of hydrocarbons occurrence based effective sealing provided by caprocks directly over crest upon the analysis of southern Baltic Sea, Polish Offshore £eba of the deposit and a contrastingly intensive fracturing and Block (B)”, run in the years 1999–2002 by the “Petrobaltic”, fissuring on more deformed limbs and periclines of the nine prospective areas were selected for further petroleum structure, as well as along the fault zones closing the trap. exploration (Fig. 1). These areas were enumerated according to 6. Faults and positive deep structures manifest themselves as neotectonic features in the sea-floor morphology, i.e., as the rising exploration risk, considering the geochemical and scarps and uplifts together with an increased content of sandy hydrochemical indicators along with expected entrapment coarse grain fraction in Recent sediments. The contours of capability of a given recognized geological structure. petroleum deposits are accompanied by sea-floor gas seeps Further analysis of the pattern of specific sea-floor ano- and subsurface water submarine springs manifested by malies, geological structure and tectonics allowed to select geisers, craters and effusive cones, as well as by thermal and 30 smaller zones and sites prospective for hydrocarbon density disturbances of stratification of the sea water column. exploration (Fig. 1). These localities were given the names 7. Submarine geochemical and seismic identification of of former warships of the Polish Navy, which on week days barren and petroliferous traps requires the relevant, deta- helps and closely cooperates with the “Petrobaltic”. iled studies including the recommended geochemical The selected prospective zones embrace not only the survey of the sea-floor in the area of structures localized most promising fault-related elevations occurring along and contoured with seismic data. If such survey confirms the upthrown walls of longitudinal and meridional faults, the occurrence of possible petroleum accumulation in a but also alternating, drag anticlines on downthrown walls, given structure, geochemical data may provide an impor- as well as structural terraces and noses located upon central tant contribution to the location of a wildcat well and may part of tectonic blocks. even enable the contouring of a future petroleum field. Correlation of the results of geochemical and seismic Considering the above conclusion coming from the survey, and the location of petroleum deposits already regional studies completed in the years 1999–2001 in the discovered in the Cm2 sandstones in the offshore part of the area of the offshore £eba Block (Tkachenko, 2001), the £eba Elevation allow to conclude the following: detailed geochemical survey was run within the B5 structure 1. The pattern of relative variations in hydrocarbon that had been seismically contoured on the western concentrations in sea-floor sediments and bottom waters is upthrown wall of the northern extension of KuŸnica generally closely related to the subsurface geological struc- (W³adys³awowo) Fault (Fig. 1). The survey covered the area ture of the area. Hence, the sea-floor geochemical anoma- of 150 km2 applying 1x1 kilometre grid of submarine geo- lies are mostly of deep origin and, consequently, the chemical profiles. The concentric halo pattern was found of regularities in their distribution can be applied to petroleum contouring methane and higher gaseous hydrocarbons ano- exploration, thus following the results of seismic survey. malies sampled from bottom sediments and waters with the 2. The principal venues for vertical hydrocarbon migra- characteristic damping zone over the top of petroliferous tion from reservoir rocks to the sea-floor are faults and structure. The distribution of submarine hydrocarbon ano-

798 Przegl¹d Geologiczny, vol. 52, no. 8/2, 2004 malies led to the prediction about the occurrence of an oil Mat. Konf. „Nauki o Ziemi w badaniach podstawowych, z³o¿owych deposit with a possible gas cap since the lack of saturated i ochronie œrodowiska na progu XXI wieku”. Jubileusz 50-lecia WGGiOŒ, Kraków 28–29 czerwiec 2001: 357–361. hydrocarbons among higher gaseous hydrocarbons, typical GEODEKIAN A.A. (ed.) 1976 — Geologicheskoe stroenie i perspek- of free gas deposits was observed there (Tkachenko, 2001). tivy neftegazonoznosti centralnoj Baltiki. Izd. Nauka, Moskva: 110. The offshore B5-1/01 well (sea depth 88 metres) drilled GÓRECKI W., NEY R. & STRZETELSKI W. 1979 — Rozwój pale- ostrukturalny kambru w aspekcie poszukiwañ z³ó¿ wêglowodorów from a