THICK-BEDDED DEEP-MARINE
SANDSTONES IN OUTCROP AND
SUBSURFACE
Sequence architecture and reservoir modelling aspects
IN OUTCROP AND SUBSURFACE
Sequence architecture and reservoir modelling aspects
DIK-GELAAGDE DIEPMARIENE ZANDSTENEN IN ONTSLUITING EN ONDERGROND
Interne opbouw en aspecten van reservoirmodellering (met een samenvatting in het Nederlands)
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van Rector Magnificus Prof.dr.ir. J.T. Fokkema, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag 4 oktober 2005
door
Clemens Alfonsus VISSER
doctorandus in de wiskunde en natuurwetenschappen geboren te Zwolle
Prof. Dr. S.M. Luthi
Samenstelling promotiecommissie Rector Magnificus, voorzitter
Prof. dr. S.M. Luthi, Technische Universiteit Delft, promotor Em. Prof. dr. ir. K.J. Weber, Technische Universiteit Delft Prof. A. Hurst, University of Aberdeen
Prof. S. Flint, University of Liverpool
Prof. dr. S.B. Kroonenberg, Technische Universiteit Delft Dr. M.E. Donselaar, Technische Universiteit Delft Dr. T. Dreyer, Norsk Hydro, Norway
Prof. dr. ir.R.J. Arts, Technische Universiteit Delft, reservelid
Thick-bedded deep-marine sandstones in outcrop and subsurface – Sequence architecture and reservoir modelling aspects / Clemens Alfonsus Visser
Thesis Technische Universiteit Delft. – With Ref. – With summary in Dutch. ISBN 9064642729
© Copyright 2005 by C.A. Visser, Delft University of Technology
All rights reserved. No part of this publication may be reproduced, stored in a retrieval sys-tem, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission of the copyright holder.
Printed with generous financial support of : – Norsk Hydro Research Centre
Acknowledgements 11
Chapter 1 Introduction 13
1.1. Deep-marine sandstone reservoirs 15
1.2 Geological reservoir models 15
1.3 Thesis objectives 17
1.4 Thesis outline 18
1.5 Data and metods 19
Chapter 2 Thick-bedded deep-marine sandstones 21
2.1 Introduction 23 2.2 Definition 24 2.3 Research methods 24 2.3.1 Outcrop studies 24 2.3.2 Core studies 26 2.3.3 Modern systems 26 2.3.4 Seismic surveys 27 2.3.5 Flume studies 30 2.3.6 Numerical modelling 31 2.3.7 Comparison of results 31
2.4 Problems, shortcomings, gaps, hiatuses, questions, topics of debate 32
2.4.1 Processes 32
2.4.2 The link between processes and lithofacies 34
2.4.3 Depositional models 35
2.4.4 Classification schemes for TBDS 36
2.5 Implications for subsurface reservoir characterisation 38
2.6 Possible solutions 38
Chapter 3 Thick-bedded deep-marine sandstones from Guipuzcoa,
northern Spain 41
3.1 Introduction 43
3.2 Geological setting 44
3.2.1 The Guipuzcoa Basin 44
3.2.2 Study area 46
3.3 Data description and analysis 48
3.3.1 Data set 48
3.3.3 Bed continuity - the Charlicun level 53
3.3.4 Bed continuity - the Mitxitxola level 54
3.4 Quantitative analysis 58
3.4.1 Methods 58
3.4.2 Charlicun level - Lobe sheets 59
3.4.3 Charlicun level - Isolated channels 62
3.4.4 Charlicun level - Coalesced channel sheets 64
3.4.5 Mitxitxola level - Coalesced channel sheets 66
3.5 Depositional model 66
3.6 Architectural hierarchy 69
3.7 Discussion 71
Chapter 4 Thick-bedded deep-marine sandstones, Marnes Bleues, Vocontian
Basin 73
4.1 Introduction 75
4.2.1 Geological setting 76
4.2.1 Basin formation 76
4.2.2 Thick-bedded sandstones of the Marnes Bleues Formation 77
4.2.3 The study area 78
4.3 The G5 sandstone in the Bourdeaux area 81
4.3.1 The eastern Bourdeaux outcrop 81
4.3.2 The western Bourdeaux outcrop 90
4.3.3 The G5 sandstone in the rest of the Bourdeaux area 93
4.3.4 The G5 sandstone - interpretation 95
4.3.5 Depositional model 98
4.4 The G3 sandstone in the Bourdeaux area 102
4.4.1 Introduction 102
4.4.2 The Le Poët-Célard outcrop 102
4.4.3 The G3 sandstone in the rest of the Bourdeaux area 103
4.4.4 Interpretation 103
4.5 The G1 sandstone in the Bourdeaux area 108
4.5.1 The Crupies exposure 108
4.5.2 The G1 sandstone in the rest of the Bourdeaux area 108
4.5.3 Interpretation 110
4.6 The G3 sandstone of the St-André de Rosans outcrop 111
4.6.1 Introduction 111
4.6.2 Outcrop description 112
4.6.3 G3 sandstone in the rest of the area 113
4.6.4 Interpretation 114
4.7 Marnes Bleues depositional model 114
sandstones: application to the Fram and Heimdal fields 121
5.1 Introduction 123
5.2 Data and methods 124
5.2.1 The Fram field 124
5.2.2 The Heimdal field 124
5.2.3 Methods 126
5.3 Geological setting 128
5.3.1 Geological setting of the Fram Field 128
5.3.2 Geological setting of the Heimdal Field 129
5.4 TBDS lithofacies in core 130
5.4.1 Description of TBDS lithofacies 130
5.4.2 Interpretation - depositional processes 135
5.5 Associated lithofacies 135
5.6 Vertical organization of lithofacies trends - lithofacies associations 138
5.6.1 Development stages 138
5.6.2 Lithofacies associations 140
5.6.3 Sequence stratigraphic significance of lithofacies associations 141
5.6.4 Summary 147
5.7 The Fram fiel 149
5.7.1 Seismic and biostratigraphic analysis 149
5.7.2 Sequence stratigraphic analysis 149
5.7.3 Reservoir zonation 149
5.7.4 Depositional model - hierarchy of reservoir elements 151
5.8 The Heimdal field 155
5.8.1 Seismic observations 155
5.8.2 Regional correlation 157
5.8.3 Field correlation 157
5.8.4 Depositional units 163
5.8.5 Depositional model - hierarchy of reservoir elements 167
Chapter 6 Archictecture modelling of TBDS deposits 169
6.1 Introduction 171
6.2 Sequence architecture parameters from the study areas 171
6.2.1 Basin setting 171
6.2.2 Dimensions and sand content 171
6.2.3 Hierarchy of sequence architecture 173
6.2.4 Stacking patterns 174
6.3 Deposition and sequence architecture of TBDS 178
6.3.1 Basin setting 178
6.3.2 Deposition of TBDS 179
6.3.3 Confinement 179
6.3.4 Progradation - aggradation - retrogradation 180
6.3.5 Stacking patterns 181
6.4 Static modelling of TBDS reservoirs 182
6.4.1 TBDS content 182
6.4.2 Overall versus local confinement - N/G trends 184
6.4.3 Flow units 185
6.5 Channels and sheets 187
Chapter 7 Summary and Conclusions 189
Published references 195
Unpublished references 200
Samenvatting (summary in Dutch) 201
Curriculum vitae 205
Appendix I: Core descriptions 207
Appendix II: Flow characteristics 216
Appendix III: Brushy Canyon Formation, Delaware Basin, West Texas 220 Appendix IV: Skoorsteenberg Formation, Karoo Basin, South Africa 227
The research presented in this thesis could not have been carried out without the support I re-ceived, in many different ways, from many different people and institutions. It is my pleasure to take this opportunity to thank these people and institutions here.
