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(1)Akademia Górniczo-Hutnicza im. Stanisława Staszica Wydział Geologii, Geofizyki i Ochrony Środowiska Katedra Geofizyki AGH University of Science and Technology Faculty of Geology, Geophysics and Environment Protection Department of Geophysics. ROZPRAWA DOKTORSKA DISSERTATION. KOMPLEKSOWA CHARAKTERYSTYKA WŁASNOŚCI ZBIORNIKOWYCH DLA MODELOWANIA PRZEPŁYWU MEDIÓW W ZŁOŻU GAZU ZIEMNEGO Z W ZAPADLISKU PRZEDKARPACKIM INTEGRATED RESERVOIR CHARACTERIZATION FOR FLUID FLOW MODELING OF THE Z GAS DEPOSIT AT THE CARPATHIAN FOREDEEP. M.Sc. MAN HA QUANG Hanoi University of Mining and Geology, Vietnam Promotor/Supervisor: PROF. DR HAB. INŻ. JADWIGA JARZYNA. KRAKÓW, 2011.

(2) ABSTRACT The purpose of this study was to use many statistical and geostatistical methods to integrate data of various scales: starting from microscopic scale (core plugs) through mesoscopic scale (logs) to megascopic (seismic) for improved reservoir characterization and reservoir modeling in order to generate a reliable geological model which was then used for dynamic simulation. This case study was performed for the Z gas reservoir depositional environment, of very complicated deltaic facies distribution which belongs to a group of the Miocene gas reservoirs in the northern-eastern part of the Carpathian Foredeep of Poland. The Hydraulic Flow Unit (HU) method was used to subdivide the reservoir into hydraulic flow units on the basis of conventional core data and conventional statistical methods. Two statistical approaches, namely Alternating Conditional Expectation (ACE) and Linear Multiple Regression (LMR) were applied to predict HU_log from core and log data. Since there is no rock type (RT) descriptive data available in the study area, unsupervised K means clustering was used for RT classification based on log data. Building static geological model both deterministic and stochastic geostatistical methods including Unsupervised Neural Network (UNN), Sequential Indicator Simulation (SIS), and Sequence Gaussian Simulation (SGS) were used to integrate 2D seismic and log data to perform static model. The innovative approach of the methodology developed and demonstrated in this study was building the pseudo 3D seismic cube from all available 2D seismic lines which were used as conditional data for hydraulic flow unit modeling. In order to verify the static geological model, history matching was performed constrained by the hydraulic flow unit distribution in the reservoir. The study has shown that even without 3D seismic and with limited well log control, the new hydraulic flow unit method can be used successfully to integrate reservoir data from different domains and from a wide range of scales. The resulting robust 3D property model can be validated by history matching. This methodology can be usefully extended to the upper part of the study area or other hydrocarbon reservoirs in Poland where dense networks of 2D seismic data are commonly available.. i.

(3) SUMMARY IN POLISH Geologiczna, petrofizyczna i geofizyczna charakterystyka złoża węglowodorów jest cennym źródłem informacji dla zrozumienia procesów zachodzących w formacji skalnej podczas eksploatacji. Przedstawienie wzajemnych relacji między porowatością efektywną i przepuszczalnością oraz zaileniem z uwzględnienie facjalnych zmian w trójwymiarowym ośrodku skalnym jest znakomitym ułatwieniem analizy procesów przepływu mediów w złożu. Szybki rozwój metod matematycznych i technik informatycznych oraz specjalistycznego oprogramowania stosowanego w modelowniach geofizycznych i geologicznych oraz inżynierii złożowej znacznie ułatwił parametryzację i opis procesów geologicznych niezbędnych przy poszukiwaniu i eksploatacji złóż węglowodorów. Wymusił także konieczność przedstawienia w postaci wzorów związków pomiędzy opisywanymi wielkościami. Do tego celu bardzo przydatne okazały się procedury geostatystyczne oraz sieci neuronowe pozwalające powiązać koncepcje geologiczne ze ścisłą, liczbową charakterystyką procesów. W pracy przedstawiono charakterystykę petrofizyczną i geofizyczną wybranego złoża gazu w zapadlisku przedkarpackim oraz zaproponowano podejście geostatystyczne do modelowania facji sejsmicznych oraz modelowania procesu przepływu mediów w złożu gazu w przypadku ograniczonej liczby danych. Celem pracy było połączenie danych geologiczno-geofizycznych pochodzących z różnych źródeł: punktowych wyników badań laboratoryjnych (skala mikro), jednowymiarowych danych geofizyki otworowej (skala mili) oraz sejsmiki 2D (skala makro) dla udokładnienia opisu złoża gazu oraz dla utworzenia poprawnego statycznego, trójwymiarowego modelu złoża w celu wykonania modelowania przepływu. Końcowym efektem pracy było sprawdzenie wyników symulacji przepływów z danymi otworowymi uzyskanymi podczas produkcji. Wykorzystane procedury wymagały spełnienia następujących warunków: . dostępność danych i oprogramowania oraz efektywność finansowa,. . możliwość obliczenia wskaźników Flow Zone Index, FZI, oraz podziału obszaru złoża na jednostki hydrauliczne, HU,. . dysponowanie danymi sejsmicznymi 2D (jeśli byłyby dostępne dane sejsmiczne 3D wyniki będą dokładniejsze),. ii.

(4) . dostęp do danych umożliwiających poprawne skalowanie modeli statycznych i dynamicznych od poziomu mikro do skali makro,. . sprawdzenie. poprawności. wyników. na. podstawie. historycznych. danych. eksploatacyjnych. Kolejne kroki wykonane w pracy są przedstawione na diagramie (Fig. 1.1). Diagram ten może być podstawą organizacji podobnych prac przy spełnieniu warunków wymienionych powyżej. Praca doktorska składa się z 7. rozdziałów. Pierwszy rozdział stanowi wprowadzenie do tematu konstrukcji statycznego i dynamicznego modelu ośrodka skalnego w warunkach ograniczonej dostępności danych. Zawiera również krótki opis treści pozostałych rozdziałów. W rozdziale 2. przedstawiono zarys budowy geologicznej złoża gazu ziemnego Z w zapadlisku przedkarpackim. Złoże należy do typowych wielowarstwowych nagromadzeń gazu ziemnego w utworach sarmatu w północno-wschodniej części zapadliska. Do prac modelowych wybrano fragment cienkowarstwowej formacji piaskowcowo-mułowcowoiłowcowej,. powstały w warunkach. sedymentacji. deltowej.. Zwrócono uwagę na. skomplikowany charakter utworów zbiornikowych i pokazano, że cechy obserwowane na anomaliach geofizyki otworowej oraz widoczne na przekrojach sejsmicznych (amplitud oraz innych atrybutów) wskazują na określone warunki sedymentacji. W rozdziale 3. przedstawiono koncepcję konstrukcji jednostek o jednakowych zdolnościach do przepływu (Hydraulic Units, HU) na podstawie parametru Flow Zone Index, FZI, wyznaczonego tylko na podstawie porowatości i przepuszczalności. FZI oraz HU zostały wyznaczone na podstawie wyników badań laboratoryjnych dostępnych w 10. otworach na złożu gazu Z (560 analiz). W rozdziale 4. omówiono wyniki połączenia danych laboratoryjnych i profilowań geofizyki otworowej w celu utworzenia jednowymiarowych modeli zmian HU w profilach otworów. W tym celu zastosowano regresję wielowymiarową, Linear Multiple Regression, LMR, oraz metodę Alternating Conditional Expectation, ACE. Nie dysponowano geologicznym opisem facji w interwałach, z których pobrano rdzenie, zatem zastosowano statystyczną metodę K mean do wydzielenia charakterystycznych warstw w badanej formacji, czyli wyznaczenia Rock Types. Uzyskane wyniki podziału mioceńskiej formacji cienkowarstwowej na 6 typów. iii.

