New stratigraphic scheme for Zechstein rocks in the Pogorzela High (Foresudetic Monocline) and its significance for hydrocarbon exploration

New stratigraphic scheme for Zechstein rocks in the Pogorzela High

(Foresudetic Monocline) and its significance for hydrocarbon exploration

Krzysztof Kwolek

, Zbigniew Miko³ajewski

A b s t r a c t . Results of an analysis of new 3D seismic data, obtained from the part of the Wolsztyn High in the Pogorzela High area (SW Poland), allow to test the existing knowledge regarding the geologic framework of the Zechstein rocks in this area. A characteristic arrange-ment of seismic reflectors within pinched-out Zechstein deposits on slopes of the high shows that they are overlapped in relation to the distinct surface of angular unconformity related to the base of the Zechstein — the Z1’seismic boundary. 3D seismic data seems to show that PZ1 strata are absent in the close vicinity of the Pogorzela High with the lower part of the PZ2 cyclothem also absent across the crest. This suggests that the interpretation of the stratigraphy of Zechstein deposits in the Pogorzela–1 and Pogorzela–2 wells (located on the crest of the high) is, in the light of 3D seismic data, questionable. Probably, the initial stages of the Zechstein transgression did not reach the most elevated part of the high, so that the Carboniferous basement is directly overlain by rocks of the Main Dolomite (Ca2), not by the Zechstein Limestone (Ca1) as was pre-viously thought. The lack of Ca1 strata across the crest of the Pogorzela High opens new explo-ration perspectives in this interval and explains the apparent negative results of boreholes drilled in 1970s. Presumably it also explains differences in formation of these rocks in comparison with the central and western part of the Wolsztyn High (the Koœcian–Nowy Tomyœl area).

Keywords: Foresudetic Monocline, Wolsztyn High, Pogorzela High, Zechstein Limestone (Ca1), 3D seismics

The Pogorzela High makes up the south–eastern part of the Wolsztyn High, a Variscan paleohigh in the Per-mo–Mesozoic basement of the northern part of the Foresu-detic Monocline across which Upper Rotliegendes rocks are absent trough erosion (Fig.1). Possibilities of hydrocar-bon discovery are connected here with two carhydrocar-bonate mem-bers within the Zechstein: the Zechstein Limestone (Ca1) and the Main Dolomite (Ca2). The Zechstein Limestone perspectivity is confirmed by large thicknesses of this inte-rval identified in several boreholes (maximum up to 66 m, well Bu³aków–1) drilled in the 1970s, and also by intere-sting results of the Ca1 sampling carried out by drill stem testers: significant brine inflow with combustible gas shows of flow rates from about 1.8 m3

/h (Pogorzela–4) up to about 7.4 m3/h (Bu³aków–1). Perspectivity of the Main Dolomite deposits is evidenced by the Siedmiorogów–1 well (1992) which, in the central and top part of Ca2, found intercalations of porous oolitic dolomites related to a barrier depositional environment. Testing of this interval with a drill stem tester resulted in an inflow of gas-cut brine with combustible gas (lack of H2S) at a flow rate of about 9 m

3

/h (Kwolek, 2000; Kwolek & Piotrowska, 2002; Kwolek & Miko³ajewski, 2006).

The Pogorzela High is located in SW part of the Polish Zechstein Basin (Fig. 2). The Pogorzela–1 and Pogorze-la–2 wells found a thinning of Zechstein deposits which was unparalleled in this part of the basin: reduced to 134 m and 85.5 m in Pogorzela–1 and Pogorzela–2, respectively. In these boreholes only one carbonate interval was encountered within the Zechstein (P–1 — 19 m; P–2 — 20.5 m) deposited directly on Carboniferous rocks. This carbonate interval was included into the Zechstein Lime-stone (Protas, 1975; Peryt, 1978a, b; Peryt & Protas, 1978). Hence, it was considered that there was an enclave on the

Pogorzela High within which Main Dolomite rocks were absent, the interpretation being originally based only on interpolated and extrapolated borehole data (G³owacki [In:] Antonowicz & Iwanowska, 1992). Attempts to speci-fy the extent of the enclave were made on the basis of the results of 2D seismic data acquired in 1998–1999 (Drabo-wicz, 2000; Zubrzycka, 2000; Kwolek [In:] Kwolek, 2000). Seismic studies from that time helped to model precisely the geologic framework of Zechstein deposits pinching out on the margins of the Pogorzela High. It was suggested that, under a favorable spatial configuration of the Ca2 surface, there was a possibility of stratigraphic or structural-stratigraphic traps within the Main Dolomite deposits (Kwolek, 2000). After the analysis of all available geophysical-geological data, an exploration play concept was worked out for the Ca2 and Ca1 deposits (Kwolek, 2000). However, it was accepted that due to insufficient good-quality seismic data, hence, serious interpretation ambiguity, that verification of the play concepts by drilling would be very risky. Therefore, 3D seismic imaging was proposed (Kwolek & Piotrowska, 2002), and implemented in 2004 (Filo, 2005). Analysis of the 3D seismic data, together with reinterpretation of the well data, suggests a changed view of the geologic framework of the Zechstein deposits within the Pogorzela High. Authors of the present paper intend to show the possible importance of this chan-ge for further exploration for natural gas in the Zechstein Limestone deposits.

