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The application of well logging and seismic modeling to assess the degree of gas saturation in Miocene strata (Carpathian Foredeep, Poland)

Marzec P., Niepsuj M., Bała M., Pietsch K.

Acta Geophysica

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2014

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Vol. 62, no. 1

83--115

EN

The presence of natural gas in the pore space of reservoir rocks results in a significant decrease in P-wave velocity. Even low gas saturation can generate seismic anomalies (DHI) and false image of gas accumulation of economic importance. This article presents an attempt to evaluate gas saturation from 2D seismic section in the Miocene sandstone strata in the south-eastern part of the Carpathian Foredeep. The ESTYMACJA program and the Biot–Gassmann model were used to study the dependence between elastic parameters and saturating fluids (water and gas) recorded in wells. Series of calculations was carried out using a method of fluid substitution for various gas saturation. The applicability of seismic data for evaluating gas saturation of reservoir beds was assessed with the use of 1D modelling (synthetic seismograms) and 2D modelling (theoretical seismic section) calculated for different gas saturation. The proposed methodology can be used to identify low and high gas-saturated zones and contour the reservoir.

2

Application of 2-D forward seismic modelling for impro- ved imaging of sub-salt Rotliegend strata in Polish Basin

Marzec P., Niepsuj M., Słonka Ł., Pietsch K.

Annales Societatis Geologorum Poloniae

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2013

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Vol. 83, No. 1

65--80

EN

Forward seismic modelling can aid seismic studies of the pre-Zechstein strata in areas of developed salt tectonics, such as the Obrzycko–Szamotuły region, NW Polish Basin. The results not only can be used for seismic interpretation, but also can support the planning of survey methodology and the workflow of seismic data processing. This paper presents the results of modelling that was carried out, before the acquisition of the regional-scale, seismic line Obrzycko-1–Zabartowo-1–Zabartowo-2 (Górecki, 2010). An interpreted, seismic transect was used to build a basic, seismic-geological model. The modelling was based on seismic ray theory. The zero-offset mo- delling (theoretical wave field) for different geometries of salt structures showed that an increase in salt thickness resulted in a pull-up of reflection events, related to the sub-salt horizons. The incorporation of faults and salt overhangs into a model significantly complicated the seismic wave field. The results of offset modelling, pre- sented in this paper as seismic ray tracing and common-shot gathers, proved that (1) the seismic response of the Rotliegend (Permian) formations can be recorded, despite the presence of the overlying salt pillows and diapirs, if offsets several kilometres long are used, and (2) the complex configuration of seismic reflectors (diapirs with salt overhangs, faults) gives rise to complicated, seismic ray paths that may cause difficulties in common-depth-point stacking and therefore decrease the quality of the seismic records.

3

Estymacja wybranych parametrów petrofizycznych w oparciu o atrybuty sejsmiczne oraz dane geofizyki otworowej

Niepsuj M., Pietsch K.

Prace Naukowe Instytutu Nafty i Gazu

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2012

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nr 182 wyd. konferencyjne

367--373

PL

Dolomit główny (Ca2) jest jedną z podstawowych formacji zbiornikowych w NW Polsce. Stanowi on drugi poziom węglanowy cyklicznej sekwencji ewaporatowej cechsztynu na Niżu Polskim. Ze względu na duże urozmaicenie dna basenu sedymentacyjnego cyklotemu PZ1, osady dolomitu głównego wykształciły się w zróżnicowanych strefach paleogeograficznych: basenowej, podnóża platform węglanowych, bariery węglanowej i równi platformowej. Skutkiem tego jest nie tylko zróżnicowanie miąższości skały zbiornikowej, ale również pionowa i pozioma zmienność jej parametrów petrofizycznych. Właściwe ich rozpoznanie jest jednym z głównych zadań przy poszukiwaniu złóż węglowodorów. Wyznaczenie rozkładu porowatości dolomitu głównego umożliwia zaawansowana interpretacja danych sejsmicznych oraz danych geofizyki otworowej. W tym celu określono zależności pomiędzy parametrami petrofizycznymi Ca2 a parametrami, od których w sposób bezpośredni zależy zapis sejsmiczny. Umożliwiło to wyznaczenie wzorów zależności, które posłużyły w późniejszym etapie do wyznaczenia porowatości z danych sejsmicznych. Prace interpretacyjne, analizy i obliczenia, zarówno dla danych geofizyki otworowej, jak i danych sejsmicznych, wykonano w systemie Hampson-Russell Software.

EN

Main Dolomite (Ca2) is one of the main reservoir formations in NW Poland. It is the second carbonate level of the evaporates sequences of Zechstein in the Polish Lowlands. Because of the large diversity of the sedimentary basin bottom of cyclothem PZ1, Main Dolomite was formed in different palaeogeographical zones: basin in floor, toe of slope, carbonate barriers and platform flat. The result is not only the diversity of reservoir rock thickness, but also the vertical and horizontal variability of the petrophysical parameters. Their correct recognition is one of the main tasks in the search of hydrocarbon deposits. Determination of the Main Dolomite porosity distribution enables advanced interpretation of seismic data and well log data. For this purpose relations between Ca2 petrophysical parameters and parameters, on which seismic record is dependant, were estimated. It allows to determine the formulas which were used to calculate the porosity of the seismic data at the later step. The interpretation work, analysis and calculations for both, well log data and seismic data, were carried out in Hampson-Russell Software.