marine platform located about 45 km east from the w po³udniowej czêœci Ba³tyku. PAN Oddz. Kraków, Kom. Nauk Geol., already producing B3 oil deposit and about 100 kilometres 103: 56. from Gdañsk was started in July of 2001. The consecutive GÓRECKI W. & STRZETELSKI W. 1984 — Uzasadnienie perspek- tyw roponoœnoœci kambru starej platformy. IV Konferencja Kraków lithologies included: Quaternary (thickness 8 metres), 25–26X1984. Wyd. AGH, Kraków. Devonian dolomites, claystones and mudstones (thickness GÓRECKI W., STRZETELSKI W., DZIENIEWICZ M., SECHMAN H. & REICHER B. 1995a — Surface geochemical survey of gas accu- 494 metres), Silurian claystones (thickness 1,243 metres), th Ordovician marls and marly limestones (thickness 90 mulations in Polish Lowlands. 7 EAPG/54-th EAEG Conf., Glasgow, Scotland 29 May-2 June 1995. Extended Abstracts, 2: 544. metres), Middle Cambrian (Cm2, thickness 171 metres) and GÓRECKI W., STRZETELSKI W., DZIENIEWICZ M. & SECHMAN Lower Cambrian (Cm1, thickness 136 metres) sandstones, H. 1995b — Methods and results of surface geochemical survey as mudstones and claystones as well as Eocambrian (Upper adopted to petroleum exploration in Permian structure of Polish Low- land. „East Meets West” Conf. Cracow, Poland, 12–15 Sept. 1995. Ediacaran), sandstones and conglomerates (thickness 5 Ext. Abstr. Book, PC-10. metres). At 2,236 metres depth, the drilling encountered the GÓRECKI W., STRZETELSKI W. & SZWEJKOWSKI J.M. 1977 — Precambrian crystalline basement (granitic gneisses). The Geneza szczelin odprê¿eniowych w piaskowcach kwarcytowych kambru œrodkowego jako kryterium okreœlania dolnej granicy wieku aku- top part of Cm2 sandstones of porosity 7.5–13.5% accumu- mulacji wêglowodorów. Acta Geol. Pol., 27: 4 97–516. lates light oil at depth 1,951–1,990 m (Dom¿alski et al., GUDELIS W.K. & JEMIELIANOW J.M. 1982 — Geologia Morza 2002). Ba³tyckiego. Wyd. Geol. JARMO£OWICZ-SZULC K. 2001 — Badania inkluzji fluidalnych w This discovery was simultaneously the first successful spoiwie kwarcowym kambru œrodkowego na obszarze bloku £eby w application of the sea-floor geochemical survey as correla- Morzu Ba³tyckim implikacje diagenetyczne, izotopowe i geochemicz- ted with seismic data for petroleum exploration purposes. ne. Biul. Pañstw. Inst. Geol., 399: 73. KARNKOWSKI P.H. 2003 — Modelowanie warunków generacji The results obtained provide a decisive argument for exten- wêglowodorów w utworach starszego paleozoiku na obszarze zachod- sion of such combined surveying into the adjacent, Gdañsk niej czêœci basenu ba³tyckiego. Prz. Geol., 51: 756–763. (C) and Kuronian (D) offshore blocks to the east and into LASZKOVA L.N. 1979 — Litologia, facji i kolektorskie svojstva kem- the S³upsk offshore block (A) in the west with the links to brijskich otlozhenij Juzhnoj Pribaltiki. Izdatelstvo Niedra, Moskva. LENDZION K.,1988 — Kambr na Pomorzu i przyleg³ym akwenie surface geochemical onshore survey of coastal area. Ba³tyku. Kwart. Geol., 32: 555–564. The sea-floor geochemical studies revealed an impor- MODLIÑSKI Z. 1976 — Niektóre zagadnienia strukturalne zachodniej tant ecological aspect. It appeared that geogenic (i.e., natu- czêœci syneklizy peryba³tyckiej. Biul. Inst. Geol., 270, Z badañ geolo- gicznych Ni¿u Polskiego, II: 37–44. ral) hydrocarbons constitute an important contribution to PO¯ARYSKI W. & NAWROCKI J. 2000 — Struktura i lokalizacja the pollution of marine environment, presumably compara- brzegu platformy wschodnioeuropejskiej w Europie Œrodkowej. Prz. ble to the anthropogenic pollutants. The solution of these Geol., 48: 703–706. SIKORSKA M. 1998 — Rola diagenezy w kszta³towaniu przestrzeni problems is the aim of a new research project “Geochemi- porowej piaskowców kambru w polskiej czêœci platformy wschodnio- cal studies on sediments of the southern Baltic Sea aiming europejskiej. Pr. Pañstw. Inst. Geol., 164: 66. to the analysis of geogenic pollution and petroleum explo- STOLARCZYK F. 1979 — Powstanie lokalnych form tektonicznych w polskiej czêœci syneklizy peryba³tyckiej na tle rozwoju geologiczne- ration”, planned for the years 2004–2007 as a joint research go ca³ej jednostki. Acta Geol. Pol., 27: 519–558. of the Polish Geological Institute, Department of Fossil STRZETELSKI W. 1977a — Zwi¹zek pomiêdzy szczelinowatoœci¹ Fuels, the AGH University of Science and Technology in ska³ zbiornikowych a ich rozwojem litofacjalnym. Spraw. z Posiedze- nia Komisji Nauk (Geol.) Oddz. PAN w Krakowie. Kraków and the “Petrobaltic” Company Gdañsk. 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