(5) skalnych (litofacji), RT, okazały się spójne z wynikami uzyskanymi wcześniej na drodze wykorzystania ACE do przeniesienia rozkładu HU na pełne profile geologiczne w badanych otworach. W rozdziale 5 opisano sposób wyznaczenia statycznego, trójwymiarowego modelu ośrodka skalnego na bazie wcześniejszych wyników oraz 25 profili sejsmicznych 2D. Do połączenia wyników profilowań geofizyki otworowej i danych sejsmicznych wykorzystano metody deterministyczne, Unsupervised Neural Network, UNN, oraz geostatystyczne, Sequential Indicator Simulation, SIS, i Sequence Gaussian Simulation, SGS. Zastosowano innowacyjne przejście od danych sejsmicznych 2D do modelu trójwymiarowego (Pseudo 3D Seismic Cube). Następnie wykorzystano trójwymiarowy rozkład facji sejsmicznych w połączeniu z litofacjami, RT, wyznaczonymi wcześniej na podstawie geofizyki otworowej do utworzenia trójwymiarowego modelu jednostek przepływu, HU. W celu udokładnienia informacji pozyskiwanej na podstawie danych o zróżnicowanej pionowej rozdzielczości podczas tworzenia trójwymiarowego modelu statycznego wprowadzono dodatkowo więzy oparte na wielkości stosunku Net to Gross. Ostatecznym sprawdzianem poprawności przygotowanego modelu było porównanie wartości współczynnika przepuszczalności wyznaczonego w jednostkach jednakowego przepływu w modelu, K_HU i wartości przepuszczalności wyznaczonych na podstawie danych laboratoryjnych i geofizyki otworowej, K_log. W. rozdziale. 6. przeprowadzono. modelowanie. przepływu. gazu. na. podstawie. trójwymiarowego modelu opracowanego w rozdziale 5. Jako weryfikację poprawności modelowania zastosowano historyczne dane produkcyjne w 3. otworach na badanym obszarze złoża Z. Zastosowanie jednostek jednakowego przepływu okazało się skuteczne pod wieloma względami - i: uzyskano bardzo dobre korelacje między porowatością i przepuszczalnością w jednostkach HU, ii: dzięki użyciu HU możliwe było zredukowanie obliczeń w programie Eclipse do jedynie tych jednostek, które miały wysoki współczynnik FZI, czyli dużą porowatość i przepuszczalność oraz wysoki stosunek NTG, iii: funkcja kompakcji była dobrze dopasowana do modelu porowatości w każdej jednostce HU. W rozdziale 7 podsumowano wykonane prace oraz podkreślono innowacyjne aspekty modelowania statycznego – tworzenia trójwymiarowego ośrodka geologicznego z podziałem na jednostki HU oraz dynamicznego dla obliczenia przepływu mediów. Zwrócono uwagę na celowość proponowanego połączenia danych laboratoryjnych i geofizyki otworowej poprzez. iv.

(6) wyliczanie parametrów FZI oraz HU. Włączenie do obliczeń jednostek litologicznych (Rock Types) wyznaczonych na podstawie geofizyki otworowej stanowi dodatkowy element skalowania danych i przejścia od wartości punktowych do ciągłej informacji wzdłuż osi otworu. Włączenie danych sejsmicznych 2D, dzięki wykorzystaniu metod geostatystycznych i deterministycznych do konstrukcji facji sejsmicznych, pozwoliło na utworzenie statycznego modelu trójwymiarowego (Pseudo 3D Seismic Cube). Obliczenie porowatości i przepuszczalności w trójwymiarowym modelu statycznym zostało dodatkowo uzupełnione informacją na temat Net to Gross. Wyniki modelowania przepływów zostały zweryfikowane na podstawie danych historycznych. Ocena niepewności uzyskanych wyników jest najtrudniejszym do wykonania elementem przedstawionego schematu postępowania, zalecanego do wykorzystania w obszarach, gdzie do dyspozycji pozostaje ograniczona liczba danych geofizycznych i geologicznych. Stosowanie metod statystycznych i geostatystycznych powoduje konieczność wykorzystania dużej ilości danych laboratoryjnych dla uzyskania wiarogodnych wartości średnich i odchyleń standardowych porowatości i przepuszczalności. Stosowanie procedur statystycznych w modelowniach skutkuje brakiem powtarzalności przy powtarzaniu obliczeń, a nie uwzględnienie szczegółowych informacji o elementach tektonicznych w badanym obszarze z powodu. niepełnych. danych. geologicznych. powoduje. ograniczenie. wiarogodności. ostatecznych wyników modelowani. Ograniczenie wiarogodności predykcji zachowania złoża w przyszłości wynika z braku historycznych danych z długiego okresu produkcji złoża. Zatem, ograniczona wiarogodność uzyskanych wyników wynika z ograniczeń zastosowanych metod i dostępu do danych.. v.

(7) ACKNOWLEDGMENTS I would like to take this opportunity to thank all the family and friends who have helped and inspired me during my doctoral study! I would like to express my sincere thanks to Prof. dr Jadwiga Jarzyna, who patiently supervised the progress of my work and it is for her patient supervision, useful advice and discussions that led to the completion of this project. Many thanks also go to Prof. Le Hai An for his helpful counsels and arguments. I owe my deepest gratitude to the Rector of AGH University of Science and Technology, Cracow, Poland for granting me the scholarship and also Hanoi University of Mining and Geology for their support to my accomplishing this study. I am grateful to Prof. Jacek Matyszkiewicz, the Dean of the Faculty of Geology Geophysics and Environmental Protection AGH UST, Cracow, Poland. My special thanks to the Polish Oil and Gas Company Ltd., Warsaw, Poland for the permission to use the data. Petrel®, Eclipse®, Interactive Petrophysics® were used thanks to the university’s grant donation by Schlumberger to AGH UST. STATISTICA software was used thanks to AGH UST, Cracow, Poland license. I am also grateful to dr Jerzy Ziętek and his family, Ms. Maria Cicha and all of staff in Geophysics Department for helping me to adopt myself into Polish life style and other official matter. Without their helps this project would not have been completed. Thanks also go to Mr. Graham Dryden from Subsurface Consultants & Associates, Houston, USA for help with the English corrections and also helpful discussions. A special thanks to many of the students at AGH University. In particular, thanks to Wojciech Machowski, Michał Michna, Paulina Krakowska for the friendship we have developed. Also thank to all my Vietnamese friends in Cracow for their invaluable support and consideration during my stay here. I would like to thank my dear parents, parents-in-law and my brothers, sisters for their encouragement and support during the last 4 years when I was absent from home. Finally, I am deeply indebted to my dear wife Thu and daughter Minh Hằng for their love and their encouragement to my course-choosing and study.. vi.

(8) DEDICATION This thesis is dedicated to my Grandparents, my Parents, my Parents-in-law, my wife Thu and my daughter Minh Hằng. vii.