Stratigraphical and lithofacial characteristics of Zechstein deposits in the Pogorzela High area Zechstein strata in the Pogorzela High lie on folded and erosionally truncated Carboniferous rocks or Upper Rotlie-gendes coarse-grained clastic deposits which pinch out on the eastern margin of the High and are underlain by Lower Rotliegendes volcaniclastics. Thicknesses of Zechstein deposits range from ca 100 to 400 m (Fig. 3). Their sedi-mentation proceeded in relatively calm tectonic conditions (Wagner, 1994; Kwolek & Protas, 2001). The observed

1_{Polskie Górnictwo Naftowe i Gazownictwo SA w Warszawie,}

Oddzia³ w Zielonej Górze, Dzia³ Poszukiwania Z³ó¿, Plac Staszica 9, 64-920 Pi³a; krzysztof.kwolek@pgnig.pl; zbigniew.mikolajewski@pgnig.pl

thickness variation within these rocks is a consequence of relatively weak subsidence relative to adjacent areas and to differences in paleorelief of the post-Zechstein surface, toget-her with a substantial uplift of the area causing a different

geo-metry of transgressive-regressive carbonate-evaporite cyc-lothems (Wagner, 1994).

In the central, most elevated part of the high, where the top of the Carboniferous is at depth of only 1750 m, the Zechstein is significantly thinner than in the adjacent areas. In the Pogorzela–1 and Pogorze-la–2 wells the Zechstein is only 134 m and 85.5 m thick, whereas on the slopes of the high, the thickness varies from 268 m in the well Siedmiorogów–1 (in the northern part) to 379 m in the Pogorzela–6 (in the southern part). The Zechstein sections encountered in all boreholes dril-led along the crest of the high (Siedmiorogów–1, Pogorzela–1, –2, –4) do not contain PZ1 and PZ2 cyclothem salt members, with the PZ3 cyclothem salt member also absent in the Pogo-rzela–1 and Pogorzela–2 wells. Notably, only a single carbonate interval some 20 m thick lying directly on Carboniferous rocks, was encountered in both latter wells,. Up to now this carbonate interval has been referred to the Zechstein, both in the literature (Peryt, 1978a; Peryt & Protas, 1978; Kwolek & Protas, 2001), and in different studies by the Polish oil industry (e.g., Protas, 1975; Antonowicz & Iwanow-ska, 1992; Kwolek, 2000). Hence this is also the interpretation held

C_E N_T R_A L B_A S I N 50 km study area playa lithofacies

aeolian lithofacies gas field in Rotliegendes

fluvial lithofacies

Weissliegendes clastic deposits (S³upsk Basin) DEPOSITIONAL ENVIRONMENTS S o_u t h e_r n E r_g

Lower Rotliegend rocks

alluvial lithofacies Pre-Permian

E a s t e_{r n} E rg W O L S ZT Y N R I DG E

Fig. 1. Paleogeography of the late Upper Rotliegendes of Poland (A. Buniak — unpublished; after H. Kiersnowski, PIG and PGNiG SA) Szczecin Koszalin Gdañsk Olsztyn Bydgoszcz Poznañ Wroc³aw Kraków Kielce £ódŸ Warszawa Lublin Bia³ystok S L O V A K I A C Z E C H R E P . U K R A I N E BELARUS GERMANY R U S S I A L T H U A N I A 200 200 400 600 800 1000 1400 1200 1500 600 800 400 200 1000 600 600 600 400 400 200 400 600800 10001200 1400 1500 1500 200 600 200 400 PZ3 PZ2 PZ4 PZ2 PZ3 PZ1 _PZ1 PZ3 PZ1 PZ2 PZ4 PZ1 PZ1 PZ1 PZ2 PZ4 PZ3 PZ2 PZ1

SOUTH BALTIC LAND

POMER ANIAN BA_Y WOLSZTYN HIGH SILESIAN

TROUGH _{PODLASIE BAY}

HOL Y CROSS LAND S O U T H - _{P O} L I _{S H} L A N D PERY BALT ICLA ND M I D - P_O L I S H T R O U G H E A S T E_U R O P E A N C R A T O N S L O P E 0 25 50 75 100km study area A A' 0 m 200 400 600 800 1000 1200 1400 1500

Fig. 2. Paleothickness of Zechstein deposits in Poland (Wagner, 1994). Broken lines, marked as PZ1, PZ2, PZ3 and PZ4, indicate primary extent of lithostratigraphic units (cyclothems)

in the PITAKA borehole database.