My first words of gratitude must go to Prof. Dr. Ir. K.J. Weber for his support, for sharing his ideas, and for encouraging me to finalize my thesis. Koen, you enthusiastically accepted the role of thesis supervisor and I took great pride in being your last PhD student. Unfortunately, the expiration of your “ius promovendi” came too early for me, but I am honoured to have you in my examination committee.
Prof. S.M. Luthi is gratefully acknowledged for being prepared to take over from Koen We-ber in the final stages of preparation of this thesis, for carefully reading the manuscript, and for giving valuable suggestions for improvement.
I am very much indebted to Dr. M.E. Donselaar, who deserves a special place in these ac-knowledgements. Rick, from our time in Utrecht onwards you have always had a lot of con-fidence in me and you took me on board at the GeoRes group after a dip in my professional career. You managed to find sponsorship for a series of exciting research projects, which took us to a variety of places from Scandinavia to Utah and the Mediterranean. I am grateful to you for being a skilful field geologist, a fine colleague and a friend. Additionally, many thanks for all the efforts you have put into the realization of this thesis project.
I also thank all the other colleagues and students I met at the Applied Geosciences group in Delft. You all contributed to making my time at the university a most enjoyable experience. This particularly holds for Cees Geel and Jan Kees Blom, two excellent colleagues and trav-el companions.
Norsk Hydro Research Centre in Bergen, Norway, is thanked for letting me participate in their Deep-Marine Systems research programme, and for supporting my turbidite projects. I very much enjoyed working with Tom Dreyer and his colleagues in the stimulating working environment of the Research Centre. I am also very grateful to Tom for taking a seat in the examination committee.
The other members of the examination committee, Prof. A Hurst, Prof. S. Flint, Prof. S.B. Kroo-nenberg and Prof. R.J. Arts are all thanked for showing interest in my research. Special thanks go to Steve Flint for valuable suggestions that have improved the final version of this thesis. Financial support for the research presented in this thesis was provided by Norsk Hydro Re-search Centre. The Pieter Langerhuizen Lambertuszoon Foundation of the Koninklijke Hol-landsche Maatschappij der Wetenschappen provided financial support for a data acquisition campaign in Basque Country.
Norsk Hydro Research Centre and Shell Exploration & Production Europe are gratefully ac-knowledged for their generous contribution to the printing costs.
I want to finish with expressing my very special gratitude to Judith, the most important per-son in my life. Judith, I wholeheartedly take the risk of using a cliché, but really: this would not have been accomplished without your continuous love, encouragement and support.
Chapter 1
1. Deep-marine sandstone reservoirs
Deep-marine sandstones and associated deposits have received overwhelming interest from the oil industry over the past two decades. The prime reason for this popularity lies in the fact that the deep-marine depositional setting hosts a significant proportion of the newly discov-ered hydrocarbon occurrences worldwide in the same period. Moreover, the interest will con-tinue for at least two more decades to come, as many future discoveries are expected to be found in similar depositional systems.
The economically viable exploitation of deep-marine reservoirs is hindered by two compli-cating circumstances. Firstly, the reservoirs often occur in deep-water regions. The techno-logical challenges for operating in such an environment are high, with consequently high associated costs. Secondly, a large part of the deep-marine reservoir rocks has been found to comprise thick-bedded, massive sandstone facies. Both the dominance of this facies type, and the inferred three-dimensional architecture of the reservoirs, are difficult to explain with ex-isting models for the deep-marine depositional environment.
These circumstances have left the oil industry with a strong need for new and adequate de-positional models for thick-bedded deep-marine sandstones. Such models should have pre-dictive capacity with respect to the locations where reservoir quality sand concentrations can be found, the size of these concentrations, and their expected reservoir behavior. The more adequate the depositional models, the more adequate the geological reservoir model, the more adequate the economical risk analysis, the more confident the conclusion whether or not high investment costs are justified by predicted recovery and earnings.
1.2 Geological reservoir models
Geological reservoir models are constructed by combining information from various sources, and at various scales:
1. Large-scale, general information on regional geology, tectonic setting, and basin configu-ration;
2. Intermediate scale information from seismics on the overall character of the depositional system, and the location of reservoir units. Depending on the type and quality of seismic data, more detail may be obtained on the internal architecture;
3. Small-scale information from wireline logs and cores, providing detailed information on rock type and fluid flow parameters.
The general methodology is outlined schematically in Fig. 1.1. The first step is to deter-mine the depositional environment of the reservoir interval. This is done through detailed sedimentological analysis of core and log material. Based on this analysis, a suitable depo-sitional model is selected from a library of previously established models. The depositio-nal model is combined with the information from seismics and possible other sources to form a geological model of the reservoir. Such a model is a conceptual representation of the internal architecture of the reservoir, in which flow units and permeability barriers
are defined. The geological reservoir model is used to define and test production scena-rios.
During the lifetime of a field, more and more data become available that allow for adjustment and fine-tuning of the reservoir model. This new information may come from newly drilled development wells, from pressure monitoring during well testing and production, from
Fig. 1.1 Flow chart for the geological modelling of clastic reservoirs (modified after Weber,
lapse seismic surveys (4-D seismics), or from evolving geological insights. Mature, or de-tailed, reservoir models are essential for the detection and recovery of remaining oil. The sketched methodology is by no means a straightforward task. It is a rather intuitive pro-cedure that requires the creative input of experienced modellers from different disciplines. Reservoirs comprising thick-bedded deep-marine sandstones provide the modellers with a suite of additional challenges, because:
1. The link between transport and depositional processes and resulting lithofacies is not completely understood;
2. The link between architectural elements (sedimentary body types) and constituent lithofa-cies types has not been fully established;
3. Architectural element types are very poorly defined with respect to their geometries, di-mensions, spatial relationships and stacking patterns;
4. Existing depositional models for deep-marine environments often cannot explain obser-vations from the subsurface.
5. The number of wells available for analysis tends to be small because of the high cost of deep-water drilling.
1.3 Thesis objectives
The need for improved depositional models for deep-marine systems in general, and for thick-bedded deep-marine sandstones in particular, has boosted the research efforts enor-mously. Academia and industry have bundled capacity and budgets to form research consor-tia that are fully dedicated to the subject. A wide variety of approaches and methods is followed, ranging from classical outcrop sedimentology and vessel-borne ocean floor sur-veys to numerical simulation of gravity currents. The combined efforts have resulted in a wealth of published data, theories, models and ideas. These new data have gradually led to a better understanding of deep-marine depositional systems. But too often, there appears to be a tendency to focus on the variability and complexity of such systems, instead of focusing on similarities and common characteristics.
Many of the older published outcrop studies are mainly descriptive and do not quantify the dimensions of sedimentary bodies in a three-dimensional sense. This hampers the practice of reservoir modelling, in which geological variation and complexity are to be narrowed down as far as possible, and quantified in hard numbers. A number of recent outcrop studies from extensive and very well-exposed areas have tried to bridge the gap. These will be described briefly and referenced where appropriate.