(9) TABLE OF CONTENTS ABSTRACT…………………………………………………………………………………….i SUMMARY IN POLISH……………………………………………………………………....ii ACKNOWLEDGMENTS……………………………………………………………….........vi DEDICATION…………………………………………………………………………...….. vii Chapter 1……………………………………………………………………………………...1 INTRODUCTION Chapter 2 GEOLOGICAL SETTING 2.1 Introduction to the geology and tectonic framework of the study area…………………….5 2.2 Lithology and sedimentological environment…………………………….………………10 2.3 Core Analysis……………………………………………………………………………..13 2.4 Relationship between reservoir parameters and sedimentary environment………………17 2.5 Organization of succession, its division and correlation diagram……………...………...20 2.6 Conclusions……………………………………………………………………….………28 Chapter 3 HYDRAULIC FLOW UNIT 3.1 Concept of Hydraulic Flow Unit……………………………….…………………………29 3.2 Hydraulic Flow Unit classification technique…………………………………………….35 3.2.1 Histogram……………………………………………………………….….…..36 3.2.2 Probability plot………………………………………………………….……...38 3.2.3 Ward's algorithm approach…………………………………………….…….....39 3.3 Critical review on determining HU in the Lower Miocene reservoir of Z gas field……..40 3.3.1 Preliminary outcomes…………………………………..………………...…….40 3.3.2 Global Hydraulic Elements………………….…………………………..……..41 3.4 Conclusions……………………………………………………………………………….46. viii.

(10) Chapter 4 CORE – LOG INTEGRATION 4.1 Core - log depth matching…………………………………………...……………………47 4.2 Hydraulic flow unit prediction…………………………………………………….……...50 4.2.1 Methodology……………………………………………………………..…......50 4.2.2 Flow Zone Indicator prediction………………………………………….……..52 4.2.2.1 Linear Multiple Regression (LMR)…………………………….…….52 4.2.2.2 Alternating Conditional Expectations algorithm (ACE)…….….…….56 4.2.2.3 Comparison of LMR and ACE algorithm…………………….………59 4.2.3 Hydraulic Flow Unit Prediction (HU_log) from FZI_pre_ACE……..….……..60 4.2.4. Validation of results………………………………………..………………..…63 4.3 Rock types classification………………………………………………………..…….…..66 4.3.1 K means clustering background ………………………………………..……….66 4.3.2 Applying K means for the data group 3 (G3: Z-76, Z-81, Z-82)……….………69 4.4 Relationship between Hydraulic Flow Unit (HU) and Rock Types (RT)……...…………69 4.5 Conclusions…………………………………………………………………………….…77 Chapter 5 STATIC MODELING 5.1 Reservoir modeling overview………………………………………………….…………79 5.1.1 Reservoir modeling workflow……………………………………….………...…79 5.1.2 Deltaic facies and spatial relationship………………………………..…….…….80 5.1.3 Geostatistical methods overview…………….…………………..….………...….83 5.2 Structure modeling……………………………………………………………….……….86 5.2.1 Miss-tie correction for 2D seismic survey…………………………..……….…...87 5.2.2 Horizons picking and 3D grid…………………………………….….………......87. ix.

(11) 5.3 Rock Type and Hydraulic Flow Unit Modeling…………….……………..……………..91 5.3.1 Convert 2D seismic to pseudo 3D seismic……………………….…….……...…93 5.3.2 Rock Type modeling constrained by seismic facies model…………………..….96 5.3.2.1 Seismic facies extraction volume…………………………………….96 5.3.2.2 Seismic facies classification………………………….……………...100 5.3.2.3 Rock Type modeling………………………………..………..……...104 5.3.3 Hydraulic Flow Unit modeling constrained by Rock Type model..….……...….107 5.4 Properties modeling constrained by HU model………………………………..………..112 5.4.1 Porosity and permeability modeling……………………………….…………....112 5.4.2 Water saturation and Net to Gross modeling………………………………...….116 5.5 Conclusions……………………………………………………………………….……..120 Chapter 6 HISTORY MATCHING UNDER HYDRAULIC FLOW UNIT CONTROL 6.1. History matching under Hydraulic Flow Unit control………………………………….123 6.1.1 History Matching Overview……………………………………..……….……..123 6.1.2 History matching under Hydraulic Flow Unit control………………….…..…..125 6.2. Upscaling...…………………….……………………………………………...…….….129 6.3. Reservoir initial condition………………………………………………………………134 6.4 History matching and discussions……………………………………………………….137 6.4.1 Results…………………………………………………………………..………137 6.4.2 Discussion……………………………………………………………..………..138 6.5 Conclusions……………………………………………………………………………...143 Chapter 7 CONCLUSIONS AND RECOMMENDATIONS 7.1 Conclusions…………………………………………………………………………...…144 7.2 Recommendations……………………………………………………………………….147. x.

(12) REFERENCES.......................................................................................................................151 LIST OF FIGURES AND TABLES………………………………………………….....…..158 LIST OF ABBREVIATIONS…………………………………………….…………...…….168 APPENDIX A…………………………………………………………………….…………169. xi.

(13) Chapter 1 INTRODUCTION In the upstream petroleum industry, reservoir characterization and description plays an important role giving geologists and geophysicist a more accurate and detailed understanding of the subsurface reservoirs. Understanding key reservoir properties such as porosity, permeability, their relationship as well as their spatial distribution by having precise and realistic reservoir model, assists petroleum engineers in improving the characterization of the reservoir, and to enhance the development and performance of the reservoir throughout its life. In recent years, with the robust development of the mathematics and computer science such as modeling, geostatistics, and neural networks, geological ideas have become easier to realize and be verified, especially with the advanced tools available through specialized software in the petroleum industry like Schlumberger Petrel and Eclipse. The need to bring the latest developments to the petroleum industry has led to one of the major motivations of this study. This thesis looks at various aspects of reservoir characterization and modeling where the geological and geophysical data available is rather limited. The aim of this thesis is to integrate G&G data from diverse disciplines; from core plugs through wireline logs to seismic. To this aim we develop suitable methodologies for improved reservoir characterization and reservoir modeling with rather limited data (mainly 2D seismic available) in order to generate a reliable geological model which was then used for dynamic simulation. On top of that, this study has successfully verified a recently emerged Hydraulic Flow Unit approach in various scales from micro scale of the core plug to mega scale of the seismic within a constraint of dynamic data through history matching processes. The methodologies developed in this study were designed to be: . reliabile and cost-effective,. . applicable to rocktyping by mean of hydraulic flow unit in reservoir characterization,. . applicable to dealing with limited data availability, especially where no 3D seismic data available,. 1.

(14) . applicable and reliable in upscaling from micro to mega scale in the framework of geological reservoir modeling constrained by dynamic data.. The workflow, which while by no means universally applicable, illustrates a practical approach to handling sparse data in a deltaic environment gas reservoir. The workflow was used throughout the course of this study, and is summarized in figure 1.1.. Core, Logs. 2D Seismic. Production. Qg, BHP, PVT,…. FZI_core. FZI_log. K-mean Clustering. Pseudo 3D Seismic. HFU_core. HFU_log. RT_log. 3D Seismic Structure Facies. 3D Rock Type Modeling. 3D HFU Modeling Dynamic Modeling. History Matching. 3D Properties Modeling (PHI, K). Fig. 1.1 The main workflow in this study for integration of static and dynamic models The rest of the thesis consists of 6 further chapters. Chapter 2 reviews the geological setting, including geology, tectonic framework, lithology and sedimentological environment of the Z gas deposit belonging to a group of Miocene gas reservoirs in the northern part of the Carpathian Foredeep of. This chapter also addresses the relationship between reservoir parameters and deltaic sedimentary environment due to the complexity of the structure of the Miocene (Sarmatian) succession in study area.. 2.