Zechstein deposits are built of four evaporitic cyc-lothems: PZ1, PZ2, PZ3, and PZ4. The PZ1, PZ2 and PZ3 cyclothems comprise relatively simple carbonate-evapori-te sequences, formed during transgressive-regressive cyc-les of the Zechstein Sea. The PZ4 cyclothem is more complex and is divided into subcyclothems named PZ4a to PZ4e, it contains terrigenous-evaporitic sequences formed as a result of cyclic climate changes, from dry to more humid. The overall trend of the Zechstein cyclothems has a regressive character: the PZ1 deposits have the largest late-ral, while younger cyclothems are progressively more restricted in extent (Wagner, 1994).

The basal unit of the PZ1 cyclothem in the Pogorzela High area comprises the Zechstein Limestone (Ca1). Occurrence of the oldest formal Zechstein member, the Kupferschiefer, has not identified, as is characteristic for basement uplift. However, on the south-west margin of the Pogorzela High in the Pogorzela–6, a thin layer (0.2 m) of Basal Conglomerate deposits was encountered, equivalent to the Weissliegendes in the uplifted areas such as the Wolsztyn High.

The PZ2 cyclothem commences with the Main Dolo-mite (Ca2). In the area under study this member usually represents a shallow shelf environment: that is, it is develo-ped in carbonate platform facies (Kwolek et al., 2002). In the area of the Pogorzela High the PZ2 cyclothem is conti-nuous only in the zone located northwestward of the Sied-miorogów–1 — Bu³aków–1 wells. In the crestal part of the high (area of the Pogorzela–1 and Pogorzela–2 wells), almost the whole of the Zechstein interval is represented by a stratigraphic hiatus (Fig. 3).

In the area of the Pogorzela High, with exception of its top axial part, the PZ3 cyclothem usually has a complete profile. It is commenced with the Grey Pelite (T3), overlaid by the Main Anhydrite (A3), and followed by the Younger Halite (Na3). The Screening Anhydrite (A3r) (ca 1 m thick) underlain by the Younger Halite is very much discontinu-ous: it was only found in the profiles of several wells.

The PZ4 cyclothem is characterized by terrige-nous–evaporite deposits formed as a result of periodic changes of continuously more humid climate, narrowing connection with the Upper Permian Sea and progradation of continental terrigenous deposits into the basin (Wagner 1994). These deposits accumulated on a surface partially smoothed by deposition of older Zechstein rocks. In the Pogorzela High area, likewise across the whole Wolsztyn High, the PZ4 cyclothem is commenced with the Lower Red Pelite (T4a), overlain by the Pegmatite Anhydrite (A4a), and the Youngest Halite (Na4a). The PZ4 section terminates with the so called Top Terrigenous Series (PZt) which, according to R. Wagner (op. cit.), is a transitional member between Zechstein and Buntsandstein deposits. In the Polish oil industry its equivalent is possibly called the Transitional Claystone Series (IP).

Proposal for Zechstein local stratigraphy change — seismic evidence

Results of 3D seismic studies accomplished in 2004 applied to the Pogorzela High area require a review of exi-sting knowledge on geologic framework of the Zechstein deposits. Structural interpretation of Zechstein seismic boundaries, perhaps with the exception of the Z3 boundary related to the top Main Anhydrite, is very difficult even using 3D data. Admittedly this difficulty is mainly in the zone around of the crestal part of the Pogorzela High, but because of possible occurrence of structural-lithological traps in the Zechstein carbonate deposits this area is highly important for hydrocarbon exploration. The interpretation difficulty results mostly from the complete pinch-out of salt members in the lower part of the Zechstein, whose pre-sence determines the formation of intra-Zechstein seismic reflectors at the boundary of anhydritic members (Z2, Z1). Additional impediment to generation of seismic reflections in the zone mentioned are the convergence, thickness reduction and facial changes of the carbonate members. METERS MD TVDSS METERS MD TVDSS METERS MD TVDSS METERS MD TVDSS METERS MD TVDSS -1850 -1900 -1950 -2000 -2050 -2100 -2150 -2200 -2250 -2300 -2350 -2400 -2450 -1750 -1800 -1850 -1900 -1950 -2000 -2050 -1500 -1550 -1600 -1650 -1450 -1500 -1550 -1600 -1650 -1700 -1750 -1800 -1300 -1350 -1400 -1450 -1500 -1550 -1600 -1650 -1700 2000 2050 2100 2150 2200 2250 2300 2350 2400 2450 2500 2550 2600 1900 1950 2000 2050 2100 2150 2200 1650 1700 1750 1550 1600 1650 1700 1750 1800 1850 1900 1400 1450 1500 1550 1600 1650 1700 1750 1800 GR 0—5000 NEGR 0—42000CAL I 80—620 DT 100—700 GR 0—3600 NEGR 0—42000CAL I 80—620 DT 100—700 GR 0—5000 NEGR 0—42000CAL I 80—620 DT 100—700 GR 0—5000 NEGR 0—42000CAL I 80—620 DT 100—700 GR 0—5000 NEGR 0—42000CAL I 80—620 Lower Anhydrite PZ1 Anhydrites Zechstein Limestone