This thesis aims to contribute to the general understanding of thick-bedded deep-marine sandstone deposits, and to the construction of adequate reservoir models of thick-bedded deep-marine sandstones. This is attained by compiling and presenting new, quantitative data sets from two well-exposed outcrop examples, and by giving a detailed account of
observa-tions on core material from two thick-bedded deep-marine reservoirs. The basic data and ob-servations are on the architectural element scale. Working at this scale provides a means for comparison of individual deep-marine systems. The second, more theoretical objective is to propose concepts and guidelines that can aid in reservoir modelling. In this, it will be tried to narrow down supposedly complex matters to essentials that can be used to tackle practical problems in a time- and cost-efficient way.
General emphasis will be on the applicability of reported data and concepts at a production geological scale. This practical and quantitative approach forms part of the research philoso-phy of the Applied Earth Sciences department at Delft University of Technology. It narrows down the scope of the presented research, and also provides a rationale for keeping away from some of the controversial theories about details of the depositional processes. These the-ories are interesting enough, but do not directly influence features such as sand body geome-try or reservoir architecture.
1.4 Thesis outline
Chapter 2 elaborates on the objectives of this thesis. Definitions are given, and concepts re-lated to thick-bedded deep-marine sandstones explained. The chapter presents an overview of the common research methods and of the problems associated with combining results from different methods. Possible ways to overcome these problems are suggested. One of these is the architectural element approach that is followed in this thesis.
Chapter 3 is a study of well-exposed thick-bedded deep-marine sandstones in Guipuzcoa, northern Spain. The degree of exposure in the study area is such that quantitative data could be acquired of various types of architectural elements, and their stacking patterns determined. Such data can be used to construct 3-D geological (reservoir) models. This is illustrated by a stochastic computer model of the study area.
Chapter 4 describes observations on thick-bedded deep-marine sandstones from the Vocon-tian Basin, southeastern France. Quantitative data on these deposits could not be acquired because of the poor degree of exposure. Nevertheless, they provide insight into the stacking patterns and internal architecture of sand-rich deposits in a deep-marine depositional system. Chapter 5 is concerned with the reservoir architecture of TBDS deposits from the Fram field and the Heimdal field in the Norwegian part of the North Sea. Fram has reservoir rocks of Oxfordian age, Heimdal of Eocene age. Both are dominated by thick-bedded deep-marine sandstone. The chapter illustrates methods to subdivide very massive, sand-rich and thick-bedded sequences into their constituent architectural elements. Detailed sedimentolog-ical
descriptions of core are with data from other sources (seismics, wireline logs, biostrat). The Fram field presents an example of how such information can be used to construct a geologi-cal model in an early appraisal/development stage. The Heimdal field is at a mature develop-ment stage.
compared and discussed. This gives rise to the formulation of guidelines for the modelling of TBDS reservoirs.
Finally, in Chapter 7, generic statements are made with respect to the deposition and se-quence architecture of TBDS, and the main observations and interpretations from the study areas summarized.
1.5 Data and methods
The acquisition and interpretation of field data in the Guipuzcoa and Vocontian Basin outcrop areas has been carried out by the author, with assistance from graduate and PhD students. This work included sedimentological description and characterization, interpretation of pho-to panels, tracing of sedimentary units in outcrop and on areal phopho-tographs, and measurement of the dimensions of sedimentary units. These outcrop data sets were combined with pub-lished information on the structural and stratigraphic setting of the study areas to come to the conclusions and interpretations presented in this thesis.
The subsurface studies are centred around detailed description and interpretation of core ma-terial and wireline data by the author. Additional information on seismic interpretation, re-gional stratigraphic schemes, detailed palynological analysis and well test data have been provided by Norsk Hydro Research Centre. The integration of core data and additional infor-mation into depositional and sequence architecture models have been carried out by the au-thor. They do not necessarily represent the views of Norsk Hydro.
Chapter 2
2.1 Introduction
In section 1.3 it was mentioned already that the research efforts on the subject of deep-marine depositional systems are worldwide and extensive. This is expressed in:
● The foundation of a large number of research consortia in which industry and academia have bundled their budgets and research capacity, respectively;
● The organisation of research conferences that are specially dedicated to the subject; ● The appearance of all kinds of special publications and other thematic issues/editions; ● The publication of a continuous stream of new papers on the subject, in a variety of
pro-fessional and scientific magazines.
An important related aspect is that oil companies seem very much willing to share their re-search and data bases with the earth science community, probably more than they used to. This has definitely been catalytic to the achievements so far, and to their worldwide commu-nication. A major advantage of these developments is that a continuous stream of inspiring data and ideas becomes available for evaluation. For a thesis like the current one, it also im-plies a major disadvantage. Because the developments are in full swing, any attempt to com-pile, in retrospect, a comprehensive overview of thick-bedded deep-marine sandstones, and of the applicability of published models for reservoir development purposes, is deemed to be premature. Too many questions are still open, too many aspects are being discussed, to be able to present a definite synthesis.
Consequently, this chapter takes the form of an instantaneous cross-section through the pres-ent stage of knowledge. A number of aspects that relate to the deposition of thick-bedded deep-marine sandstones will be discussed. It is not intended to present a complete historical overview of the achievements that have been made so far. This has been done in an excellent way by other workers already (eg. Shanmugam, 2000; Stow et al., 1996; Mutti, 1996), and the reader is assumed to be acquainted with the state-of-the-art. Only where required for a full understanding of the discussions, some theory will be given. But the emphasis will be on identifying major weaknesses and hiatuses in our knowledge, and on providing an overview of what other students of deep-marine deposits have identified as problems in the under-standing of thick-bedded deep-marine systems.
This chapter starts with a definition of what is meant by thick-bedded deep-marine sand-stones in this thesis. This will be followed by a short description of the variety of research methods that are in use, each with its own advantages and limitations. Subsequently, an overview is given of the problems and shortcomings that still exist in our understanding, and of suggested strategies to solve these problems. The identification of problems and the sug-gested guidelines for further research will set the scene for the various studies on thick-bed-ded deep-marine deposits presented in Chapters 3 to 5.
2.2 Definition
Thick-bedded deep-marine sandstones, or TBDS from now on, are defined here as very thick sandstone beds, usually without primary sedimentary structures, that occur in association with other deep-marine sediments such as Bouma-type turbidites and hemipelagites. This definition is adapted from the definition of deep-water massive sands (or DWMS in acronym) by Stow & Johansson (2000). Their terminology puts emphasis on the massive na-ture of the beds under consideration, with the word ‘massive’ essentially meaning ‘devoid of sedimentary structures’. The here-proposed TBDS term rather puts emphasis on the thickness of the beds. This is preferred since many TBDS do exhibit sedimentary structures upon close examination. These structures may be primary, very subtle grain size and sorting variations. Still, they can be recognized, so the term massive seems to be less appropriate. This holds even more for TBDS with obvious structures, of which examples will be described in this the-sis.
‘Very thick’ refers to a generally accepted bed thickness of 1 m or more. The definition is not limited by a maximum thickness. However, it is a common observation that the sandstones occur in units of much greater thickness, locally up to 50 m or more, but that a large part of these units is composed of thinner TBDS beds ranging in thickness from 1 to 3 m.
‘Deep-water’ refers to waters deeper than storm wave base, usually to the bathyal water depth environment of 200 m and deeper.
The addition ‘in association with . . . and hemipelagites’ in the above definition serves to dis-tinguish TBDS from, for example, sandy shelf deposits. It also infers that TBDS are not Bouma-type turbidites. That specific term is usually reserved for deposits resulting from rel-atively low-density turbidity currents, with a mixed sand-mud sediment load. These can be easily identified by the characteristic sequence of sedimentary structures described by Bouma (1962). The formation of TBDS is not completely understood, and is still under de-bate. Several types of processes have been proposed, that differ from the low-density turbid-ity current concept. Depositional processes will be briefly touched upon in the following.