(15) Chapter 3 introduces the concept of the hydraulic flow unit (HU) and applies this approach to determine HU throughout the field based on conventional core data and conventional statistical methods. The results show that using all available 570 core plugs from the study area resulted in a reliable model of 6HUs. These were consequently used for the reservoir simulation. Chapter 4 discusses the application of two statistical approaches, namely Alternating Conditional Expectation (ACE) and Linear Multiple Regression (LMR), which were applied to predict hydraulic flow units (HU_log) from cores and logs. Since there is no rock type (RT) descriptive data available in the study area, unsupervised K means clustering was used for facies classification based on log data. The results demonstrate that statistical methods are useful, flexible, and effective, and that they delivered acceptable results. Unsupervised K means clustering is effective for classifying six rock types. The ACE method is good for predicting FZI from core and log. Chapter 5 proposes an appropriate approach in building static geological modeling for the reservoir in study area. Both deterministic and stochastic geostatistical methods including Unsupervised Neural Network (UNN), Sequential Indicator Simulation (SIS), and Sequence Gaussian Simulation (SGS) were used to integrate 2D seismic and log data to perform static modeling. The innovative approach of the methodology developed and demonstrated in this chapter was building the pseudo 3D seismic cube from all available 2D seismic survey for use as conditional data for hydraulic flow unit modeling. The derived results show clearly that there is a good correlation between permeability log (K_log) and 3D permeability model (K_HU) was and also confirm the advantage of applying the HU approach in this study. Due to lack of SCAL data for reservoir rock parameters, the 3D NTG model was calculated directly from 3D HU and it again showed the advantages of HU methods. In Chapter 6, in order to verify the static geological model built in chapter 5, history matching was performed constrained by the hydraulic flow unit distribution in the reservoir. Some recent accomplishments in jointly integrating static and dynamic information into a reservoir flow simulation model and history matching were reviewed. History matching was tested by manually adjusting a few reservoir model parameters through a trial-and-error procedure. Manual history matching was performed by running the simulator over the static 3D reservoir model for the historical production period, and then the results were compared. 3.

(16) with known field performance. The history matching results of three cases from 3 wells (Z74, Z75 and Z76) showed good results for gas flow rate for a few years production. As the study evolved, it became abundantly clear that when comparing the results of the traditional method (two stages) to the HU method (four stages), there are three distinct advantages of the HU approach: (i) at each HU we have very good correlation between porosity and permeability that is good for classification of cells in fluid flow for Eclipse simulation, (ii) from 6HUs distribution we can easy control NTG model for Eclipse simulation cutting off HU1 with lower PHI and K while reducing number of cells in the 3D grid to increase CPU processing speed, and (iii) the rock compaction function can respond to the porosity model (PHI) and each hydraulic flow unit. Chapter 7 summaries all of the innovative aspects of the thesis, the conclusions from various studies in the thesis and gives some recommendations and suggestions for future work that might benefit the reservoir in the study area.. 4.

(17) Chapter 2 GEOLOGICAL SETTING 2.1 Introduction to the geology and tectonic framework of the study area The multi-pay gas field, Z-L, is located in the north-eastern part of the Polish Carpathian Foredeep (Fig. 2.1). The Carpathian Foredeep is a peripheral molasse sedimentary basin which has been being overthrust in a northerly direction. The most hydrocarbon prospective sedimentary section is Miocene age (Karnkowski, 1999). The Miocene sediments can be up to 3500 m thick in the south part of the Carpathian Foredeep and are thinner in the northern part (Fig. 2.2).. Fig. 2.1 General overview of the Carpathians and Carpathian Foredeep (Oszczypko, et al., 2006) Two main geological components are important in the Carpathian Foredeep: first – the structural Precambrian - Palaezoic unit – constituting the base for the Neogene sediments, whilst in the Z-L region the unit is comprised of Proterozoic rock. The second structural unit is formed by the autochtonous Miocene formation including Lower Badenian, Middle Badenian and Upper Badenian and Lower Sarmatian and Quaternary sediments.. 5.

(18) Fig. 2.2 Schematic map showing the submarine deposits diversity in the eastern part of the Carpathian Foredeep (Myśliwiec, 2004) The Precambrian basement is made of fyllitic Table 2.1 Top of the Precambrian in shales. The rocks are ductile, with common study area cracks and fissures, most of which are filled with. Well. Depth [m]. clay minerals or calcite. Also present are thin. Z2. 1283. laminae of hard sandstone. In the study area the. Z24. 1223. top of the Precambrian was observed in four. Z72. 1237. wells as follows in Table 2.1.. Z75. 1097. In the Z72 well, the Precambrian formation consists of mudstones of low or moderate cohesion, fissured and cracked, and moderately to poorly lithified. These beds dip at 45o. In the Z74 well, the Precambrian rocks comprise mudstones and sandy-mudstones having a. 6.

(19) wackestone texture. In the D2 well the Precambrian rocks comprise massive mudstones and claystones. The mudstones sporadically include thin layers of fine-grained sandstone lenses, and more rarely continuous laminae. Sandstones textures include subarkose or arkose arenites, and rarely wackestones. The dense network of fissures, fractures and cracks range from closed to partially-open and are filled with quartz, chlorite and kaolinite. In the Z77 well the Precambrian rocks comprise claystones and mudstones. The Precambrian formation was also found in the Ch.D. 1, 2 and 3 wells and in the G.D. 1 and 3 wells. Generally, the Precambrian succession is covered by the autochtonous Miocene sediments, but in the southern part of the central element of the Z gas field, in the cores in the Z76 well, weathered fragile mudstones, claystones and fine grained quartzite sandstones with carbonateclay cement, and calcite cement are found. Organic matter is also present. There is no paleontological documentation, although by way of analogy with other formations in the Carpathian Foredeep it was assumed that the sediments are of Paleogene age. In the most wells in the Carpathian Foredeep the basement is covered by the Badenian and Sarmatian sediments. To the south of the Z-L gas field there is a zone without the Lower and Middle Badenian sediments, the so called Rzeszów Iceland. Its northern border coincides with the southern edge of the central element of the Z-L gas field. The Miocene formations of the Carpathian Foredeep have many years been divided into the Lower Badenian sub-evaporitic series, Middle Badenian evaporites, and the Upper Badenian and Sarmatian supra-evaporitic formations. Newer and more precise stratigraphic studies slightly modify the age determined for the above-mentioned principal series of sediments occurring within the Z-L bed. The former divisions are still used, however, because evaporites are the most important reflecting horizon identified in seismic sections in the foredeep (Myśliwiec, 2004, 2006a). The gas field Z-L consists of three components: the eastern component, central component and western component (Fig. 2.3). To the south there is another gas field, G.D., and to the east there is the Ch.D. gas field. The studied gas field Z-L and the surrounding fields are formed on the uplifts of the older base of the Miocene succession (Myśliwiec, 2004, 2006a).. 7.

(20) Fig. 2.3 Location of Z gas-field with cored and logged wells; tectonic elements marked in red (after Myśliwiec, 2006b and Myśliwiec et al., 2004). Lower Badenian This formation comprises grey-green shales deposited in the outer shelf environment and green glauconite sandstones from the shallow shelf environment. It has not been thoroughly explored, because most of the wells were drilled only to the bottom of the Sarmatian. Sandstones are average in terms of sorting and hardness, but they are fractured. However they are not important as a reservoir because they are generally of insufficient thickness (Dziadzio 2000). In the Z74 well, the Lower Badenian (Baranów beds) were penetrated from 1199 to 1204 m, in the Z72 well: 1228 to 1237 m, and in the Z77 well: 1215 to 1220 m. According to core descriptions from the Z72 well, the Baranów beds comprise clay shales and mediumgrained sandstones, medium-sorted, with only slightly rounded grains. Middle Badenian In the eastern and southern part of the central element of the Z gas field, the Middle Badenian comprises anhydrite with clays, salts and calcites, including also epigenetic calcite (Mysliwiec 2004). The anhydrite is about 10 – 25 m thick. In the west central element, the Middle Badenian comprises hard and massive limestones, with mudstone intercalations. The rocks are 10-15 m thick and are deformed. This evaporite series was formed during the shallowing of marine environment in which the reservoir rocks were being deposited, and this resulted in its partial isolation (Oszczypko, 1999). The evaporites in the eastern part of the foredeep are called the Krzyżanowice formations (Table 2.2) Their age, determined on the basis of nanoplankton, was established as the top of the Upper Badenian (Olszewska, 1999), thus. 8.

