Oil- and gas-bearing sediments of the Main Dolomite (Ca2) in the Międzychód region: a depositional model and the problem of the boundary between the second and third depositional sequences in the Polish Zechstein Basin

The Polish Zechstein Basin was a tideless sea dominated by storms. Main Dolomite deposits of the Miedzychód region were deposited: a) on the carbonate platform (in the environments of the outer barrier, inner barrier and high- and low-energy platform flat); b) on the platform slope; c) at the toe-of-slope; d) on the basin floor. The best reservoir properties are recorded in shallow-marine deposits of the outer and inner barriers and in deep-sea sediments of the toe-of-slope (turbidites and debrites). Rich reserves of crude oil and natural gas were discovered both on the carbonate platform (the Miedzychód and Grotów deposits) and at its toe-of-slope (the Lubiatów deposit). The Main Dolomite sediments are wholly included in the second depositional sequence (PZS2 sensu Wagner & Peryt, 1997). The maximum flooding surface of the PZS2 sequence within the platform, its slope and toe-of-slope, runs along the A1g/Ca2 boundary. In the basinal zone, its correlative equivalent is a hard ground observed within the Main Dolomite carbonate rhythmites. The boundary between the second and third (PZS2/PZS3) depositional sequences (corresponding to the ZS3/ZS4 sequence boundary in the German Basin) runs on top of the Main Dolomite carbonates (on the platform slope, at the toe-of-slope and on the basin floor) and above top of the Main Dolomite carbonates, within the lower part of the Basal Anhydrite (on the platform)

Microfacies and diagenesis of the main dolomite (Ca2) strata in the Międzychód barrier area (Grotów Peninsula, Western Poland)

Utwory dolomitu głównego stwierdzone w rejonie Międzychodu stanowią fragment dolomitowej bariery oolitowej obejmującej zachodnią część półwyspu Grotowa. Profile z otworów wiertniczych Międzychód-4, Międzychód-5 i Międzychód-6 składają się z bogatego spektrum odmian mikrofacjalnych, wskazujących na depozycję w zróżnicowanych subśrodowiskach szeroko pojętej strefy bariery węglanowej. Przeobrażenia diagenetyczne oraz rozwój przestrzeni porowej zachodziły wielostopniowo i związane były zarówno z etapem depozycyjno-diagenetycznym (eodiageneza), jak również z etapem pogrzebania (mezodiageneza). Niektóre z nich przyczyniły się do obniżenia potencjału zbiornikowego (kompakcja, cementacja, neomorfizm), inne w znacznym stopniu go poprawiły (rozpuszczanie, szczelinowatość). Najbardziej niekorzystnie na zabudowę przestrzeni porowej wpłynęła cementacja dolomitowa i anhydrytowa. Także halit, minerały ilaste oraz impregnacja bitumiczna przyczyniły się do jej ograniczenia. Neomorfizm doprowadził niekiedy do całkowitego zatarcia pierwotnych cech strukturalno-teksturalnych skały. Rozpuszczanie i szczelinowatość w znacznym stopniu polepszyły właściwości zbiornikowe (porowatość, przepuszczalność). W wyniku rozpuszczenia niestabilnych bioklastów oraz jąder ziarn węglanowych doszło do powstania porowatości moldycznej.The Main Dolomite rocks found in the Międzychód area are associated with a fragment of a dolostone oolite barrier comprising the western part of the Grotów Peninsula. The sections from the Międzychód-4, Międzychód-5 and Międzychód-6 wells contain diverse microfacies types characteristic of deposition in differentiated subenvironments within the carbonate barrier zone. Diagenetic transformations and development of pore space occurred as multistage processes. They were associated with both the diagenetic-depositional zones (eodiagenesis) and the burial stage (mesodiagenesis). Some of them lowered the reservoir potential (compaction, cementation, neomorphism), whereas others significantly improved it (dissolution, fracturing). Dolomite and anhydrite cementation, halite, clay minerals and bitumen impregnation reduced the pore space filtration properties. In some places neomorphism affected primary structural-textural features of the rocks. Dissolution and fracturing significantly improved reservoir properties (porosity, permeability). Dissolution of unstable bioclasts and cores of carbonate grains resulted in formation of moldic porosity

Criteria of identification of lithofacies objects as potential hydrocarbon traps in the Main Dolomite (Ca2) strata at the toe-of-slope of carbonate platforms and microplatforms in the central-western Poland