Lower Bundsandstein Upper Rotliegend Lower Rotliegend

Tp1 P1d P1g Zechstein 4 Younger Halite Main Anhydrite Screening Anhydrite Older Halite Main Dolomite Upper Anhydrite IP PZ4n Na3 A3 A2r Na2 Ca2 A1g

Screening Anhydrite Grey Pelite Basal Anhydrite Oldest Halite

A3r T3 A2 Na1 A1d ZECHSTEIN: Carboniferous C A1 Ca1 Transitional Claystone Series Pogorzela High IP POGORZELA-1 PZ 2 Tp1 C PZ2 + PZ3 A2 + A 3 Ah Ah 2343,0 Na 1 PZ 1 A1 d ROTLI E G END P1 d CARB ONI -FERO U S IP IP SIEDMIOROGÓW-1 Tp1 Na 3 Ca 2 Ca 1 A1 A3 A2 T3 Tp1 Na 3 A3 Na 2 A2 A2r Ca 2 WYCIS£OWO IG-1 PZ 2 PZ 3 PZ4n PZ4n PZ 3 PZ 2 PZ 1 IP IP POGORZELA-6 ROGO¯EWO-1 Tp1 Tp1 A1 d A1 g A1 g C Na 1 Ca 2 Ca 2 A3 A3 Na 3 Na 3 A2 Na 1 P1g T3 T3 PZ4n PZ 3 PZ 2 PZ 1 PZ 2 PZ 1 PZ 3 CARB ONI -FERO U S CA RB ON I-FE ROUS

Fig. 3. Correlation of Permian rocks in the area of the Pogorzela High (reference level — top Zechstein) (Kwolek & Protas, 2001 — modified). Additional explanations in the profile of the Pogorzela–1 well: Ah — anhydrite; S.A–I — anhydritic-clayey unit

The result is that, beneath a minimum thickness of a reflec-tive layer, it becomes “invisible” in the seismic record.

Characteristic arrangement of reflectors within thin-ning Zechstein deposits on the slopes of the high suggests

that they are overlapped in relation to the distinct surface of angular unconformity related to the base of the Zechstein — the Z1’ seismic boundary, connected with the top rocks of the sub-Permian basement (Fig. 4). A considerable

1000 1000 L337 L300 L250 L200 L158 L163 L189 T150 T189 T225 T258 T300 T350 T400 ArbitraryLine W E W ArbitraryLine E W ArbitraryLine E SW ArbitraryLine NE S N ArbitraryLine 1000 1000 L337 L300 L250 L200 L158 L163 L189 T150 T189 T225 T258 T300 T350 T400 ArbitraryLine W E W ArbitraryLine E W ArbitraryLine E SW ArbitraryLine NE S N ArbitraryLine

A

Pogorzela-1 Pogorzela-2 Pogorzela-4 Bu³aków-1 Pogorzela-7

Zechstein top Zechstein top

Zechstein base Zechstein base

B

Pogorzela-1 Pogorzela-2 Pogorzela-4 Bu³aków-1 Pogorzela-7

Zstr Z3 Z2 Z1’

C

Fig. 4. Arbitrary section of the Pogorzela High (A) and its reflection strength (B) — the Siedmiorogów–Pogo-rzela 3D seismic image (Geofizyka Kraków 2005); C — section locality versus structural map of the base Zechstein in the Pogorzela High area (after Kwolek & Kowalczak [In:] Kwolek & Miko³ajewski, 2006)

improvement of 3D seismic data quality compared to the two-dimensional method (2D) has been achieved, with better resolution of both Z1’ and Ca2sp (base Main Dolo-mite) surfaces (Kwolek, 2000). This has made easier the identification of the Ca1 reflection related to porous rocks of the Zechstein Limestone, which lies between the Z1’ and

Ca2sp events. The validity of the interpretation of the Z1’s-eismic boundary is confirmed by analysis of sections in the form of one of seismic attributes, namely reflection strength (Fig. 4B).