(21) these formations are not as old as previously thought (Middle Badenian). The thickness of the evaporites fluctuates around ten meters or so. Upper Badenian - Lower Sarmatian The sedimentation associated with the Upper Badenian and Sarmatian subsidence formed thick shaly-sandy layers, overlaying evaporites or, whenever the latter are absent, lying directly on the substrate rocks. There are many divisions concerning these formations, but they are most often called the Machów formations (Table 2.2). In the area of the Z gas field, these sediments consist of hard shales rarely laminated with fine-grained sandstones and mudstones. The sediments dip at from 5 to 30o and their thickness increases to the south. Separating the formations of the Upper Badenian from the Lower Sarmatian is difficult and usually made on the basis of increased sand content in the Sarmatian formations. In the places where the Middle Badenian is absent, the Upper Badenian is deposited directly on the Precambrian basement. The thickness of the Upper Badenian sediments in the central element of the Z gas field varies from 10-40 m (Z82). The greatest thickness is where the Middle Badenian is absent. Sarmatian The most important and thickest deposits of the autochtonous Miocene are the Sarmatian sandstones and shales, which are about 1100 m thick in the area of the Z gas field. The Sarmatian formations are most prospective in the study area because of significant accumulations of bitumens in the Z-L, G.D., and Ch.D. gas fields. The origin of the geobodies, was based on sedimentological investigation of cores and dipmeter interpretation (Documentation of wells: Z75, 79 and 84). Sedimentary environment studies of the Sarmatian, as well as the studies of the geometry and the lithology and facies of the geobodies enable us to recognize relationships between the lithofacies and gas reservoir rocks of Miocene age. These studies also show that the Miocene sediments form stacks of layers, which vary from very thin (several centimeters thick) up to 10 meters thick. These studies also indicate that these particular sandy bodies are comparable to those of other Miocene gas deposits. These studies have also contributed to our knowledge about horizontal and vertical variation in lithology and reservoir properties, and allow us to predict which sand bodies are likely to be the best gas prone reservoir rocks.. 9.

(22) Table 2.2 Stratigraphy of the Miocene formation (Rögl, 1996; Martini, 1971). 2.2 Lithology and sedimentological environment A characteristic feature of the Miocene deposits in the Carpathian Foredeep is the considerable diversification of facies. Facies changes related to the differentiation of a sedimentation environment are distinctly visible in the Sarmatian rocks. The depositional environment strongly influences the reservoir parameters of rocks (Dziadzio et al., 1997). In terms of their lithology, petrography and particularly facies characteristics, the Miocene reservoir rocks are extremely diversified. It occurs so often that each region, bed or even a well or single gas-bearing horizon, have their own specific characteristics. For this reason it is very difficult to systematize the data relevant to them. However, it is possible to classify them into several such types which, although of different age, lithology and origin, are - at the same time – of major economic importance (Myśliwiec, 2004; Myśliwiec, 2006a; Matyasik et al., 2007). In the Carpathian Foredeep, four main litho-facies have been identified. These are, starting from the deepest sediments: 1. A complex of turbidite sediments of the lowest Sarmatian, 450-500 m thick. It comprises sandstones of submarine fans and heteroliths of sandstone-mudstone-claystone from the basin plain. This sequence is organized as about 50 m cyclothems. A characteristic feature is the presence of a thin or very thin laminae. Some of these laminae are over one meter thick. Sandstone sequences do not cover very large areas and on their edges, they grade into mudstones. Heteroliths are good reservoir rocks; their porosity exceeds 20% and 10.

(23) permeability reaches 500 mD (Myśliwiec, 2006a; Dziadzio, 2000). These rocks appear to have been developed on the slopes of vast prograding accumulation levees typical of the distal parts of submarine complex. This type of prograding or accreting submarine fan complex, with thin laminae suggests that the environment of deposition was distal from the source, where hydrodynamic energy was almost exhausted. 2. A transitional sediment complex about 220 m thick. It comprises sandstone laminae fining upwards into sandstone-mudstones. In the upper part of the complex, sandstones prevail over fine-grained sediments. The complex is a result of progradation of a deltaic depositional system and is transitional between turbidites and deltaic sediments. 3. A complex of deltaic sediments is the most diverse in terms of a sedimentation environment and facies. Its most important feature is a vertical cycling. The 300 m thick complex comprises several parasequences (cyclothems) each 20-30 m thick. The parasequences have a complicated structure, but in each, one can observe sets of laminae of coarse grained sandstones with the thickness of each set of laminae increasing towards the top of the parasequence. Parasequences start with sediments of prograding accumulation levees (piles or bars). Inside of these sediments one can select parts of various sedimentation environments: delta slope, estuary levees (piles/bars) and deltaic plain. Sandstones in parasequences occur in the form of several meters thick packets bordered by underlying heteroliths of similar thickness. In the region of the Z gas field, not all cyclothems underwent typical or complete development, particularly in the middle and upper part of the deltaic sediment complex. Deltaic sediments are built in a zone where the river and the sea meet.They include a considerable amount of organic matter. This fact together with prograding deltas, is important from a hydrocarbon prospecting point of view because: 1) organic matter can be converted into hydrocarbons, which means the Sarmatian deltaic deposits are mature source rock for hydrocarbons and 2) the maximum flooding surfaces (MFS) indicated by the muddy-shaly upper limits of the parasequences form hydrocarbon seals for the reservoirs below. In fact deltaic sediments, especially those built in estuary ostiary levees (piles/bars), are good reservoir rocks. The sandstones have porosities ranging from 15-32 % and permeabilities of around 900 mD (Myśliwiec, 2006a, Śmist, 2003).. 11.

(24) 4. A complex of shallow shelf, littoral sediments represents the final stage of sedimentation in the Carpathian Foredeep. Deposits are poorly sorted and include fine grained lithofacies and muddy-shaly litho-facies. There is great vertical and horizontal variation in facies, which makes it difficult to recognize, classify and correlate them. Despite these litho-facial complexities, these sediments include gas accumulations in the shallow horizon (nr I) of Z gas field, representing sediments of a near shore coastal environment and open shelf and in the horizon nr II, as sediments from sandy barriers delimiting lagoons (Myśliwiec, 2004). The sandstones of the Sarmatian deltaic formations, and particularly the sandstones occurring in heterolithic or clayey formations, are characterized by low textural and mineral maturity. This first feature means that there is a relatively high content of a clayey-shaly matrix in the cement, and low rounded detritus. The second feature shows as a varied petrographic composition with minerals of platy crystal habit, plagioclases and various rock fragments. The deltaic sandstones of the Sarmatian usually contain large quantities of dispersed calcium carbonate, thus they are sometimes called marly sandstones. The total calcium carbonate content is increased also by the mud derived from from erosion of limestone rock, which, together with shale, forms cement. The grain matrix of sandstones is typically fine- to medium-grained with an admixture of silty-shaly material. Using the typical classification of lithoclasts, the deltaic sandstones include lithic and sublithic wackes, quartz-mica wackes, and – when better-sorted – sublithic arenites. The dominant mineral components include quartz and fragments of other rocks (limestones, dolomites, radiolarites, quartzites, quartz-muscovite shales, inter alia), as well as plagioclases, micas, calcium feldspars, glauconite, and carbonized plant detritus. Also present are bioclasts – foraminifera, fragments of bryozoans, and bivalves. The cement is made of shaly material, limestone mud, and terrigenous silt. Incidentally, crystalline calcite and dolomitic cements occur, filling single pores. On the basis of geological and geophysical studies it may be stated that the trap for natural gas in the Z gas deposit, which consists of a number of gas-bearing horizons, is of a structuralstratigraphic type. It is an anticline formed above an uplift of substrate rocks. In some horizons, the factors creating the trap also include lithological changes and more precisely, changes in facies together with local variations in porosity and permeability.. 12.