2.3 Research methods
A variety of methods and data sources is in use for the study of deep-marine depositional sys-tems. Each of these approaches provides specific keys for improved understanding of TBDS. The following overview summarizes the type of data and the limitations associated with the various methods.
2.3.1 Outcrop studies
Outcrop studies in general can provide sedimentological data at various scales (Fig. 2.1). At the bed scale, information can be acquired on grain size distributions, sedimentary structures, the nature of bed boundaries, and so on. Examples include Schuppers (1995), Dreyer et al.
(1993) and many others. On a larger scale, vertical sequences can be studied, as well as lat-eral changes in sedimentological characteristics and architectural aspects of sedimentary bodies. The vertical aspect is important, because it witnesses the changing depositional con-ditions through time. It is also possible to measure petrophysical parameters, such as spectral gamma radiation and permeability, with hand-held devices (Hartkamp-Bakker & Donselaar, 1993).
Another major asset from the study of outcrops is that it sets the mind for the possibilities and impossibilities regarding architectural aspects of depositional systems at all scales. This is es-sential for interpreting depositional architecture from indirect observations such as seismics (see below).
Limitations are mainly associated with the degree of exposure. The presence of vegetation commonly biases our observations towards the sandy units, whilst the covered silty/shaly de-posits also contain valuable information. This has been demonstrated in the Permian Tanqua-Karoo basin floor deposits studied in the NOMAD project (Hodgson et al., in press). Weathering of the sandstones may obscure subtle grain size trends and sedimentary struc-tures, but may also accentuate features that would otherwise have remained undetected. The
main limitation, however, lies in the fact that outcrops are two-dimensional and of limited ex-tension. The limited extension means that only features smaller than the dimensions of the outcrop can be interpreted with certainty. The dimensional nature implies that only two-dimensional sections through three-two-dimensional geological features can be observed. Three-dimensional outcrops do not exist. ‘Good 3-D exposure’ in an outcrop area refers to the presence of at least two two-dimensional exposures with different orientations. Only when the spacing between individual exposures is smaller than the dimensions of the architectural elements of a depositional system, and the total outcrop area is larger than those elements, can a good 3-D picture of the elements be obtained.
Some of the world’s famous TBDS outcrop areas are sufficiently large and sufficiently well-exposed to circumvent the above limitations. Examples are the Tanqua-Karoo Basin in South Africa (described in recent publications by Deville Wickens & Bouma, 2000, and Johnson et
al., 2001), and the Delaware Basin in West Texas (described by Gardner & Borer, 2000, and
Carr & Gardner, 2000). The results of these studies are summarized in Appendices III and IV, to present reference material for the TBDS studies in this thesis.
2.3.2 Core studies
Slabbed core is highly suitable for the detailed investigation of sedimentological features in a continuous (sub-)vertical sequence (e.g. Lowe & Guy, 2000; Haughton et al., 2003; Shan-mugam et al., 1994). Weathering is not a problem thanks to the fresh cut, and the cored se-quence is evenly well-accessible. This contrasts with some steep field exposures in which only the lowermost few metres are easily accessible. Similar to outcrop studies, the vertical aspect of the data marks the changing depositional conditions through time, and thus marks the dynamics of the depositional system. Cores are the starting point for reservoir cha-racterization (Fig. 1.1) and serve to calibrate information from other sources. Wireline log responses can be calibrated for lithofacies type, which allows for reliable lithofacies inter-pretation in wells that have not been cored. Petrophysical measurements on core samples allow for the accurate determination of fluid flow parameters from log responses. Sometimes, even seismic reflections can be linked to specific lithological boundaries. Some scales of ob-servation in core studies are illustrated in Fig. 2.2.
The limitations of core studies obviously lie in the one-dimensional nature of the material. It is almost impossible to infer dimensional information from core. Even the exact nature of bed boundaries, be they erosional or non-erosional or loaded, may be difficult to discern, because only a very small part of such surfaces is exposed in core. A practical problem with cored se-quences of TBDS is that they sometimes are poorly consolidated. This may hamper the ob-servation of sedimentary features, although freezing and on-site resination techniques may be quite effective.
2.3.3 Modern systems
Studies on modern deep-marine systems have provided important information on the plan view geometry and dimensions of the constituent elements (Habgood et al., 2003, Fig. 2.3A; Liu & Bryant, 2000; Piper et al., 1999). Two main limitations to this source of information
should be stressed. First, the results are two-dimensional with only limited info on the thick-nesses of elements in the depth dimension. The obtained plan view geometries represent the architecture of the studied system at present, i.e. at one specific point in time. The situation at present is characterized by relatively high sea level, relatively low sediment supply rates, and relatively low fan activity. Most ancient systems were deposited during periods of sea-level lowstand.
The second limitation is that usually only scarce information on sediment composition and sedimentary structures is obtained. This hampers establishing the link between the geometry of morphological elements and their constituent lithofacies. Methods to overcome these lim-itations include shallow seismics (Fig. 2.3B) and the acquisition of shallow cores (Fig. 2.3C).
2.3.4 Seismic surveys
Seismic surveys are well-suited for outlining the dimensions of deep-marine depositional systems, for defining proximal-distal-lateral relationships, and for determining large-scale architecture (Weber, 1993; various deep-water examples in Weimer & Davis, 1996). The de-tailed interpretation of seismic data was greatly enhanced by the development of seismic se-quence stratigraphy concepts. These concepts link the stratal patterns of seismic reflectors with specific stages in the development of depositional systems, and thus allow for determin-ing the 3-D architecture. New developments in the acquisition, processdetermin-ing and analysis of seismic data are going fast. Time-lapse seismics, multicomponent data, attribute mapping, seismic inversion and AVO-analysis are but a few examples of these developments, with highly promising applications (Fig. 2.4).
The limitations of seismic surveys are ruled by the resolution of the methodology. Under ide-al conditions, features with a thickness of around 20 m can be detected, but these conditions are hardly ever met. This means that details on lithology, bed boundaries and stacking pat-terns cannot be resolved from seismic data. A potential danger lies in the careless application
Fig
. 2.3
Shallo
w surv
ey
methods for studying recent deep-marine systems:
A.
Sidescan sonar mosaic (upper) and interpretation (lo
wer) from the Gulf of Cadiz (Habgood
et al ., 2003); B. Shallo w seismic surv ey of the Zaire F an (Droz et al ., 2003); C.
Piston core of massi
v
e gra
v
elly sand layer with silty top, scale in cm subbottom (Hesse
et al.
Fig
. 2.4
Seismic
meth-ods for impro
v ed visuali-sation of subsurf ace systems. A. T otal ener gy attrib ute at 400 m depth sho
w-ing meanderw-ing chan- nel course (Sv
anes et al. , 2004); B. Seismic in v ersion (a:
seismic data, b: block
ed acoustic
im-pedance model, c: synthetic seismogram from block
ed
imped-ance model (Chapin et al.
, 1996);
C.
Synthetic seismics (a: architecture model coded by interv
al v
e-locity
, b: synthetic
seismic traces gener- ated by the model, c: actual seismic traces (Chapin
et al.,
of sequence stratigraphic concepts, which may lead to forced, model-driven interpreta-tions.
