Sequence Stratigraphy

EDITED BY

Dominic Emery and Keith Myers BP Exploration, Stockley Park Uxbridge, London

Sequence Stratigraphy WITH CONTRIBUTIONS FROM

George Bertram Cedric Griffiths Nick Milton Tony Reynolds Marcus Richards Sitnon Sturrock

• A Blackwell "-II Publishing

EDITED BY

Dominic Emery and Keith Myers BP Exploration, Stockley Park Uxbridge, London

Sequence Stratigraphy WITH CONTRIBUTIONS FROM

George Bertram Cedric Griffiths Nick Milton Tony Reynolds Marcus Richards Sitnon Sturrock

• A Blackwell "-II Publishing

List of Contributors

G EO R G E BE R T RAM Stratigraphic Research International, Braehead Avenue, Milngavie, Glasgow. UK

DOMINIC EMER Y BP Exploration. Uxbridge One, 1 Harefield Road. Uxbridge, London, UK

CEDRIC GRIFFITHS NCPGG, Thebarton Campus. University of Adelaide, Adelaide. Australia

NICK MIL TON BP Norge, Forus, Stavanger. Norway

KEITH MYERS BP Exploratzon. Uxbridge One, 1 Harefield Road, Uxbridge, London, UK

TONY REYNOLDS BP Exploration, Sunbury-onThames. London, UK

MARCUS RICHARDS BP Exploration, Anchorage. Alaska, USA

SIMON STURROCK 56 Gloucester Court, Kew Road, Kew, London, UK

IV

Preface

In 1989, the chief geologist of BP Exploration and his senior colleagues recognized the need to expand the company's resource of sequence stratigraphers, and created the Stratigraphic Studies Group. This group initially included a few experts, but was composed chiefly of a mixture of willing geophysicists, sedimentologists and biostratigraphers, who were to train as sequence stratigraphers, but more importantly, were to bring the expertise from their own disciplines to bear on sequence stratigraphy. This merging of geological disciplines with sequence stratigraphic principles first saw the light of day as BP's 'Introduction to Sequence Stratigraphy' course. This course has been given internally since 1991, and has been presented in whole or in part to over a dozen national oil companies and at several international geological and geophysical conferences. The 'Introduction to Sequence Stratigraphy' course manuals formed the basis for this book, because, as we naively thought, it would not be too much trouble to recast the manuals in the form of a textbook, which could form the basis of university and professional courses. Inevitably, it was more difficult than we had imagined. Sequence stratigraphy is a rapidly evolving subject, new terminology was being added as we wrote, and the jargon we sought to demystify continues to grow. This book must thus be seen as a sequence stratigraphic synthesis for the mid-1990s, and not the final word on the subject. It is above all a practical guide and contains tools and techniques that the authors have found useful in their daily work. We have arranged the book to cover four main themes; a brief history of sequence stratigraphy, concepts and principles, sequence stratigraphic tools and finally the application of sequence stratigraphy to different depositional systems. For the last theme, we have tried to emphasize the importance of seeing sequence stratigraphy in its sedimentological context, and it is recommended that the reader should have some familiarity with sedimentological processes before tackling the last five chapters. Otherwise,

the book covers all the basics of sequence stratigraphy, and is intended to be a broad text suitable for undergraduate geologists of all years, MSc and PhD sedimentologists and stratigraphers, and for oil company geoscientists who wish to broaden their knowledge of the stratigraphic methods avaiiable for solving problems with which they are routinely faced. We hope you find it interesting and useful.

Acknowledgements Bob Jones contributed several sections on biostratigraphy and Neil Parkinson contributed to Chapter 2. We are grateful to the following reviewers who have helped Improve the book at various stages of its development: David Roberts, Henry Posamentier, Maurice Tucker, Dan Bosence, Mike Bowman, Andy Horbury and Andy Fleet. We also acknowledge the support of BP Exploration, particularly David Roberts, Bob Rosenthal, John Wills and Peter Melville and we are grateful to BP's partners for permission to publish information on many of the areas and fields mentioned in the text. The following companies are also acknowledged for their assistance in providing seismic data; Lynx Information Systems, Trans-Asia Oil and Mineral Development Corp., Balabac Oil Exploration and Drilling Co. Inc., Crestone Energy Corp., Coplex (Palawan) Ltd., Oriental Petroleum and Minerals Corp., The Philodrill Corp., Seafront Resources Corp., Vnioil and Gas Development Co. Inc., Vulcan Industrial and Mining Corp.