(25) In the Z gas deposit several types of gas-water contact were identified depending on the facies and lithology of the reservoir rocks (Geological Documentation of the Z-L gas field, 2007). In terms of different positions of separation surfaces between parts saturated with natural gas and those saturated with formation water, in the horizons where edge water occurs, the deposit is partly a thinly bedded sheet-deposit, whereas it is partly a more massive deposit in those horizons where bottom water occurs. In the sandstone horizons, and particularly these occurring in the upper parts of deltaic prograding accumulation levees, sealed by shaly sediments of the basin bottom, or those in sand-filled canals of the upper undersea fans, seismic profiling shows a visible gas-water boundary. The reservoir drive mechanism in these types of sediments is either edge drive in the case of thin laminated reservoirs, or bottom water in the case of thick and massive sandstone reservoirs When the gas is accumulated in heterolithic sediments of the delta plain or in deposits in the bays of a shallow shelf/near-shore coastal environment zone each thin sandstone or shale layer may have has its own individual gas-water contact. 2.3 Core Analysis Results of porosity and permeability obtained by laboratory measurements on cores are presented on the examples from 3 wells: Z75, Z84 and Z78. Core samples in other wells from the Z gas field provided similar results. Porosity distribution was measured using an Auto Pore 9320 mercury porosimeter. A scanning electronic microscope, SEM, and Roentgen microanalyser were used to examine the pore structure. Information was also obtained on cement type and distribution. Porosity in Z75 ranged commonly from 23 to 28 percent with rare samples having a range of 16-20 percent. The grain diameters fall within the 0.032 – 2 mm range, with the dominant fraction in the range of 0.033 – 0.125 mm (Fig 2.4). The average diameter of a pore throats is high and ranges from 0,2 to 2 μm. Over 50% of throat diameters exceed 1 μm. Polished sections impregnated with blue resin and images from the scanning microscope illustrate the nature of porous space perfectly (Fig 2.5). The porosity of this sample is 20.85%. The grain material is loosely packed and the proportion of diagenetic cement is low. Envelopes of quartzite cement and aggregations of clay materials are seen on scanning microscope images along with single quartzite crystals (Fig 2.6). Porosity is distinctly macropore in character. Pores with a diameter greater than 1 μm total some 80 to 95 percent, which means they. 13.

(26) possess good filtration abilities. The SEM showed that there is not much cement among the mineral components. It was also shown that porosity is inter granular or inter crystalline and pores are in communication, not isolated, suggesting high permeability. There is also a minor secondary porosity related to pores and micropores in grains of feldspar and plagioclases and in the lithoclasts. In some cases porosity exceeded 30 percent with permeability higher than 1 Darcy. These excellent reservoir characteristics were observed in well sorted sediment deposited in a high energy sedimentary environment, typically in the channel axes at the head of the delta or in distributary channels.. Number of data (%). 45 40 35 30 25 20. Fig.2.4 The result of grain size. 15. analysis of powdered sandstone. 10. taken at 677m, the depth of the. 5 0 0.031. Z75 well 0.063. 0.125. 0.25. 0.5. 1. 2. Grain size (mm). Fig. 2.5 Photomicrographs of a polished section impregnated with blue resin, from a sample obtained at a depth of 855.20m in well Z75; crossed nicoles (left – magnification 14X, right – 120X) (Documentation of wells: Z75, 79 and 84). 14.

(27) Fig. 2.6 Scanning microscope photomicrographs of a polished section from a sample obtained at a depth of 855.20m in well Z75; magnification 600X (Documentation of wells: Z75, 79 and 84). Samples in the Z84 well were highly diversified. Many of them, taken from the shallower part of the borehole, were too soft and shaly to make measurements. Only samples from the depth section of 720-729 m enabled regular measurements. Porosity in that section ranged from 24 to 25 percent and permeability in some samples was higher than 1 Darcy. Most of the samples revealed permeability of 100-500 mD. However, a few samples with a high porosity of 20 percent - show permeability lower than 2 mD. SEM investigations showed that rocks are porous and permeable, and that there is a small amount of carbonate-shaly matrix cement. SEM investigation showed conclusively that even when there was intergranular and intercrystal porosity with clear interpore communication, the presence of significant amounts of matrix cementation, degraded porosity and permeability so that these rocks were probably not capable of producing hydrocarbons. In the Z78 well, the reservoir parameters were also very diversified. In the upper part, from 528-537 m, the rock displayed high porosity (almost 30 percent) and high permeability (more than 100 Darcy). In other sections there are also good reservoir parameters except for 717-726 m, where extensive cementation lowers the porosity to 5-6 percent. Average porosity is equal to 23-26 percent and permeability about 100 mD. Similar to previous wells, porosity values of about 20 percent with permeability close to zero, were observed. Such samples are common in the lowest part of the Sarmatian. Geological core descriptions were also used for lithofacies calibration (Mastalerz et al., 2004). The short descriptions of the cores in two selected depth sections in the Z76 well are presented as example:. 15.

(28) 691-700 m Lithology: the upper part of the section comprises thinly bedded heteroliths of mudstones and sandstones, whilst the lower section exhibits a higher proportion of sandstone heteroliths. Also observed were the frequent synsedimentological deformations, mostly in one directional slope of diagonal laminae, rarely bimodal, there are frequently observed small nodules. In the lower part of the section, the normal sorting of grains is observed, and in some parts frequent bioturbacies have been detected. In addition lithoclasts are occasionally deposited in waves. In other areas nodular concretions are present. Facies: The environment of deposition is characterized by traction currents with a considerable amount of suspension; suspension currents, high movement of deposits in an area with a soft nonconsolidated bottom, a lower share of hemipelagic suspension deposition; probably sporadic influence of wave action and tidal currents; this sedimentation being typical of submarine deltaic slope. Other: Dip is generally horizontal or slightly inclined (0-5o); transportation is generally from one direction, in parts bimodal; the cores from that interval were correlated with logs in the depth section 689 – 698.5 m. 700-709 m Lithology: the upper part of the section is mostly heterolithic with mudstones and sandstones, of various lamina thickness; the dip direction in the sandstones is primarily in one direction, , rarely bimodal; frequent small nodules are visible, current ripple marks are deformed synsedimentologically; in thicker sets a normal grain size distribution is present, and not the frequent modifications due to wave activity; in the middle part of the interval sandstone clasts become more common; laminae are thicker and grains are coarser; in the lower part poorly sorted material occurs, and coarse and medium-coarse sandstones with few mudstone laminae are observed. Facies: the upper part of this section is similar to the previous one (691-700 m). In the lower part there is observed influence of weak suspension currents and traction currents. A considerable result of traction currents is also observed in the upper part.. 16.