2.3.5 Flume studies
Flume experiments are well-suited for the study of transport and depositional processes (Simpson, 1997; Lamb et al., 2004). These processes cannot be observed in nature because of their catastrophic and unpredictable nature. Observation under controlled laboratory con-ditions allows for studying the influence of individual factors on both the behaviour of the mass flow and on the resulting deposit (Fig. 2.5). Such factors may be the gradient of the transport path, the composition of the sediment carried by the mass flow, the morphology of the receiving basin, and so on.
The main limitations are due to the fact that the downscaling of high-density flows to exper-imental conditions is problematical. The volume of the generated mass flows and the
dimen-Fig 2.5 Experimental setup (A) and lacquer peel with interpreted line drawing (B) of flume
studies by Baas et al., 2004. The line drawing shows a cross-section through a channel-levee system, with the central channel in red and levees in yellow.
sions of the resulting deposits are restricted, as is the time span under consideration. There are practical constraints to maintaining a mass flow with a fair sediment load for a longer pe-riod of time. This means that differences between short pulses of sediment supply and more sustained mass flow currents cannot be simulated in a laboratory setting.
2.3.6 Numerical modelling
Tries to capture the fluid dynamics and the mass balance of mass flow processes in mathe-matical equations and boundary conditions. Computer simulations in which these equations are solved as a function of, for example, sediment concentration or slope gradient, provide insight in the behaviour of the subject depositional system (e.g., Garcia, 1993; Kneller & Buckee, 2000). The main advantage is that the effect of individual factors on transport and deposition can be analysed through experiments that are very difficult to realize in flumes.
Limitations are due to the complexity of the mathematical/physical basics behind mass flow processes. Full appreciation of this complexity requires demanding programming efforts and enormous computer power. In practice, this leads to proxies and concessions such as the depth-averaging of flow parameters or reduced dimensionality, and consequently to less re-alistic experiments.
2.3.7 Comparison of results
The listing of methodologies shows that no single approach can cover the full range of scales and aspects that is required for a full understanding. The common factor they share is their suitability for the study of TBDS. However, it is more important to be aware of the differ-ences, which are related to the following aspects:
● The scale of the observation, and the associated resolution;
● The method of observation, i.e. direct through visual observations on sediment/rock, or in-direct through measurement of physical parameters;
● The nature of the observed parameters;
● Dimensionality of the observations, e.g. 1-D wireline logs versus 3-D seismics.
Research groups are using the whole range of methodologies, and the complete range of scales and parameters is covered (see Table 2.1). This might imply that a synthesis of all the results of these research efforts should ultimately lead to an in-depth and detailed level of knowledge. However, it is generally felt that this level has not been achieved to date. Main reasons are the varying scales of observation and the varying nature of the measured param-eters, which hampers the meaningful integration of results. But research methods and ap-proaches are more and more combined to overcome these problems.
2.4 Problems, shortcomings, gaps, hiatuses, questions, topics of debate
The statement in section 2.3.7 that many questions are still open in our understanding of TBDS systems can also be found, in different formulations, in recent review articles on the subject (e.g., Weimer et al., 2000; Mutti et al., 2003; Bouma, 2000; Shanmugam, 2000). From these articles, and from own experience in working with TBDS, a list emerges of chal-lenges in the interpretation of TBDS. They relate to:
● The processes of transport and deposition of TBDS;
● The link between processes and resulting sedimentary features; ● Adequate depositional models for TBDS;
● Classification scheme for depositional systems with TBDS; ● Criteria for comparison of deep-marine systems.
These challenges have to be dealt with when outcrop-derived concepts and core descriptions are used for understanding subsurface geology. The associated problems and open questions are outlined in the following.
2.4.1 Processes
The two most frequently cited processes capable of transporting and depositing TBDS are de-bris flows and turbidity currents (e.g., Shanmugam et al., 1995; Hiscott et al., 1997). The es-sential differences between the two are in the type of flow and in the mechanism supporting the sediment in the flow (Fig. 2.6). In plastic debris flows, the cohesive yield strength of the transporting flow is capable of carrying coarser sediment. With non-plastic turbidity currents, it is the upward component of the turbulent eddies that keeps the sediment into suspension. Turbulent flows are often subdivided into high-density and low-density turbidity currents.
Table 2.1: General characteristics of the main research methods
research method scale of dimension of type of observations
observations (m) observations
outcrop studies vertical: 10-1- 103 2-D * detailed sedimentology lateral : 100- 105 (pseudo 3-D) * architectural information
core studies vertical: 10-3- 102 1-D * detailed sedimentology
lateral: 10-1 * petrophysical data
seismic surveys vertical: 101- 103 3-D * basin configuration
lateral: 103- 105 * large-scale architecture
* hydrocarbon indicators
modern systems vertical: 100- 102 2-D * plan view geometry
lateral: 103- 104 * shallow lithology
flume experiments vertical: 10-3- 101 1-D * fluid flow parameters lateral: 10-2- 101 (2-D, 3-D) * erosion/sedimentation patterns
numerical modelling any scale 1-D * fluid flow parameters
Low-density turbidity currents transport relatively low concentrations of fine-grained sedi-ment, leading to deposition of the so-called “classical Bouma-type turbidites”. High-density turbidity currents are capable of transporting higher concentrations of coarser grade sedi-ment, resulting in TBDS deposition.
Other sediment support mechanisms have been described that can be locally and temporarily active in combination with fluid turbulence and fluid yield strength. In grain flows, or sandy debris flows, it is the dispersive pressure resulting from the bouncing of grains (Bagnold, 1954). In liquified and fluidised flow, sediment support results from excessive pore pressure or from the upward flow of escaping pore fluid (Fig. 2.6).
It is broadly accepted that the transport and deposition of TBDS do not necessarily result from one single mechanism, but rather from a combination of several processes and mecha-nisms. This is because of the following reasons:
● A distinction should be made between the processes transporting large amounts of sedi-ment from the source area to the deep-sea, and the processes responsible for precipitation from the transporting flow;
● Gravity flows are likely to undergo changes as a result of changing flow conditions during their lifetime, e.g. changes in slope gradient or sea floor sediment type. These changes may force debris flows to dilute and transform into turbulent flows, or any other flow transfor-mation;
● It is probable that a continuum of processes exists, with debris flows and turbulent flows being amongst the suite of potential end members.
Fig. 2.6 Types of gravity flows as suggested by Lowe (1979), based on flow rheology and
● Flow transformation may occur in one and the same gravity-driven current. This has been described for slurry flows (Lowe & Guy, 2000), which may show periodic concentration of sand-sized clay clasts into vertically separated layers. The disintegration of the clay clasts leads to plastic flow behaviour of that specific layer, while the rest of the current be-haves as a turbulent flow;
● Different flow behaviour may occur at different locations within the flow. Such locations are the head, the main body, the dilute upper boundaries, or the basal layer with highest sediment concentration and in closest contact with the sea floor. The former three areas may be dominated by turbulent flow. The basal high-concentration sand-rich layer, also re-ferred to in high-density turbidity currents as traction carpet, probably uses other sediment support mechanisms in combination, such as dispersive pressure and fluidisation.
The described processes and mechanisms are mainly based on theoretical concepts. The va-lidity of these concepts has been demonstrated with flume studies and numerical modelling. However, the inherent restricted spatial and/or time scales do not allow for direct application of experimental results to the larger scales of a depositional basin. Direct observation in na-ture is of course hampered by their unpredictable occurrence and catastrophic nana-ture. These practical limitations explain why much confusion still exists about which (combination of) processes and mechanisms can lead to the deposition of TBDS, and on which temporal and spatial scales they can be active.