George Bertram, Dominic Emery, Cedric Griffiths, Nick Milton, Keith Myers, Tony Reynolds, Marcus Richards and Simon Sturrock BP Exploration London, Sunbury-on-Thames, Glasgow, Stavanger and Anchorage

v

CHAPTER ONE

Historical Perspective

1.1 What is sequence stratigraphy? 1.2 The evolution of sequence stratigraphy

1.1 What is sequence stratigraphy?

1.2 The evolution of sequence stratigraphy

Sequence stratigraphy is a subdiscipline of stratigraphy, the latter being defined broadly as 'the historical geology of stratified rocks'. There have been many definitions of sequence stratigraphy over the years, but perhaps the simplest, and that preferred by the authors, is 'the subdivision of sedimentary basin fills into genetic packages bounded by unconformities and their correlative conformities'. Sequence stratigraphy is used to provide a chronostratigraphic framework for the correlation and mapping of sedimentary facies and for stratigraphic predictiorr. Several geological disciplines contribute to the sequence stratigraphic approach, including seismic stratigraphy, biostratigraphy, chronostratigraphy and sedimentology. These are discussed in more detail in forthcoming chapters. Note that lithostratigraphy is not considered to contribute usefully to sequence stratigraphy. Lithostratigraphy is the correlation of similar lithologies, which are commonly diachronous and have no time-significance (Fig. 1.1). Lithostratigraphic correlation is useful provided the sequence stratigraphic boundaries enveloping the interval of interest are constrained.

Sequence stratigraphy is often regarded as a relatively new science, evolving in the 1970s from seismic stratigraphy. In fact sequence stratigraphy has its roots in the centuries-old controversies over the origin of cyclic sedimentation and eustatic versus tectonic controls on sea-level. Much of this early debate has been summarized recently in a set of historical geological papers edited by Dott in 1992 (1992a), entitled 'Eustasy: the Ups and Downs of a Major Geological Concept', and the interested reader is referred to this volume for more detail. Other historically important collections of sequence stratigraphic papers include American Association of Petroleum Geologists (AAPG) Memoir 26, published in 1977, and Society of Economic Paleontologists and Mineralogists (SEPM) Special Publication 42, published in 1988.

Fig. 1.1 The difference between sequence stratigraphy, which has a geological time significance, and lithostratigraphy, which correlates rocks of similar type. A lithostratigraphic correlation would correlate conglomerate units 1 and 2, sandstone units 3, 4 and 5 and mudstone units 6, 7 and 8. A sequence stratigraphic correlation would correlate time lines A ~ A', B--- Brand C-C

Sacred theories The Deluge and the story of Noah is the most well-known of the earliest references to sea-level change. To the early investigators of sea-level change, the veracity of the Deluge

-----18' q)

E F

3

Concepts and Principles

CHAPTER TWO

Concepts and Principles of Sequence Stratigraphy

2.1 Introduction 2.1.1 Basin forming processes 2.1.2 Basin-margin concepts 2.2 Relative sea-level, tectonics and eustasy 2.2.1 Definitions of sea-level 2.2.2 Accommodation 2.2.3 Accommodation through time 2.2.4 Orders of cyclicity and global correlation 2.3 Sediment supply 2.3.1 Principles of clastic sediment supply 2..1.2 Filling of accommodation 2.3.3 Basin architecture

2.4 Sequences and systems tracts 2.4.1 Sequences and sequence boundaries 2.4.2 Systems tract definition 2.4.3 Lowstand systems tract 2.4.4 Transgressive systems tract 2.4.S Highstand systems tract 2.4.6 Type 2 sequence boundary and the shelf-margin system, traer 2.4.7 Lowstand systems trans on a ramp margm 2.4.8 Controls on system'. tract boundaries 2.4.9 Other possible systems tracts within a relative sea-level cycle 2.4.10 Composite (second and third

2.1 Introduction The stratigraphic signatures and stratal patterns in the sedimentary rock record are a result of the interaction of tectonics, eustasy and dimate. Tectonics and eustasy control the amount of space available for sediment to accumulate (accommodation), and tectonics, eustasy and climate interact to control sediment supply and how much of the accommodation is filled. Autocydic sedimentary processes control the detailed facies architecture as accommodation is filled. The purpose of this chapter is to introduce the principles that govern the creation, filling and destruction of accommodation. It then shows how these principles are used to divide the rock record into sequences and 'systems tracts', \vhich describe the distribution of rocks in space and time. The chapter uses siliciclastic systems to introduce the concepts and principles of sequence stratigraphy. Carbonate systems differ from clastic systems in their ability to produce sediment 'in situ', and they respond in a different manner to accommodation changes. Carbonates are therefore discussed separately in Chapter 10.