In terms of oil reserves of Poland, the Lubiatów oilfield in the Międzychód area is the second largest after BMB oilfield. The discovery of this structural-lithological trap in a toe-of-slope position in relation to the Main Dolomite (Ca2) carbonate platform opened a new hydrocarbon opportunities in this unit. One of the conditions for trap occurrence in the "Lubiatów-type" deposits of the toe-of-slope zone in relation to Ca2 carbonate platforms and microplatforms is the presence of appropriate reservoir facies. These facies are characterized by occurrence of thick packets of porous Main Dolomite rocks (Ca2 lithosomes) composed mostly of sublittoral, redeposited carbonate sands (packstones, floatstones, sporadic rudstones), deposited below wave-base and resulting from activity of traction and suspension currents and gravity flows. The only method to detect such facies is via high resolution seismic images, preferably from three-dimensional (3D) surveys. Unfortunately, intensive exploration studies carried out in this zone between 2005-2008 did not bring measurable effects, that is discovery of new hydrocarbon deposits. Several boreholes drilled in Ca2 lithosomes found water-bearing horizons only. In several other dry holes, none of the facies assumptions made prior to drilling were confirmed. Comparison of seismic images of area of those dry holes made it posssible to find a geometric criterion for seismic detection of Ca2 lithosomes located at the toe-of-slope of carbonate platforms and microplatforms. This geometric criterion, along with the seismological one, should constitute an important interpretational premise in identifying or excluding areas with potential hydrocarbon traps in the Main Dolomite rocks in the area under study

Barrier microfacies and diagenesis of the main dolomite (Ca2) strata in the West Pomerania area

Przeprowadzono mikrofacjalną i diagenetyczną charakterystykę czterech profili reprezentujących środowisko bariery zewnętrznej cechsztyńskiego dolomitu głównego (Ca2) w rejonie platformy węglanowej Kamienia Pomorskiego (Benice-3, Ciechnowo-5) oraz platformy pomorskiej (Czarne-1, Gardomino-1). Analizowane profile składają się z dolomitów wykształconych w następujących mikrofacjach: greinstony i pakstony ooidowe, wakstony bioklastyczne, flotstony, rudstony intraklastowe, madstony stabilizowane mikrobialnie oraz utwory biogeniczne - maty mikrobialne i stromatolity. Bardzo często występuje biostabilizacja mikrobialna. Wyróżniono dwa etapy diagenezy: eodiagenezę - w nieznacznym stopniu wpływającą na pierwotnie wysoką porowatość, oraz związaną ze stopniowym pogrzebaniem osadu mezodiagenezę, która odegrała znacznie większą rolę w obniżeniu potencjału zbiornikowego tych skał. Na tym etapie oddziaływały procesy związane z neomorfizmem agradacyjnym, z kompakcją chemiczną oraz zróżnicowaną cementacją. Miejscami doszło także do znacznego zeszczelinowacenia skał, co spowodowało powstanie potencjalnych dróg migracji dla węglowodorów i roztworów zmineralizowanych. Przeprowadzone badania potwierdzają opinię, że utwory dolomitu głównego związane ze strefami barier na obszarze Pomorza Zachodniego charakteryzują się generalnie słabymi właściwościami zbiornikowymi.Microfacies and diagenetic characteristics of four profiles representing outer barrier environment of the Zechstein Main Dolomite (Ca2) have been performed in the area of the Kamień Pomorski (Benice-3, Ciechnowo-5) and Pomorze (Czarne-1, Gardomino-1) carbonate platforms. The profiles contain the following types of the dolostone microfacies: ooid grainstones and packstones, bioclastic wackstones, intraclast floatstones and rudstones, microbially stabilized mudstones and biogenic forms such as microbial mats and stromatolites. Microbial biostabilization is very common. Two stages of diagenesis have been distinguished: eodiagenesis - which only slightly affected the high primary porosity and mesodiagenesis associated with the gradual burial which played an essential role in lowering of the reservoir potential of the rocks. Aggradational neomorphism, chemical compaction and diverse cementation processes also acted at this stage. Fracturing of the rocks happened in some places causing formation of the potential migration routes for hydrocarbons and mineralized solutions. The studies confirmed previous opinions that the Main Dolomite rocks deposited in the barrier zones of West Pomerania have in general poor reservoir properties

Hydrocarbon fields in the Zechstein Main Dolomite (Ca2) on the Gorzów Block (NW Poland)