2.4.2 The link between processes and lithofacies
Another problem in assessing the importance of transport and depositional processes and sediment support mechanisms is the link with the resulting deposits. In an ideal case, each individual depositional mechanism would leave a distinct fingerprint in the resulting deposit, preferably in the form of a specific combination of bed characteristics, sedimentary structures and textural features (Fig. 2.7). Unfortunately, this ideal situation does not seem to occur in nature. Many diagnostic criteria have been proposed by different workers. Examples are clay percentage, inverse grading, the presence of large floating clay clasts, the absence of sedi-mentary structures, and so on. It is often seen that, once a certain phenomenon has been pro-posed as typical for let’s say debris flow deposits, this is followed by a series of reactions explaining the occurrence of the same phenomenon in turbidity current deposits. Conse-quently, a generally accepted set of diagnostic criteria does not exist. There is no dispute as to the recognition of thin-bedded Bouma-type turbidites, or of typical debris flow deposits such as muddy conglomerates. Problems rise with the thicker beds that solely consist of sand-grade sediment, or, in other words, with TBDS.
Agreement does exist on the lithofacies types that can be distinguished in TBDS and associ-ated deposits. Examples of lithofacies schemes are published in Mutti (1985), Ghibaudo (1992), Pickering et al. (1986), and more recently in Stow & Johansson (2000). These schemes present a broadly comparable suite of lithofacies. It may be clear from the above that less agreement exists on the processes responsible for deposition of these lithofacies types.
2.4.3 Depositional models
Many published depositional models for deep-marine systems are based on outcrop studies
(e.g., Mutti and Ricci Lucchi, 1972), on modern systems (Normark, 1970), or a combination
(Walker, 1978). The limitations associated with outcrop studies have been discussed in sec-tion 2.3.1. Many of these outcrop areas are located in rather small and elongate basins in thrust-fold belts. This basin type is but one example of the total range of dimensions, shapes and tectonic settings in which deep-marine depositional systems can develop. This means that the application of these depositional models as generally valid passepartouts should be carried out with caution.
Depositional models based on observations from modern systems also have their limitations. Such systems tend to be clay-rich and dominated by channel-levee complexes. Derived mod-els are difficult to apply to ancient sequences because of the limitations mentioned in section 2.3.3.
Depositional models based on sequence stratigraphic concepts (Posamentier et al., 1988; Vail and Sangree, 1988) also have limitations. Such models assign an important role to eustatic
Fig. 2.7 Typical
de-posits from the flow types of Fig. 2.6. Ar-rows indicate that gradual transitions between lithofacies types occur (after Lowe, 1982).
sea level variations in controlling the architecture of deep-marine systems. Overestimating this role, and neglecting the influence of, for example, autocyclic variation, tectonic activity and climatic variations, can also lead to misinterpretations.
2.4.4 Classification schemes for TBDS
One way to bring order in the observed variability in deep-marine depositional systems is to develop a suitable classification scheme, based on external characteristics such as shape, dimensions, lithological composition, stacking pattern of architectural elements, and so on. Such a scheme can only be adequate when all the controlling factors are known. This is where existing classification schemes appear to fail. They all use a limited number of classification criteria, which sometimes are poorly defined. Moreover, the amount of in-formation required for the correct classification of a deep-marine depositional system is often unrealistically high. The following example may serve to illustrate these classification problems:
The popular classification scheme of Reading & Richards (1994) uses volume and grain size of the available sediment, and nature of the feeder system as distinguishing criteria (Fig. 2.8 for rich systems). Deep-marine depositional systems are either conglomeratic, sand-rich, mixed sand-mud, or mud-rich. Their feeder systems can be described as either a slope apron linear source, a multiple source ramp or a submarine fan point source. Together, the classification criteria are thought to define diverse system characteristics such as slope gradi-ent, facies distribution, flow frequency, feeder channel stability, vertical organisation, down-current length/width ratio, etcetera.
One difficulty with applying this classification scheme to TBDS systems is that the term sand-rich is not easy to define. The term can refer to the depositional system as a whole, but is also used for certain architectural elements within more clay-rich systems. Another problem relates to the factor time. The scheme describes the areal distribution of morpho-logical features at one specific instant in time. Subsurface reservoirs or outcrop examples re-sult from a prolonged period of time, during which the volume and calibre of the sediment and the nature of the feeder system may well have changed significantly. This is expected to lead to combinations of different deep-marine system types in one and the same deposition-al system.
Other classification schemes have been published, based on different criteria. Examples are the distinction between coarse-grained, sand-rich and fine-grained, mud-rich systems (Bouma, 2000), between high-efficiency and low-efficiency systems (Mutti, 1985), or be-tween slope fans and basin floor fans (e.g. Mitchum et al., 1993). Each of these systems may contain TBDS beds. The classification schemes all share the recognition that the composition of the sediment is an important control on the architecture of deep-marine depositional sys-tems.
Fig. 2.8 Block diagrams illustrating the range of sand-rich deep-marine systems: (a) aprons,
2.5 Implications for subsurface reservoir characterisation
In the foregoing, a general inventory was presented of the problems related to the interpreta-tion and classificainterpreta-tion of TBDS deposiinterpreta-tional systems. This thesis specifically aims to con-tribute to the construction of reservoir geological models of subsurface TBDS systems. The general methodology for subsurface reservoir characterisation was outlined in section 1.2. Available data usually comprise background information on regional geology, 3-D seis-mic surveys, wireline logs, core material and well test results.
The regional geology provides general information on the tectonic setting of the basin under consideration, the location of the basin relative to potential source areas, and so on. Seismic surveys give insight in the basin configuration, sediment transport fairways, depositional pat-terns, the nature of potential hydrocarbon traps, etcetera. Under favourable conditions it fur-ther allows for the recognition of sand-rich intervals, and provides the depositional context for the interpretation of reservoir units. Well data should lead to detailed interpretation of the depositional environment, and the changes through time as expressed in the vertical develop-ment of reservoir units. Core material is further analysed to establish a stratigraphic zonation. Such a zonation can be biostratigraphical, but may also be based on chemical element analy-sis.
Apart from establishing the depositional environment in as much detail as possible, the above information also needs to be analysed to obtain reservoir geological features such as: ● Time lines to identify correlatable units;
● The lateral extension of reservoir units;
● The stacking pattern of reservoir units, and the nature of their boundaries; ● The fluid flow parameters of the units;
● Trapping conditions and reservoir interconnectivity.
2.6 Possible solutions
Several approaches can be followed to arrive at a better and completer understanding of deep-marine depositional systems.
Ideally, the focus should be on very large-scale and very well-exposed outcrop areas, where the detailed architecture of TBDS deposits can be revealed from direct observations (e.g., Johnson et al., 2001; Gardner et al., 2003). But only a limited number of these areas exist, which represent only part of the whole spectrum of geological settings where TBDS can be formed. Hence, additional sources of information have to be tapped.
Another approach is to look into the various research methods and data sources to try to im-prove the comparability of acquired data. Examples are 1) the collection of (deep) cores in modern systems in order to lithologically calibrate the results of shallow seismic surveys (e.g., Goldfinger et al., 2003), 2) drilling and logging ‘behind-the-outcrop’ in order to gain in-sight in the subsurface expression of exposed features (Donselaar, in press; Hodgson et al., in press), or 3) forward modelling of the seismic response of sedimentary bodies in order to
im-prove the detection of such bodies from seismic surveys (Badescu, 2002; Coleman et al., 2000). Many efforts along this line are currently being undertaken.