2.1.1 Basin forming processes Tectonism represents the primary control on the creation and destruction of accommodation. Without tectonic subsidence there is no sedimentary basin. It also influences the

order) sequences and systems tracts 2.4.11 Genetic stratigraphic sequences

2.5 High-resolution sequence stratigraphy and parasequences 2.5.1 Introduction 2.5.2 Parasequences and their continental equivalents 2.5.3 Parasequence sets 2.5.4 Parasequence thickness trends 2.5.5 Sequence houndaries 2.5.6 Maximum flooding surfaces 2.5.7 Ravinement surfaces 2.5.8 Problems and pitfalls of highresolution sequence stratigraphy

rate of sediment supply to basins. Tectonic subsidence results from two principle mechanisms, either extension or flexural loading of the lithosphere. Figure 2.1 illustrates theoretical tectonic subsidence rates in extensional, foreland and strike-slip basins. These curves in effect govern how much sediment can accumulate in the basin, modified by the effects of sediment loading, compaction and eustasy. Extensional basins form in a variety of plate tectonic settings, but are most common on constructive plate margins. In extensional basins, tectonic subsidence rates vary systematically through time, with an initial period of very rapid subsidence caused by isostatic adjustment to lithosphere stretching, followed by a gradual (60-100 million years) and decreasing thermal subsidence phase as the asthenosphere cools. This systematic change in tectonic subsidence rate has a strong influence on the geometry of the basin-fill, such that it may be possible to divide the stratigraphy into pre-, post- and syn-rift phases (these phases have been termed megasequences; Hubbard, 1988). In the simple syn-rift megasequence model the sediments are deposited in the active fault-controlled depocentres of the evolving rift and can show roU-over and growth into the active faults. Differential subsidence across the extensional faults may exert a strong control on facies distributions. In the post-rift megasequence, any remaining riftrelated topography is gradually buried beneath sediments that fill the subsiding basin and onlap the basin margin, creating the typical 'steers head' geometry Ct\.1cKenzie,

11

Sequence Stratigraphic Tools

CHAPTER THREE

Seismic Stratigraphy

3.1 Seismic interpretation 3.1.1 Principles of seismic stratigraphic interpretation 3.1.2 Resolution of seismic data 3.1.3 Seismic processing and display for stratigraphic interpretation 3.2 Seismic reflection termination patterns 3.2.1 Marking up a seismic section

3.2.2 Categorizing reflection terminations 3.2.3 Seismic facies and attribute analysis 3.2.4 Recognition of stratigraphic surfaces 3.3 Recognition of systems tracts on seismic data

3.1 Seismic interpretation 3.1.1 Principles of seismic stratigraphic interpretation Seismic stratigraphy is a technique for interpreting stratigraphic information from seismic data. Together with its offspring sequence stratigraphy, it is acknowledged as being among the most signific;mt developments in the earth sciences in the last 30 years. The ideas behind the technique were introduced in a number of papers in Association of American Petroleum Geologists (AAPG) Memoir 26 (Vail et al., 1977a,b,c). The fundamental principle of seismic stratigraphy is that within the resolution of the seismic method, seismic reflections follow gross bedding and as such they approximate time lines. It is important to realize that this statement does not deny in any way the physical fact that the seismic reflections are generated at abrupt acoustic impedance contrasts, nor does it dispute the fact that variations in impedance contrast will produce reflections of varying amplitude (impedance is the product of rock density and seismic velocity). The key message is that the correlative impedance contrasts represented on seismic data come

Fig. 3.1 Seismic reflections are believed to follow gross bedding surfaces. Impedance contrasts are abrupt across bedding planes and gradual across facies boundaries

3.3.1 Recognition of lowstand systems tracts 3.3.2 Recognition of transgressive systems tracts 3.3.3 Recognition of highstand systems tracts 3.4 Pitfalls in interpretation

from bedding interfaces and not lateral facies changes. At the scale of seismic resolution, facies changes in timeequivalent strata are gradual and do not generate reflections (Fig. 3.1). The axiom states that reflections can be thought of as time~lines that represent time surfaces in three dimensions, separating older rocks from younger. There are acknowledg