By discoveries of hydrocarbon deposits in the Polish part of the Southern Permian Basin, Western Poland became a key explorational area. The most spectacular discoveries have been done on the Gorzów Block which borders the Fore-Sudetic Monocline from the south and the Szczecin Swell from the north. This is the area where the biggest oil field - Barnówko-Mostno-Buszewo (BMB) - have been discovered and the group of oil and gas deposits Lubiatów-Międzychód-Grotów (LMG). Detailed litho- facies analysis of Main Dolomite carbonates and seismic surveys (mainly 3D) contributed to good recognition of depositional architecture in this part of the Main Dolomite basin. An interesting issue is the relationship of the discovered deposits with carbonate depositional zones and diverse chemical composition of hydrocarbons. Most of deposits is related to carbonate platforms and microplatforms. Less important are platform slopes but basinal zone is and was considered as the area where source rocks occur. Discovery of the Lubiatów oil deposit at the foot of the Grotów Peninsula opened new possibilities in searching for hydrocarbons. Later negative results of drillings in this zone did not affected the fact that there are still explorational hopes for this type of carbonates. Hydrocarbons which can still be discovered in this interval are still not known based on calculations done in the recent years. It seems, that better geochemical recognition and advanced look on diagenetic evolution and depositional architecture will bring about new discoveries in the Main Dolomite interval on the Gorzów Block

Permian trepostome bryozoans from the Zechstein Main Dolomite (Ca2) of Western Poland and NE Germany

The trepostome bryozoan Dyscritella microstoma Ernst, 2001 is reported for the first time from the Zechstein Main Dolomite (Ca2) of Poland and Germany. This species has been previously recognized in the older fossiliferous Zechstein Lime stone (Ca1) of the North Sudetic Basin (Poland) and in the northern part of Germany. The Polish specimens derive from a few boreholes located in the Pomerania region and in the Wolsztyn High, both in the western part of Poland. The German material comes from a borehole on Rügen Island in North eastern Germany. The present paper deals with the taxonomic description of D. microstoma and its distribution in the Main Dolomite deposits

X-ray micro computed tomography characterization of porosity in Rotliegend sandstones on the northern slope of the Wolsztyn Ridge, western Poland

Natural gas in the Polish Rotliegend Basin occurs in porous and permeable aeolian sandstones, and traps are mostly structural. Lithological traps are rare and occur on the northern slope of the Wolsztyn Ridge where fluvial and aeolian sandstones overlap with alluvial facies. Both fluvial and aeolian sandstones are reservoir rocks in this area. The X-ray micro computed tomography (MCT) results and microscopic observations indicate that the sandstones in lithologic traps on the northern slope of the Wolsztyn Ridge form highly compartmentalized vertical reservoir rocks composed of four types of sandstones. The profiles are dominated by very low-porosity fluvial sandstones (F2), and low-porosity aeolian and fluvial sandstones (A2, F1). The A1 type of sandstones with high porosity (10%) occurs only in some sections of the profiles. The most important diagenetic processes that reduced porosity were compaction and cementation by carbonate cements. All studied sandstones were subjected to the same diagenetic processes. However, each of the processes ran with varying intensity in different types of sandstones. Detailed analyses of pore distribution by MCT methods with respect to primary depositional or lithofacies effects, and secondary diagenetic effects, help to understand the 3D geometry of pores and pore shape-size distributions. The results can be used in the studies of other sandstones with a different origin and age

Mixed wetted carbonate reservoir: origins of mixed wettability and affecting reservoir properties

The greatest Polish oil fields are built of the Main Dolomite. They are mainly created as a single porosity - double permeability reservoir rocks showing mixed wettability. Mixed wettability is a result of differentiated diagenetic processes connected with basin evolution. The Main Dolomite sediments were sealed very quickly between salt and anhydrite sediments within the whole basin. Hence, reservoir rocks were also source ones when carbonate grains were mixed with organic matter. This caused that residual organic matter is still present in the Main Dolomite rocks. Presence of organic matter and way of mixing are factors affecting wettability of rocks. There is no evident correlation between the content of residual organic matter and wettability index. The main role is played by the process of mixing carbonate grains and organic matter. Rock Eval analyses were applied to indicate the type of oil wet parts within pore space. It was shown that oil wet pores occupy the range from the smallest to the greatest pores. Oil and water paths of fluid migration are practically independent. A large part of samples (70) are predominantly oil wet, 17 show predominant water wetting. There was no pure oil or water wet rocks. Such type of wettability produces irreducible oil and water in all samples

Air aerosol sampling station AZA-1000 at Polish Polar Station in Hornsund, Spitsbergen

The high volume air sampler AZA-1000 was built in the Environmental Protection Laboratory of the Andrzej Sołtan Institute for Nuclear Studies. In July 2002, this air sampler AZA-1000 was installed in the Polish Polar Observatory of the Polish Academy of Science in Hornsund, Spitsbergen. The device was built with numerous specific features which make possible to operate such station in extra difficult climatic conditions of polar region. Since July 2002, radioactive aerosols were collected at the Petrianov filter tissue FPP-15-1.5 and measured using high resolution g spectrometry in the Environmental Protection Laboratory at Świerk. The concentration of the airborn, cosmogenic radionuclide 7Be and of other natural and man maid radionuclides like 137Cs was determined. A comparison of the preliminary results with those from the ASS-500 sampling station operating at Świder is presented