(29) Other: horizontal lamination or minor dips (0-5o), transportation from one direction - of structural dip, is rarely bimodal. The cores from the interval were correlated with logs in the depth section 698.5 – 707.5 m. 2.4 Relationship between reservoir parameters and sedimentary environment The discussion presented below is based on data from the Z75, Z78, Z79 and Z84 wells selected as the typicall wells of the Z gas field. The best reservoir rocks were deposited in turbidite fans. They are thinly laminated, the laminae are equal in thickness and are parallel. Grain size is generally fine. Fining upwards sequences are common. The thickest laminae occur in coarse grained poorly sorted sandy lenses. They are formed in zones of migrating channels in the upper submarine fan. In the middle fan and between channels, sediments are more fine grained and their characteristics are more akin to typical turbidites. Turbidites in the Z gas field exhibit high porosity (up to 20 percent) and permeability (up to 500 mD). In the discussed wells sediments from the submarine fan turbidites do not contain any gas accumulations. Proximally the turbidites are replaced by a series of deltaic sediments. This transition occurs gradually, over a long distance. The transition zone is typified by laminae of fining upwards cycles typical of turbidites diminishing as they are replaced by sandy-muddy packets with a characteristic increase in grain size. Turbidites have low values of GR and rather low NPHI, whilst the occurrence of sandstones predominates over shales. An example is presented from the depth section of 925 – 1025 m in the Z84 well (Fig 2.7a). Deltaic sediments are where the richest gas accumulations are found. Thirteen of the seventeen gas horizons identified in the Z gas field are located in deltaic facies. They are cyclic and it is the reason why the Sarmatian gas deposits are found in stacked reservoirs. Changes in grain size and their sorting are related to cycles in parasequences. At the beginning of the parasequence, grain size increases except for in the top laminae where the grain size decreases. A similar pattern in terms of thickness of the laminae can also be seen. The frequent erosion of bars is typical of deltaic sediments caused by changes in the direction of the channel axes supplying the sedimentary material. The erosion cuts are filled with sands. From a hydrocarbon prospecting point of view there is an important stable geometrical shape of the deltaic sediments. It is related to a decrease in the energy of the sedimentation environment from stream outlets to the open sea. The shape of the deltaic area also depends. 17.

(30) on the mutual relationship between energy of sources, i.e. current flow in river, sea wave energy and tides. The fusion of the river environment with a considerable inflow of clastic material and the sea environment enables accumulation in the same place, of large amounts of organic matter, sandy material and shaly components. A typical parasequence consists of claystone forming the basis for levee (bar), overlying this are heteroliths Heterolithic bedding is defined as a closely interbedded deposit of sand and mud, generated in environments where current flow varies considerably. The three main types of heterolithic bedding are called flaser, wavy, and lenticular. Flaser bedding is characterized by cross-laminated sands with thin mud drapes over foresets. Wavy bedding consists of rippled sands with continuous mud drapes over the ripples. Lenticular bedding consists of isolated lenses and ripples of sand set in a mud matrix. Heterolithic sediments can be deposited in storm-wave influenced shallow marine environments, river floodplains, tidal flats, or delta front settings where fluctuating currents or sediment supply permit the deposition of both sand and mud. At the beginning of the sedimentation process the heteroliths form as a result of prograding of the bar slope. During the parasequence development phase, the genesis of heteroliths is connected with the channel fill. The top of the parasequence is built up of sandstones from the prograding head of the accumulation levee. The sequence terminates with the deposition of pelagic shales. These represent the maximum flooding surface which coincides with the beginning of the next parasequence. This pattern of deltaic sediments is well illustrated by pairs of GR and NPHI and electric logs (depth sections of 550-690 in the Z75 well) (Fig. 2.7b). In the study area the deltaic sediments are not homogeneous. A typical parasequence is observed in the Z75 well in the eastern part of the central element of the Z gas field. Boundaries between parasequences are distinct and relatively easy to discriminate. A similar picture is observed in the Z84 well. Moving to the west, the distinct picture of the parasequences is not observed. The same problem appears in the Z79 well, where distinguishing the boundaries is difficult. Sediments of the shallow shelf form the top of the Sarmatian succession. Their genesis is associated with the last stage of sedimentation in the Carpathian Foredeep, in which they filled the remaining free sedimentation space. They are not vertically organized, and are not well sorted, but they are composed of fine-grained sediments, mainly claystone and mudstone.. 18.

(31) They are not prospective for hydrocarbons. However, despite this, four of the highest horizons in the Z gas field are composed of those layers, and are located in heteroliths and mudstones intercalated with very fine-grain sandstones. GR and NPHI track each other closely which is interpreted to mean that these sediments are shaly, as evidenced by the high neutron porosity (high NPHI is observed in shaly formations due to water bound in clay minerals or in small micropores). The depth section of 410-500 m in the Z79 well presents a typical littoral deposit (Fig. 2.7c). RMS amplitude map and GR log for each well from top (~400m) to bottom (~900m) in the study area are presented in the figure 2.8.. Shallow marine shelf. Fluvial system/Open sea ?. Baries. Coarsening Upward. Fining Upward (b). Shallow marine shelf. Deltaic. (a). (c). Fig. 2.7 Well logs (GR and NPHI) showing vertical facies distribution; a) an example of turbidities from the depth section of 925 – 1025 m in the Z84 well; low value of GR and rather low NPHI, sandstones dominate over shales, b) a typical parasequence coarsening upward in deltaic sediments in the Z75 well, c) sediments of shallow shelf form the top of the Sarmatian succession (GR and NPHI are very close to one another), sediments are shaly and have high neutron porosity. 19.

(32) Fig.2.8. RMS amplitude map and GR log for each well shows deltaic facies distribution in the study area 2.5 Organization of succession, its division and correlation diagram Several analyses of sedimentological and structural conditions for the Z gas field area were provided on the basis of well log data (including dipmeter measurements and interpretation). The type example for sedimentological and structural analysis of the Miocene sediments in the Z75 and the Z76 wells is presented on the basis of dipmeter SED (Halliburton) measurements and interpretation (Mastalerz et al., 2004). Organization of a series and its structural-textural characteristics shows that in the profile we find series of sequences (cyclothems), mostly of a prograding character at the beginning and retrograding character in the next stage. In the Sarmatian sediments one can distinguish a few dozen sedimentary sequences with a hierarchic organization with varying character (part of them there are parasequences or sequences of a higher order in a sequential stratigraphical sense). Single sequences run from several to more than a hundred meters in thickness (mainly depending on their rank). A considerable part of the sequences show an almost symmetric. 20.

(33) structure: in the lower parts there is a visible general increase in the sandy character at higher elevations (it is connected with the gradual increase in the average thickness of the sandy laminae), however shallower sandstones are gradually fine grained, and laminae, especially the sandy ones, become thinner and thinner. Considerably rarer are sequences that are distinctly asymmetric (i.e. almost solely increase in grain size or almost solely decrease of grain size up to the top of sequences). Most of the sequences of a higher order are complex, i.e. they have internal subsequences of higher frequency. Generally speaking, for an accuratecorrelation of sediments in two geological profiles from the, it is important to distinguish marker correlation horizons with a characteristic structure. However in the case of the Z76 and the Z75 wells it was impossible due to their belonging to different structural units and the considerable distance between them. Additionally, the Z76 well and the Z75 well penetrate two sedimentary accumulations related to different depositional systems (as seen on well logs, especially on the arrow plot of the dipmeter) (Mastalerz et al., 2004). In the basin influenced by differential synsedimentary tectonics (as seen in the Miocene succession in the Carpathian Foredeep), a correlation of horizons between separate depositional systems is very difficult. A basic step of differentiation of a sedimentary series, due to genetic sequence stratigraphy criterions, is to find maximum flood surfaces (MFS). These are associated with periods of relative increase in water depth of the basin in which the tempo of growth of accommodation space was not compensated by an accumulation of sediments. These surfaces are connected to phases of maximum range of successive transgressions and are boundaries of genetic sequences. Such surfaces are of fundamental significance due to the chronostratigraphy of horizons in sedimentary series of transitional and shallow shelf types of environment. Parts of them can be connected with episodes of intensive subsidence in the parts of the basin or in the whole basin or with eustatic episodes. Then come the surfaces of the maximum range of progradation, named by some of the scientists as transgression surfaces, which may be treated as helpful horizons of chronostratigraphic significance. Their identification on well logs is more difficult and needs experience and a Dipmeter may be helpful here. Identification of these surfaces is indispensable for more detailed differentiation of the sedimentary series into tract systems in genetic sequences.. 21.