On the basis of interpretation of 3D seismic data it is concluded that in the close vicinity of the Pogorzela High the uppermost PZ1 strata and the lower part of the PZ2 cyclothem are missing. This means that the currently accepted stratigraphy of the Zechstein deposits in wells Pogorzela–1 and Pogorzela–2 is debatable, in the light of 3D seismic data. Thus, it can be suggested that in the most elevated part of the high, Main Dolomite deposits lie directly on the Carboniferous basement: previously, the Zechstein Limestone sediments had been interpreted to lie at this level.

Based on 3D seismic data a new model for the geologic framework of Zechstein deposits in the Pogorzela High area is proposed (Fig. 5B). The rock sequence showed in this model seems to testify that the most elevated part of the area was an island during Zechstein transgression. A similar sequ-ence was found in the Koszalin–Chojni-ce zone where Zechstein transgression did not flood all basement highs (Fig.6). These highs formed islands around which were formed algal-bryozoan bar-riers, several tens of meters of thick,

Pogorzela High C Muchelkalk (Tm) Buntsandstein (Tp) Zechstein PZ4 Anhydrites

Salts (Na1, Na2, Na3) Main Dolomite (Ca2)

Zechstein Limestone (Ca1) Carboniferous (C) Tp Ca2 Ca1 Na3 Na3 Ca2 Ca1 ? Tp C Na2 PZ4 Na3 Na3 Ca2 Ca1 Tm Na1

Potential lithologic traps in Ca1

Pogorzela High Na2 Ca2

A

B

C

Fig. 5. Model of geologic framework of Zechstein deposits in the Pogorzela High; A — based on well and 2D seismic data (Kwolek & Protas, 2001); B — based on well and 3D seismic data; C — locality of profiles which were used to create models A and B on the structu-ral map of base Zechstein in the Pogorzela area (after Kwolek & Kowalczak [In:] Kwolek & Miko³ajewski, 2006)

Na3 Na3 Ca2 Ca1 ? Tp C PZ4 Ca2 Ca1 Pogorzela High B A PZ4a PZ3

Dretyñ

1

Miastko

3

Bia³y

Bór

1

Fig. 6. Analogy of geologic framework of Zechstein deposits in the Koszalin–Chojnice zone (A) (Wagner [In:] Raczyñska, 1987), to the Pogorzela High (B). No scale

separating the lagoonal-littoral part of the basin from the open sea (Wagner [In:] Raczyñska, 1987).

Proposal for Zechstein stratigraphy change — geologic evidence

In order to find direct evidence to prove the thesis on the lack of the Zechstein Limestone deposits in wells Pogo-rzela–1 and Pogorzela–2, authors of the paper re-analyzed all available geological materials gathered in the relevant well documentation. Key evidence turned out to be a field core description from well Pogorzela–2, representing the Zechstein — basement (Carboniferous) transition. The description shows an intercalation of anhydrite lying direc-tly on Carboniferous deposits (Kupferschiefer is absent)

and below layers of dolomites assigned to the Zechstein Limestone (Fig. 7A). This anhydrite intercalation was completely omitted in the lithostratigraphical profile enc-losed in the well documentation. Because of incomplete core recovery (ca 40%), it cannot be excluded that the thickness of this intercalation up to twice higher. This assumption seems to be confirmed by log curve analysis. Presence of clasts from the basement rocks is observed in the microscopic thin-section collected from the analyzed anhydrite layer (Fig. 7B), suggesting that sedimentation took place close to the face of basement beds uplifted above the sea level. From the analyzed succession: base-ment — anhydrite — dolomite, it unambiguously shows that the latter layer cannot be included into the Zechstein Limestone. Undoubtedly, these are the Main Dolomite

B

A

1 mm 1 mm

Fig. 7. A — fragment of a page of the Pogorzela–2 well documentation with original core descriptions after K. Król; B — microphotographs of petrographic thin sections from the anhy-drite overlain by Carboniferous deposits in the Pogorzela–2 well (ca 1747.5 m depth)

Discritella microstoma

0,25 mm 1 mm

deposits underlain by anhydrites of the PZ1 cyclothem, as they are seen in the seismic sections to be pinching out on the slopes of the Pogorzela High (Fig. 4).