A more theoretical approach is to look into the classification of deep-marine systems. A good classification scheme should appreciate the observed variability in terms of tectonic setting, basin shape and dimensions, lithological composition, and so on. It should also provide in-sight into the internal architecture of the systems, and on the behaviour of systems when con-trolling factors change over time. Finally, such a scheme should give criteria for establishing the differences and the similarities between individual systems.
The approach in this thesis is to study examples of TBDS from the fossil rock record, both in outcrop and in core material, with focus on their three-dimensional (3-D) architecture. The term 3-D architecture here refers to the spatial distribution of sand bodies in a deep-marine depositional setting, in relation to its finer-grained background sediments. The building blocks of 3-D architecture are architectural elements. Architectural elements are defined here as groups of beds that are genetically related, and that can be distinguished from neighbour-ing elements or background sediment by its geometry, dimensions and/or lithofacies distri-bution. A depositional system can be fully described by quantitatively characterizing the constituent architectural elements and their stacking patterns. The studies in this thesis ex-emplify how and what type of data can be extracted from TBDS rocks to identify and char-acterize the architectural elements.
This so-called architectural element approach is also recommended by other workers (e.g. Stow & Mayall, 2000; Clark & Pickering, 1996; Weimer et al., 2000; Piper and Normark, 2001). It has a number of clear advantages:
● The scale of architectural elements coincides with the scale of reservoir flow units; ● This scale fits into the observation window of most of the previously described research
methods;
● It goes beyond the scale of individual beds, which allows for bypassing of problems asso-ciated with depositional processes.
Chapter 3
Thick-bedded deep-marine sandstones
from Guipuzcoa, Northern Spain
3.1 Introduction
The coastal area around San Sebastian in the province of Guipuzcoa, Basque Country, Spain (Fig. 3.1), offers excellent opportunity for the study of geometrical aspects of thick-bedded deep-marine sandstones. A thick Lower Eocene turbidite sequence crops out, with massive sandstone intervals of tens of metres thick. No detailed sedimentological-architectural ac-count of these sandstones has been published before. Earlier work in the area (e.g., van Vli-et, 1982; Rosell et al. 1985; Pujalte et al. 1993; Winkler & Gawenda, 1999) was mainly concerned with general stratigraphic aspects of the complete Palaeogene turbidite sequence in the basin.
This outcrop study focuses on the uppermost part of the deep-marine sequence, which com-prises an alternation of thick-bedded sandstones and background mudstones and siltstones with occasional thin-bedded turbidites. The objective is to determine the three-dimensional architecture of the thick-bedded sandstones in the study area, and to characterize the geome-try of its building blocks in a quantitative way. This is achieved through the interpretation of sedimentological observations, tracing of correlation horizons in the field, and the interpreta-tion of photo panels and aerial photography.
This chapter starts with a short introduction to the geological setting of the Guipuzcoa Basin
and the fieldwork area. It further describes the types of acquired data, and how the data were processed to obtain dimensions of architectural elements. The spatial and depositional rela-tionships between the various architectural element types are explained with a depositional model. Finally, it will be discussed what the merits and limitations of the data set are, and how depositional architecture is organized along a system of hierarchical levels.
3.2 Geological setting
3.2.1 The Guipuzcoa Basin
The Tertiary Guipuzcoa turbidite basin (Rosell et al. 1985) is situated in the western part of the Pyrenean chain (Fig. 3.1). The basin originated from rifting in the Bay of Biscay during the Early Cretaceous. Later stage collision between the Iberian and the European plate re-sulted in a transition from an extensional to a compressional stress regime. Due to the oblique direction of collision, the compression occurred diachronously along the Pyrenean chain. The eastern part of the Pyrenees was subjected to compression from the Late Cretaceous onwards. The western part was incorporated in the Pyrenean thrust and fold belt foreland basin from the earliest Eocene (Puigdefabregas & Souquet, 1986). The early stage (Cretaceous to Palaeocene) fill of the Guipuzcoa basin was dominated by pelagic limestones and thin-bed-ded calcareous turbidites derived from carbonate platforms surrounding the deeper basin. The calcareous sediments have been dated by Winkler & Gawenda (1999; Fig. 3.2) to belong to nannofossil zones NP1 to NP9 (as defined by Martini, 1971, and Okada & Bukry, 1980). The thickness of this interval is approximately 150 m (Winkler & Gawenda, 1999). The Eocene fill is associated with an increase in subsidence in the basinal parts, accompanied by the development of large-scale siliciclastic turbidite systems (Fig. 3.2). These derived their sediment from the incipient Pyrenean mountain chain. The lowermost part of the Eocene (nannofossil zones NP10 and NP11) shows a gradual transition from dominantly carbonate to dominantly siliciclastic sedimentation. From zone NP12 onwards, the sequence is domi-nated by siliciclastic turbidites and interbedded (hemi-) pelagics. The total thickness of the Guipuzcoa turbidite sequence is 2500 m (eg. Winkler & Gawenda, 1999).
Siliciclastic turbidite sedimentation in the narrow, E-W trending basin took place in three main depositional sequences. These were named by Rosell et al. (1985) the Sarikola se-quence, the Hondarribia sese-quence, and the Jaizkibel sequence (Fig. 3.3). These sequences have been interpreted by Rosell et al. (1985) to result from rapid sea level drops during which siliciclastic shelf sediment was transported into the deeper parts of the basin. Tectonic over-print on eustasy influenced sediment supply, but also the configuration of the depositional se-quences (Pujalte et al. 1993). The sese-quences are bounded by Type 1 sequence boundaries (sensu Vail, 1987).
Each sequence comprises an alternation of thin-bedded Bouma-type turbidite intervals and TBDS intervals. This alternation probably represents a 4th-order cyclicity. The thin-bedded
intervals have been interpreted by Van Vliet (1982) as fan fringe and basin plain deposits. The thick-bedded intervals represent proximal lobe deposits and lobe-channel transitions (Van
Fig 3.2 Palaeocene to early Eocene fill of the Guipuzcoa Basin (after Winkler & Gawenda,
Vliet, 1982; Rosell et al.1985). Palaeocurrent directions measured in the outcrop area indi-cate a general northern source area for the sequences. From east to west, the thick and mas-sive sandstone units gradually become thinner-bedded. This reflects a general westward proximal-to-distal trend.
3.2.2 Study area
The study area is located east of San Sebastian, between Pasajes de San Juan and Hondarrib-ia (Fig 3.3). An alternation of TBDS intervals and thin-bedded turbidite intervals crops out in a 1 - 2 km wide zone directly bordering the shoreline, with a length of approximately 15 km. The exposed rocks belong to the Jaizkibel sequence of Rosell et al., 1985 (Fig. 3.3). This se-quence contains at least seven thick-bedded sand-rich intervals. The thickness of the exposed sequence is approximately 700 m. Nannofossil dating (Winkler & Gawenda, 1999) shows the sequence to belong to nannofossil zone NP14.
Structurally, the study area consists of a single northwest-dipping monocline. The monocline has a gentle curvature, leading to steep NW-directed dip slopes in the southwestern part of the area, and gentler WNW-directed dip slopes in the northeast (Fig. 3.4). This relatively low de-gree of tectonic deformation was a major criterion for selecting the working area.
