The Prospects for Petroleum Exploration in the Eastern Sector of Southern Baltic As Revealed by Sea Bottom Geochemical Survey Correlated with Seismic Data
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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 accumu- lated 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 dis- turb thermal and density stratification of sea water column. The submarine geochemical studies strictly correlated with seismic pro- files 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