4

Analysis and modeling of petrophysical parameters of the Main Dolomite formation on the basis of well logging and seismic data

Niepsuj M., Krakowska P.

Geology, Geophysics and Environment

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2012

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Vol. 38, no. 3

317--327

EN

The Main Dolomite is one of the most prospective hydrocarbon reservoir formations in Poland. The goal was to determine the Main Dolomite zonation in selected part of carbonate platform sedimentation area and also to analyze the influence of cementation factor on water saturation by well logging and seismic data integration. Well logging interpretation quantitatively characterized petrophysical parameters. Seismic inversion presented the distribution of the parameters at vertical and horizontal scales. Basic statistical calculations of petrophysical parameters, 2D crossplots and seismic inversion were made. The central part of the Main Dolomite interval indicated the best reservoir properties. High porosity values and low P-wave velocity, low bulk density and low water saturation values were observed in the analyzed zone. Mudlogging confirmed the occurrence of gas. Determination of hydrocarbon saturation in carbonates is a challenge for well logging interpretation and geophysical interpretation. The cementation factor is one of the main coefficients in petrophysics which strongly affect water saturation. Adopting the wrong value of this parameter causes serious error in the coefficient of the water saturation value. In the paper, water saturation was modeled using the Borai equation and Shell formula. By using the computed water saturation and fluid substitution method theoretical velocities of P-waves and S-waves (also P-wave/S-wave velocity ratio) were calculated. Results of the comprehensive interpretation of logs are the basis for lithology determination but P-wave and S-wave velocity can also serve as a source information about lithology. In reservoir rocks VP/VS ratio may also work as a confirmation of gas saturation.

5

The influence of seismic velocity distribution on the depth imaging of the sub-Zechstein horizons in areas affected by salt tectonics: a case study of NW Poland

Pietsch K., Marzec P., Niepsuj M., Krzywiec P.

Annales Societatis Geologorum Poloniae

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2012

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Vol. 82, No. 3

263–-277

EN

Lateral changes in the thickness of strata and petrophysical parameters within the Zechstein succession (salt pillows and domes) can cause many problems in seismic exploration of the aeolian Rotliegend formations, which are prospective for hydrocarbons. An assessment of the influence of halokinesis on the seismic imaging of the sub-Zechstein strata in NW Poland (Obrzycko–Szamotuły area, to the SW of the Mid-Polish Swell) utilised time-to-depth conversion with different, seismic-geological models. Various, seismic velocities were used in models for the Zechstein and the Mesozoic successions, namely velocities, dependent on the thickness of particular rock successions, on their depths, and velocities, determined from seismic inversion. The results show opposite reflection patterns for the seismic section imaged in the time and depth domains. The synclinal arrangement of the strata boundaries in the depth model is represented by convex-upwards reflection events on the seismic section. The pull-up of reflection events, associated with the sub-Zechstein strata, observed on the seismic sections, is mainly a result of both the greater thickness of the Zechstein salt within the salt structures (pillows, diapirs) and the increase in velocity contrast between the salt body and the Mesozoic strata.

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Wykorzystanie tłumienia fal sejsmicznych do oceny stopnia nasycenia przestrzeni porowej gazem, na przykładzie złoża w dolomicie głównym Ca2

Niepsuj M., Pietsch K., Łańka M

Prace Naukowe Instytutu Nafty i Gazu

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2010

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nr 170 wyd. konferencyjne

543--547

PL

Jednym z zadań stawianych przed sejsmiką naftową jest poszukiwanie metod interpretacji umożliwiających ocenę stopnia nasycenia gazem. Wśród istotnych dla sejsmiki parametrów petrofizycznych, które zmieniają się pod wpływem nasycenia przestrzeni porowej gazem jest tłumienie fal sejsmicznych. W artykule przedstawiono próbę odpowiedzi na pytanie: na ile tłumienie fal sejsmicznych może być źródłem informacji o stopniu nasycenia dolomitu głównego gazem? Z powodu małej ilości badań doświadczalnych dotyczących tłumienia fal sejsmicznych w węglanach wykonano modelowania sejsmiczne. Modelowania miały na celu sprawdzenie możliwości prześledzenia zmian zapisu sejsmicznego związanych z przyjętymi różnymi wartościami współczynnika dobroci Q. Za wzór modelu ośrodka skalnego przyjęto profil sejsmiczny przechodzący przez otwór, który zidentyfikował złoże. Inwersja pozwoliła na ocenę wiarygodności otrzymanych wyników modelowań. Realizację projektu wykonano w programach: Hampson-Russell oraz Norsar 2D.

EN

One of the most important aim of seismic modeling is a determination of gas saturation in a reservoir formation. A seismic wave attenuation, one of the essential petrophysical parameters of a seismic signal, is dependent on gas saturation in a pore space. This article is an effort to answer the question, if a seismic wave attenuation can provide an information about changes of gas saturation in Main Dolomite (Ca2). Main reason to carry out a seismic modeling was lack of empirical studies about wave attenuation in carbonates. The key purpose of the modeling was the assessment of seismic signal changes related to various quality factor (Q) values. The modeling was based on the data acquired from the seismic profile passing through well, which discovered the gas field located in Main Dolomite. The seismic inversion allowed to evaluate a reliability of the results obtained from the modeling. Project was done using Hampson- Russell CE8 R2.1 and Norsar 2D Ray Modeling software.