The thin-bedded intervals are strongly vegetated. They can only be studied in detail in a few cliff faces. The thick-bedded intervals tend to be relatively free of vegetation. They form marked features in the landscape, oriented parallel to the shoreline, and can further be stud-ied in a number of small valleys oriented perpendicular to the shoreline. These outcrop con-ditions allow for a semi-3D examination of the architecture of the sandstone intervals. This study focuses on the two best-exposed sandstone intervals at the top of the exposed
Jaizkibel sequence. They are referred to as the Mitxitxola Level and the Charlicun Level, af-ter the names of two localities in the area. The Mitxitxola Level is the youngest of the two. Figure 3.4 schematically shows the outcrop patterns of the two intervals. Limited observa-tions from other levels will be used where required to complete the depositional model. The Mitxitxola and Charlicun Levels have been studied by tracing of beds on photo panels and aerial photographs, walking out of beds in the field, and through collection of an exten-sive set of bed thickness measurements. The results are summarized in the correlation panels of Figures 3.11 and 3.14.
Fig. 3.4 Outcrop pattern, tectonic dips and measured paleocurrent trends (arrows and rose
3.3 Data description and analysis
3.3.1 Data set
The data set obtained from the study area comprises observations from exposures, large-scale photo panels and detailed photographs, and measurements of tectonic dip, bed thickness and palaeocurrent direction.
Figure 3.5 gives an overview of the locations where photo panels have been taken. Names have
been given to topographic features in the area, either hills or larger valleys. Where appropriate, ‘East’ and ‘West’ have been added to the names in order to distinguish between the east and west face locations of the hills or valleys. These names will be referred to in the rest of this chapter. The same names have been used to indicate the locations where bed thicknesses have been measured. Each photo panel is covered by two or three thickness measurements. The position of a measurement at a certain location is indicated by ‘coast’, ‘mid’, or ‘top’ (e.g. ‘Charlicun East-coast’ or ‘Monte Mitxitxola West-top’). The results of the bed thickness measurements will be treated in sections 3.3.3 and 3.3.4 on bed continuity and correlation, and in the quan-titative analyses of section 3.4.
Figure 3.4 shows paleocurrent trends determined in the Charlicun and Mitxitxola Levels. Each arrow represents measurement of the length orientation of a sedimentary body. Length orien-tations were determined by combining evidence from the long axis of troughs and from scours at he base of TBDS beds. The direction of flow along the length orientations was derived from smaller-scale internals structures within the bodies, such as flute casts and cross-bedded sets. Features such as sedimentary structures, grain size trends, character of bed boundaries, etcetera, will be discussed in sections 3.3.2 to 3.3.4.
3.3.2 Lithofacies description
Figure 3.6 illustrates the general characteristics of the studied stratigraphic interval. The thick-bedded units of the Charlicun and the Mitxitxola Level are shown, which are both un-derlain by a thin-bedded, vegetated interval. The thin-bedded units start at the base with a clay-rich interval with very fine-grained, 2 - 5 cm thick silt/sandstone interbeds. These in-terbeds gradually become thicker and coarser-grained upsection (up to 10 cm thick, fine-grained). The fine-sand beds show common parallel and convolute lamination, typical for Bouma Tc,d,esequences. Flute and groove casts were frequently observed. The general
thick-Fig. 3.6 View on
location Tontorra, showing the gener-al characteristics of the studied stratigraphic inter-val. CL = Charli-cun Level, ML = Mitxitxola Level, a = thin-bedded, b = thick-bedded.
ening-upward trend continues with medium-grained beds of maximum 1 m thickness. These beds show normal grading at the base, and parallel and convolute lamination at the top, and are interpreted to represent Bouma Ta,b and Ta,b,c sequences (Fig. 3.7). The sandstone beds generally are continuous, at least at the scale of the exposures. However, towards the top of the thin-bedded units, discontinuous sandstone lenses may be interbedded locally. The total thickness of the thin-bedded units varies between 30 and 40 m.
The tops of the thin-bedded units are marked by a sharp transition to the overlying sand-dom-inated units (Fig. 3.8). The thicknesses of individual beds usually exceed 1 m, which classi-fies them as TBDS beds (Section 2.2). Two distinct sandstone facies types have been distinguished in the TBDS intervals. These are referred to as Type A and Type B. A third sandstone lithofacies Type C is commonly less than 1 m thick. As such, it does not classify as TBDS, but is included in the descriptions below because it occurs associated with TBDS types.
Type A Sandstone
Type A sandstone beds range in thickness from 0.6 to 6.0 m. Their bases are flat and
ally non-erosive. Very rarely, flute casts have been observed. The beds are fine- to medium-grained. They are dominantly massive without obvious grain size trends, although the basal 10 - 15 cm locally exhibit normal grading. Some beds show abundant dish structures (Fig. 3.9A). The upper parts of Type A sandstone beds usually show parallel to strongly convolute lamination, with variable degree of soft sediment deformation due to water escape. Com-monly, beds are amalgamated (sand-on-sand contacts), but may also be separated by 10 - 15 cm thick, discontinuous clay layers.
Type B Sandstone
Type B sandstone beds range in thickness from 0.5 to over 7 m. Their bases are highly irreg-ular due to extensive erosion and loading, with common elongate scours. Small-scale slump-ing has been observed locally. The depth of erosion is 10’s of cm, but in rare cases may reach as much as 1 m or more. Grain size ranges from fine to very coarse sand. Accessory compo-nents include rip-up clasts, organic debris, and mica flakes.
Type B sandstone beds generally have a very chaotic, massive appearance (Fig. 3.9B). This is due to the combined effects of spherical, diagenetic iron precipitation, fluidised tops, and strong weathering. These effects mask rapid internal grain size variations and normal grad-ing trends. Where the effects are less extensive, lenticular bodies with internal trough cross-bedding have been observed.
Type B sandstone beds are amalgamated, or separated by thinner Type C sandstone beds or Bouma interbeds.
Fig. 3.8 A 34 m thick, massive sandstone interval of the Charlicun Level, sharply overlying
Type C sandstone
Type C sandstone beds range in thickness from 10 - 50 cm. They are fine- to medium-grained with common mica flakes. Sedimentary structures are common, and dominated by parallel lamination and small-scale trough cross-bedding (Fig. 3.9C). The beds are associated with Type B sandstone beds.
Fig. 3.9 Three
main sandstone lithofacies types A, B and C, for discussion see text. Note ham-mers for scale (gb = graded base, pl = parallel lamination, d = dishes, cf = contortion/ fluidization).
3.3.3 Bed continuity - the Charlicun level
The thick-bedded sandstone interval of the Charlicun Level is continuously exposed. How-ever, lateral bed thickness variations are significant, and pinching-out is common (Fig. 3.10). Indicated in these figures are bed thickness measurements at a number of locations. Green lines between thickness logs represent ‘high-confidence’ correlation lines that have been walked out in the field, or traced on photographs. Discontinuous red lines represent ‘most probable’ correlation lines, with a lower degree of confidence.
The correlation panels of Fig.3.11 shows that, within the thick-bedded Charlicun Level, three separate zones can be distinguished, each with a dominant sandstone facies type. The first zone (in yellow) is dominated by Type A sandstone beds. Individual beds/ bodies have a relatively constant thickness throughout the exposed area. The second zone (in blue) is
dom-Fig. 3.10 Aerial photograph of location Maturreta West. Arrows point to pinch-outs of
iso-lated channel bodies of the Charlicun Level. DS = dip slopes, dipping 40 degrees towards the sea in the north.