(34) The basis of a more detailed differentiation may be identification of boundary surfaces for depositional sequences, i.e. distinct erosion surfaces (erosion in sub-areal conditions) or their correspondents from more distant parts of the basin. In this stage it is important to show regressive surfaces of submarine erosion and transgressive erosion surfaces (connected with retrogradation of the shoreline). In Table 2.3 there are important horizons which can be treated as candidates of correlation horizons of various rank. Table 3 and 4 present sketches of divisions in the Sarmatian series of the Z region into stratigraphic sequences (also semi-genetic). This division may form the basis of a general prognosis for the place and geometry of sandy lithosomes. An overview of sequence stratigraphy results in the study area based on 2D seismic data are shown in figure 2.9. In figure 2.10 a comparison of the results of well log correlation (the cross section correlation: Z81 - Z76 - Z82 based on GR and NPHI) and the cross section based on seismic facies classification (Chapter 5) is presented and showed facies distribution various from well to well. In Table 2.3 there are important horizons which can be treated as candidates of correlation horizons. Table 2.4 and 2.5 present sketches of divisions in the Sarmatian series of the Z region into stratigraphic sequences (also semi-genetic). This division may form the basis of a general prognosis for the areal extent and geometry of sandy lithosomes. On the basis of presented data we can say that the main factor controlling sedimentation and the means of filling a basin was local tectonic subsidence. The analyzed series consists of various rank sequences. Each sequence of the main scale is several dozen meters thick and comprises a few sub-ranked sequences of a lower scale, which frequently reveal a complete inner structure and almost symmetrical character. The degree of development of the sequences of lower rank is diverse in different areas, and as such, the correlation of inner parts of sequences can be difficult. In each of the basal sequences, there are several candidates for maximum flooding surfaces, MFS, maximum progradation surfaces, MPS, or erosion surfaces, which may be treated as surface boundaries. This means also that the development of sequences of higher rank (lower scale) was not only controlled by tectonics, or eustatics, but also authocycling (parasequencies). It is worth remembering that sequences of higher rank can range on average from a few meters to several tens of meters in thickness. Differentiation of such sequences and describing their character and variability is crucial for a credible. 22.

(35) prognosis of the extent and geometry of single reservoir lithosomes. In addition, the inner geometry of such sequences cannot be followed by routine seismic methods.. (a). (b). Upper delta. Delta lobe SB. MFS SB. MFS SB. Fault ?. MFS. SB. (c) (d). MFS: Maximum Flooding Surface SB : Sequence Boundary Fig. 2.9 Overview sequence stratigraphy in the study area based on 2D seismic data a) seismic facies classification based on seismic attributes (Chapter 5), b) balancing section a) c) some basic sequence stratigraphy interpretation, d) successive progradation of delta lobes deposits an offlap succession with a clinoform geometry (after Frazier 1974) 23.

(36) 24. Fig. 2.10 Comparison of the result of well log correlation; a) cross section correlation Z-81 – Z76 - Z-82 based on GR and NPHI, (after Mastalerz, et al., 2004) b) cross section basic on seismic facies classification (Chapter 5).

(37) Table 2.3 Important correlation horizons (event markers) distinguished in the Z75 and Z76 wells in the selected depth section according to its geological core description (after Mastalerz, et al., 2004) Depth Depth Horizon Well logging features Comments Z75 Z76 the lowest rank MFS. 481. 490 ?. distinct maximum on GR. top of fundamental sandy sequence SG9, close to depicted seismic horizon boundary not distinct in the. MFS_SG8/9 538. (525. distinct maximum on. Z76 well (at the top a. ?)/534. GR. relatively fine-grained sequence is visible). distinct maximum on MFS_SG7/8 612. 623-638 GR and dramatic change of plots in Z76. boundary not distinct in Z76, probable tectonic discontinuity. distinct maximum on MFS_SG6/7 696. 699. GR and minimum on. close to described seismic. resistivity, insignificant. horizon. extremes in other curves MFS_SG5/6 737. 733. distinct maximum on. probably boundary of lower. GR. rank not very distinct boundary in. MFS_SG4/5 777. 776. distinct maximum on GR. the Z75 well. Close to described seismic horizon (probably boundary of lower rank). distinct maximum in MFS_SG3/4 863. 850. GR and extremes in other logs. MFS of lower rank. distinct maximum in 908. 897. GR and extremes in other logs. close to depicted seismic horizon. 25.

(38) Table 2.4 Genetic sequences, SG, and other stratigraphic intervals distinguished in the Miocene succession in the Z75 well (after Mastalerz, et al., 2004) Depth Depth Seque Paleo Lithology, lamination from to nce slope. 463. 538. SG9_ Z75. Complex sequence (two distinguished sandy intervals) with almost symmetrical structure. Various lamination, mostly of medium thickness and up to thick laminae. WSW (ENE). Complex sequence of a complicated structure and with a significant part comprised of relatively fine grained 538. 612. SG8_. material – mostly heterolithic and sandy-shaly with. Z75. thinly laminated sandstones. Sandstones are mainly in. NNW. lower and middle part, mostly with middle and low thickness laminae Sequence of complicated structure and great litho –. 612. 696. SG7_ Z75. facial variability; from thinly laminated mudstones including nodules and ferrous cements (in top and in bottom about 605 m) to various in facies sandstones. NNWSSE. (thickly laminated are rare). 696. 737. SG6_ Z75. Sequence with distinct tendency of increasing average. SSE. size of grains to the top and distinct boundaries in parts. (signifi. of fine-grained sediments. Sandstones of variable. cant. characteristics with a relatively large amount of thickly. dispers. laminated ones. ion). Sequence of a complex structure (probably two. 737. 777. SG5_ Z75. equivalent sequences) and various lithofacial characteristics: from thinly-bedded mudstones to thickly bedded sandstones. At the depth of 768 m –a distinct zone MFS of ferrous cement. variabl e SSESW?. Complex sequence with almost symmetric structure. 777. 863. SG4_. Domination of sandstones (including a distinct part of. ESE-. Z75. thick and very thickly laminated) about fine-grained. SE. sediments. 26.

(39) Table 2.5 Genetic sequences and other stratigraphic intervals distinguished in the Miocene succession in the Z76 well (after Mastalerz, et al., 2004) Depth Depth Seque Paleoslope Lithology, lamination nce from to /transport Complex sequence with domination of fine grained (465). (525). SG9_. sediments, various in facies; only in the middle part. 491. 534. Z76. is there a packet 10 m thick dominated by thin bedded sandstones Sequence of a very complicated structure: several thick intervals of largely sandstone (various. (525). 623-. SG8_. lamination, mostly middle and thin-bedded). Upper. 534. 638. Z76. sandstones have much better marked progradation segments, and a relatively large proportion of fine-. SW-NE (significant dispersion). grained sediments Complex sequence with an almost symmetric 623638. 699. SG7_ Z76. vertical structure; several thicker intervals with a. NE ?. larger share of sandstones (mostly middle-size. (significant. lamination, only in the lower part are they thicker);. dispersion). generally dominated by fine-grained sediments. 699. 733. SG6_ Z76. Relatively homogeneous, slightly asymmetric sequence with hard progradation segment;. NNE. sandstones, only thin and middle laminated Complex sequence with two thicker intervals with. 733. 776. SG5_ Z76. a domination of sandstones of variable lamination (in the lower part there is a larger share of thick laminae), in the upper part fine-grained sediments. NE (great dispersion). dominate. Complex interval (at least two thick sequences). 776. 850. SG4_ Z76. with a large share of sandstones of variable lamination, increasing up to very thick in the central part (815-830 m), distinct domination of. NNE to NNW. fine-grained sediments (MFS zone) SG – genetic sequences 27.