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Polyphase dolomitization of the Wuchiapingian Zechstein Limestone (Ca1) isolated reefs (Wolsztyn Palaeo-Ridge, Fore-Sudetic Monocline, SW Poland)

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Jasionowski M. , Peryt T. M. , Durakiewicz T.

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tom Vol. 58, No. 3

503--520

EN

Dolomitisation was the main diagenetic process in the Upper Permian Zechstein Limestone of the Wolsztyn High-dolomite cementation (“over-dolomitisation”) also occurred. The rocks studied usually have a mixed mineralogy and represent a continuous spectrum from pure limestone to pure dolomite. This is due to varying degrees of dolomitisation, dolomite cementation and dedolomitisation. There are two main types of dolomite: replacement dolomite (mostly planar unimodal dolosparite mosaics that are mainly fabric-destructive) and cement dolomite (planar isopachous rims and pore-filling non-planar saddle-dolomite crystals). The timing of dolomitisation and dolomite cementation is difficult to ascertain, but comparing petrographical and geochemical data indicates that the reef carbonates were dolomitised shortly after deposition in a near-surface sabkha/seepage-reflux and then in burial systems. It seems that many of the dolomites gain their present isotopic composition when buried in relatively high-temperature conditions, as shown by low oxygen isotopic ratios ( δ18O as low as –9‰ PDB) and the presence of saddle dolomite. No isotopic support for a water-mixing mechanism is documented.

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Lokalizacja przypuszczalnych stref akumulacji gazu na poziomie wapienia podstawowego w kopalni miedzi „Rudna”

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Dec J. , Marzec P. , Pietsch K.

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tom T. 68, nr 7

72--80

PL

Kopalnia miedzi „Rudna” prowadząc rozcinkę złoża natrafiła na „pułapkę” sprężonego gazu, co było przyczyną wyrzutu do chodnika rozdrobnionego materiału skalnego oraz gazu. W sytuacji powstałego zagrożenia wyrzutami gazu istotnym stało się znalezienie metody pozwalającej na wyprzedzającą lokalizację stref potencjalnej koncentracji gazu. Powierzchniowe badania sejsmiczne to podstawowa metoda wykorzystywana do rozpoznanie budowy strukturalnej górotworu oraz do lokalizacji złóż gazu ziemnego. W związku z powyższym nad miejscem wyrzutu w kopalni wykonano powierzchniowe, trójwymiarowe zdjęcie sejsmiczne 3D. Celem badań było rozpoznanie utworów cechsztynu i stropowych utworów czerwonego spągowca oraz próba identyfikacji w zapisie sejsmicznym na poziomie P1 (granica cechsztyn/czerwony spągowiec) stref anomalnych, które mogą być związane z nasyceniem gazem dolomitów Ca1. Wykonana interpretacja wskazała obecność niewielkich uskoków przecinających stropowe utwory czerwonego spągowca i spągowe utwory cechsztynu. Uskoki mogą być drogami migracji gazu karbońskiego. Na powierzchni P1 zlokalizowano strefy o prawie zerowej amplitudzie odbić, które wskazują na obniżenie parametrów sprężystych dolomitu Ca1. Obniżenie to można wiązać ze wzrostem porowatości i szczelinowatości dolomitu oraz nasyceniem gazem (obniżenie prędkości propagacji fal sejsmicznych).

EN

The “Rudna copper mine, conducting deposit development, has encountered a compressed gas “trap” that was the reason of outburst into the drift of rock material and gas. In the situation of gas outburst hazard essential became to find a method that allows advanced location of potential gas concentration zones. Surface seismic surveys constitute the basic method used for the identification of the structure of the rock mass and for the localization of natural gas deposits. In connection with the above, over the outburst site in the mine a surface three-dimensional (3D) seismic survey was performed. The objective of investigations was the identification of Zechstein and Rotliegend formations, as well as an attempt to identify anomalous zones in the seismic record at the level P1 (Zechstein/Rotliegend boundary), which could be connected with gas saturation of Ca1 dolomites. The performed interpretation has indicated the presence of minor faults cutting through the top-level Rotliegend formations and floor-level Zechstein formations. Such faults can constitute migration channels for Carboniferous-period gases. On the surface P1 zones with nearly zero reflection amplitude were localized, which indicate the reduction of elastic parameters of the Ca1 dolomite. The reduction can be connected with the increase in porosity and fissility of the dolomite and its saturation with gas (reduction of seismic waves propagation velocity).

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The isolated Wuchiapingian (Zechstein) Wielichowo Reef and its sedimentary and diagenetic evolution, SW Poland

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Fheed A. , Świerczewska A. T. , Krzyżak A. T.

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tom Vol. 59, No. 4

762--780

EN

The development of a relatively small and isolated part of the Wuchiapingian, Zechstein Wielichowo Reef was possible owing to a progressive subsidence and frequent sea level fluctuations. Three biofacies were distinguished within the studied object: (1) a shallow-water and highly energetic Acanthocladia biofacies, dominated by bryozoans and crinoids, with poorly preserved porosity, reduced mainly by calcite cementation and compaction; (2) the Horridonia biofacies comprising numerous brachiopods, preferring a moderate depth of water, with comparably poor porosity; and (3) the Fenestella/Kingopora biofacies rich in fossils of the highest variability, related to the deepest and calmest waters, occurring on the top of the profile and showing a significant effective porosity, reaching almost 13%. Among many diagenetic processes altering the reef, several lines of evidence suggest that it was the meteoric diagenesis to enhance its porosity the most extensively. Since no stromatolites are present, the final sea level decrease is interpreted to be rapid, hence creating conditions favourable for the meteoric dissolution. Some intraparticle porosity, however, seems to be of a depositional origin.