The basis of identification of the carbonate interval in the well Pogorze-la–1, some 1,5 km away from well Pogorzela–2, was „Orzeczenie fauny cechsztyñskiej z otworów Pogorzela–6 i Pogorzela–1” written in 1973 by Prof. Jerzy K³apciñski as enclosed in the well documentation (Dokumentacja wyniko-wa otworu poszukiwyniko-wawczego Pogorze-la–1, 1973). In the rock chip collected from the basal part of the analyzed interval in well Pogorzela–1 (depth 1760,6 m) J. K³apciñski described mol-lusks from the genera Schizodus (Schi-zodus truncatus King) and Edmondia (Edmondia elongata How), claiming that the former represented Lower Zech-stein (Werra cyclothem), and the latter Lower and Middle Zechstein (but it is more common in the Lower Zechstein). Thus J. K³apciñski admitted that the car-bonate interval in the well should be inc-luded into the Werra cyclothem (PZ1) and thus should be referred to the Zech-stein Limestone deposits.

Genus Schizodus is known from

Upper Carboniferous. Therefore it

seems that it cannot be an obvious indi-cator of age of the deposits, especially of such a narrow interval of time as it is

in the case of the PZ1 cyclothem. In turn, described by J. K³apciñski, Edmondia elongata How is considered in the Euro-pean Zechstein Basin (beside Wilkingia elegans King) to be an index fossil for the Main Dolomite (Wagner, 1994). Moreover, this has been known for many years: an academic book Geological History (Makowski [In:] Makowski, 1977) states that this genus appeared in the Permian and was found both in the Zechstein Limestone and in Main Dolo-mite rocks.

An additional argument which

supports including the carbonate interval in wells Pogorzela–1 and Pogorzela–2 into the Zechstein limestonae deposits was the presence of fragments of bryozo-ans in the basal part of the Pogorzela–2 well profile (Fig. 8). Authors of the pre-sent work managed to find thin sections from well Pogorzela–2 that indicate the presence of scarce bryozoans which, according to Dr. U. Hara (Polish Geolo-gical Institute), represent species Discri-tella microstoma (pers. comm.). This form is found both in the Zechstein Limestone of the North–Sudetic Trough and in the Main Dolomite in northern Germany and on the Unis³aw Platform (Dr. U. Hara — pers. comm.). Hence, the 5 km Pogorzela-2 Pogorzela-1 Pogorzela-7 Bu³aków-1 Pogorzela-4 Siedmiorogów-1 The Bu³aków object

Fig. 9. Reflection map of the Z1’ seismic boundary in the Pogorzela High area (after Kwolek and Kowalczak [In:] Kwolek & Miko³ajewski, 2006 — based on 3D seismic data from Siedmiorogów–Pogorzela, Geofizyka Kraków 2005)

Pogorzela-2 Pogorzela-1 Pogorzela-7 Siedmiorogów-1 The Bu³aków object Pogorzela-7 5 km 1070 1200 1190 1180 1170 1160 1150 1140 1130 1120 1110 1100 1080 1090

Fig. 10. Time structure map of the Ca1 seismic boundary (top porous layer of the Zechste-in Limestone) Zechste-in the Pogorzela High area (after Kwolek & Kowalczak [In:] Kwolek & Miko³ajewski 2006 — based on 3D seismic data from Siedmiorogów–Pogorzela, Geofi-zyka Kraków 2005)

presence of scarce bryozoans in the carbonate interval ana-lyzed in the Pogorzela–2 profile does not seem to be a suffi-cient criterion corroborating the existing stratigraphic scheme.

Change of stratigraphic assignment of the carbonate layer in the base of the wells Pogorzela–1 and Pogorzela–2 from Ca1 into Ca2 results, of course, in the need for reassi-gnment of lithostratigraphic members higher in the profile.

L78 L95 L112 L129 L146 L163 L188 T100 T150 T200 T250 T300 T350 T400 T450L221 1000 1000 1000 1000 L78 L95 L112 L129 L146 L163 L188 T100 T150 T200 T250 T300 T350 T400 T450L221 SW E W E W E SW ArbitraryLine NE S N ArbitraryLine 16 tr/cm 12 IPS 16 tr/cm 12 IPS 1026m 1027m 221104 migr pstm dec2.bri SW ArbitraryLine NE S N ArbitraryLine 16 tr/cm 12 IPS 16 tr/cm 12 IPS 1026m 1027m inverted 16int.bri Ca1 Zechstein top Zechstein top Zechstein base Zechstein base

Pogorzela High

Pogorzela High

B

C

A

Fig. 11. Arbitrary time seismic section NE–SW through the Bu³aków object (A) and its seismic inversion (B). Structural interpretation — K. Kwolek (based on 3D seismic data from Siedmiorogów–Pogorzela, Geofizyka Kraków 2005); C — locality of section on the structural map of the Zechstein base in the Pogorzela area (after Kwolek and Kowalczak [in:] Kwolek and Miko³ajewski 2006)

Nota bene, that on account of very attenuated thicknesses, the lack of salt and of benchmark members (Grey Pelite,

Red Pelite), it is very difficult or even impossible to make a firm identification of the higher units (Fig. 3).

750 50 850 1050 1150 1250 1350 1450 750 850 1050 1150 1250 1350 1450 50 150 250 350 450 550 650 50 150 350 450 550 650 100 150 200 250 300 350 400 450 500 550 T200L121 T250L121 T300L121 T350L122 T400L122 T450L122 6–4–76K 23–9–98K 21–9–98K 19–9–98K 47–9–87K T100[pogor z3d] T50[pogor z3d] 54–9–88K SW NESW NE ArbitraryLine [pogorz3d] 65 tr/cm 12 IPS 32 tr/cm 12 IPS 2044m 2050m mig007007 18–9–87K 50 100 150 200 250 300 350 400 450 500 550 T200L121 T250L121 T300L121 T350L122 T400L122 T450L122 6–4–76K 23–9–98K 21–9–98K 19–9–98K 47–9–87K T100[pogor z3d] T50[pogor z3d] 54–9–88K SW NESW NE ArbitraryLine [pogorz3d] 65 tr/cm 12 IPS 32 tr/cm 12 IPS 2044m 2050m mig007007 18–9–87K Z1 Z1’ Z2 Z3 Zstr Tp2 Z1 Z1’ Z2 Z3 Zstr Tp2 Na1 Ca1 A1d Phase change at the bottom reef body (?)

Na1 A1d

Ca1 POGORZELA-6

Phase change at the bottom reef body (?)

The Bu³aków object Na1 Ca1 A1d

A

B

2D

3D

2D

3D

Fig. 12. Juxtaposition of the arbitrary 3D section (elaboration: Siedmiorogów–Pogorzela 3D, Geofizyka Kra-ków 2005) going through the Bu³aKra-ków object with the 2D section number 18–9–87K (last processing in 2000; Geofizyka Kraków) — A; B — section from figure A flattened to the seismic Tp2 boundary; C — locality of 2D and 3D sections set in figures A and B

The new model for the geologic framework of Zechste-in deposits (Fig. 5B) differs seriously from the model based on 2D seismic data (Fig. 5A). Comparison of both models reveals how considerably the change of Zechstein stratigra-phy model influences the exploration concepts in this area.

Significance of the new Zechstein stratigraphy in the Pogorzela High for hydrocarbon exploration The new model for the geological framework of the Zechstein deposits in the Pogorzela High (Fig. 5B) shows that the Zechstein Limestone interval, whose prospectivity had been considered to be much reduced following nega-tive reservoir results of boreholes drilled in the 1970s, still has high exploration importance.

The poor reservoir quality in what was considered at the time to be Zechstein Limestone deposits in wells Pogo-rzela–1 and Pogorzela–2, due to location of the Pogorzela High (Fig. 9), downgraded the whole region from further hydrocarbon exploration targeted at the Ca1 interval. Data quality improvement and increased seismic resolution

resulting from the acquisition of 3D seismics allowed for more precise and credible interpretation of the Ca1 seismic boundary, which is connected with porous and highly thick (reef-like) deposits of the Zechstein Limestone (Fig. 10).

The change of stratigraphic assignment from Ca1 to Ca2 of the only carbonate interval in wells Pogorzela–1 and Pogorzela–2, determines that the Ca1 deposits must pinch out on the slopes of the Pogorzela High, creating potential lithologic traps (Fig. 5B). This model was confir-med by thorough geophysical-geological analysis of the Werra rocks. In this analysis experience from the last dozen years in searching for porous and thick Ca1 reef bodies in the Koœcian–Nowy Tomyœl area was called upon (i.a., Dyjaczyñski et al., 1993; Dyjaczyñski & Kucharczyk 1998; Klecan & £omnicki, 1998; Górski et al., 2000). Detection of these bodies is based on knowledge of reflec-tion generareflec-tion in the Werra deposits but also on indirect paleostructural premises.

Reef bodies, as such, are included into lithologic-type traps, specifically after Levorsen (1972), into primary stra-tigraphic traps. However, the complete pinch-out of the Ca1 deposits on the slopes of the Pogorzela High (Fig. 5B, see Fig. 6), can create an exceptional (to date) mixed trap types in these deposits — sensu stricto litholo-gic-stratigraphic. One particular-ly prospective potential trap of this type, the Bu³aków feature, has been recognized from the seismic data (Fig. 9, 10). It shows some similarities to reefal forms found and recognized between Koœcian and Nowy Tomyœl.

The shape of the Bu³aków object is asymmetrical: in the cur-rently highest structural position there is south–western part (Fig. 11). This part of the object is on the edge of the morphological margin in the basement, southwe-stward from which the base Zech-stein abruptly deepens and the thickness of the PZ1 cyclothem deposits thickens drastically: in well the Pogorzela–6, located just ca 5.6 km from the margin PZ1 reaches a thickness of a much as 213 m (Fig. 12). However, the most important element of the geologic framework which differs between the Bu³aków object and Tp1 ? Pogorzela High Na3 Na3 PZ4 Ca2 NW SE Pogor zela-1 C Anhydrites The Bu³aków object Zechstein top

Fig. 13. Model of geologic frame -work of the Zechstein deposits in the Pogorzela High along with NW–SE direction (based on 3D seismic data from Siedmioro-gów–Pogorzela, Geofizyka Kra-ków 2005) Pogorzela-2 Pogorzela-1 Pogorzela-7 Bu³aków-1 Pogorzela-4 Siedmiorogów-1 The Bu³aków object 5 km

Fig. 14. Thickness map of the complex t = Z1’–Tp2 (after Kwolek & Kowalczak [In:] Kwolek & Miko³ajewski 2006 — based on 3D seismic data from Siedmiorogów–Pogorzela, Geofizyka Kraków, 2005). Map of seismic the Tp2 boundary was used after Geofizyka Kraków

the reefal forms of the Koœcian–Nowy Tomyœl area is the pinching out of Ca1 deposits at the base of the SE margin of the topmost part of the Pogorzela High (Fig. 13). Despite this difference, this object fulfills general criteria indica-ting the presence of the Ca1 reef bodies in the seismic sec-tion, namely:

‘the Oldest Halite (Na1) pinches out on the slopes of the Pogorzela High. Because of a limited extent of the Sied-miorogów–Pogorzela 3D survey, this phenomenon is best seen on the 2D line going through well Pogorzela–6 (loca-ted in far SW slope of the Pogorzela High) joined to an arbitrary 3D line going through the Bu³aków object (Fig. 12). Also seen is a phenomenon of „draping” of this possi-ble reef body by the Lower Anhydrite A1d in its slopes (Fig. 11).

‘ phase change of the Z1’ reflector from minimum to maximum in the base of the Bu³aków object is very specta-cular. This phenomenon is particularly well seen on sec-tions along with layers dip or related direcsec-tions (Fig. 11).

The Bu³aków object is on the elevation in the Zechstein basement well seen on the seismic sections flattened to the seismic Tp2 boundary (Fig. 12B). This elevation comprises an extension in the SE direction (along with the Wolsztyn High axis) of the structurally highest part of the Pogorzela High. The Bu³aków object was deposited about 100 m (ca 40 ms — Fig. 9) deeper and, at the time of Zechstein trans-gression when the crest was an island, could have been in an optimal place for shallow water, intensive sedimentation of organic carbonate rocks of the Zechstein Limestone (i.e., reefs). This fact clearly shows thickness map of the com-plex t = Z1’–Tp2 identical with the Z1’ horizon map flatte-ned to the seismic Tp2 boundary (Fig. 14). It seems that this assumption is also confirmed by inversions of seismic sec-tions, which show that the acoustic impedance within the Bu³aków object is far lower than in the surrounding anhy-drites (Fig. 11B).

The closure the NE of the Bu³aków reservoir trap is structural, with SE and SW closure being lithological, rela-ted to the facies change of the Ca1 deposits from the thicker and porous shallow carbonate reefs into thin (from one to several meters), non-porous and dense deepwater carbona-tes, representing so called condensed profiles (Peryt & Wa¿ny, 1978). On the other hand, the NW part the trap is formed by the extent of the Ca1 deposits, pinching out to SE on the slope of the central part of the Pogorzela High (Fig. 13).

Boreholes drilled so far in the Pogorzela High area, which confirmed the occurrence of thick Ca1 deposits, were located on its highly deep, north–east slope. Gas cut formation water inflows with low combustible gas have been obtained from the Ca1 interval in these boreholes. It is assumed that most of the gas exsolved from the brine accu-mulated in the Bu³aków trap located which lies on the migration path to the SW direction and updip by about 100–200 m (ca 50–80 ms) (Fig. 10, 11).

As a result of the data presented, the prospectivity of the Zechstein Limestone deposits in the Pogorzela High is still high. It must be emphasized that the crucial influence on the change of exploration value estimation of the Ca1 interval in the area was the result of 3D seismic studies and discerning analysis of their results, together with reliable reinterpretation of archived geological data.

Translation: Andy Sims (UK) Dr Miros³aw S³owakiewicz (PGI Warszawa).

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