Structural and Seismic Facies Interpretation of Fabi Field,
Onshore Niger Delta, Nigeria
Kehinde Oluwatoyin Olowoyo
DISSERTATION.COM
Boca Raton
Structural and Seismic Facies Interpretation of Fabi Field, Onshore Niger Delta, Nigeria
Copyright © 2010 Kehinde Oluwatoyin Olowoyo
All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information
storage and retrieval system, without written permission from the publisher.
Dissertation.com Boca Raton, Florida
USA • 2010
ISBN-10: 1-59942-354-5 ISBN-13: 978-1-59942-354-8
2
ABSTRACT
The Niger Delta is a prolific oil province within the West African subcontinent. Exploration activities have
been concentrated in the onshore part of this basin but as the delta becomes better understood,
exploration influences are gradually being shifted to the offshore. Although the geology, tectonics and
evolution of the Eocene‐Pliocene sequence of the Niger Delta are fairly well known, these are expected
to increase as new analytical tools and concepts evolve. This work was an integrated structural, seismic
facies and stratigraphic study conducted in the Fabi Field, onshore western Niger Delta, and targeted at
improving the present understanding of the structural development, sequence stratigraphic history,
paleo‐depositional environments and hydrocarbon reservoir potential of the field.
Five wireline logs, biostratigraphic data, 3‐D seismic section, check shot data and core data were
analysed and utilized in this study. Well log were used to determine the different lithologies, system
tracts, stacking pattern and reservoir potential of the field. Sequence stratigraphy and seismic facies
were used to identify the reflection packages in order to determine the environment of deposition.
Structural and horizon mapping results were used to generate time and depth structural map with the
aid of a derived function calculated from the check shot data.
The base of this sequences consists of massive and monotonous marine bioturbated shales, which
grade into inter‐bedding shallow marine fluvial sands with parallel‐cross bedding laminations, silt and
clays, while the upper part is a massive marine sandstone section. The gross reservoir thickness ranges
from 150ft ‐ 700ft with net thickness of 20ft ‐ 175ft. Sequence stratigraphic analysis revealed that the
succession consists of two sequence boundaries dated, 10.35Ma and 10.6Ma and two maximum
flooding surfaces, dated 9.5Ma and 10.4Ma.
The high percentage of the reflections with low to moderate amplitude/continuity of the
parallel/divergent configuration is identified as a feature of delta platform facies, while the sigmoidal –
hummoky reflections, indicate a slope facies. The system tracts from the log are the trangressive and
highstand system tracts, while growth faults(F1), antithetic faults(F3,F4) and synthetic faults
(F2,F5,F6,F7) are the identified structures which are typical of the Niger Delta reservoir sandstone.
Depositional setting of the Middle ‐Upper Miocene strata were influenced by fluvial, tidal and marine
systems. The up dip areas on the depth structure maps with closure, signify possible anticlinal structures
3
where hydrocarbons could be entrapped. These could serve as possible appraisal locations, where wells
could be sited to optimize the development of the reservoir sands within the field using the structural
model generated.
5
DEDICATION
This work is dedicated to the Almighty God. The giver of every good and perfect gift, who has
made his wisdom and grace available unto me.
6
ACKNOWLEDGEMENTS
I am grateful to Shell Petroleum Development Company (SPDC), Port Harcourt for the supply of
the subsurface data used for this project. My profound gratitude goes to Mr. Alelo Ameh of SPDC,
Port Harcourt for his sacrifice in securing the data and Mr. Adewuyi Adedayo (SPDC),Lagos for
making himself available for consultations.
My sincere gratitude also goes to My supervisor, Dr. M.E. Nton, for his constructive criticism
and suggestions which significantly improved the project. I wish to appreciate all my lectures in UI.
These includes Prof. A.I Olayinka (HOD), Prof.A.A Elueze, Dr.C.O Adeyemi, Dr.A.P Bolarinwa,
Dr.O.A Ehinola, Dr. O.A Okunlola, Dr. A.F.Olatunji, Dr. Boboye, Dr. Adeigbe, Dr I.M Akaeaigbobi, Mr
M.A Oladunjoye, Mr I.A Oyediran, Mr A.M Adeleye, Mr Oshinowo and Prof D.R Adeleye(H.O.D) of
Ajayi Crowther University.
I wish to acknowledge Williams (Oyeka) ,Ugochuku, Afolabi, Kolade and Emelda for their
endless contributions to the success of this project. I also wish to thank my parent Chief and Mrs.
Olowoyo for their financial and moral support during my course of study, My sisters and in‐laws:
Mr and Mrs. Ogundele and Mr and Mrs. Ojo for their constant encouragement and assistances. I
remain thankful to all my friends and classmate that made my stay in UI a memorable one and my
pastor Dr. Felix, and My sister In‐law Mrs. Folashade Aina for her support and encouragement.
This appreciation can never be complete without saying a Big thank you to my dear husband,
Mr. Nicholas Ajogwu for his patience,understanding, encouragement and love during the course
of this work.
7
TABLE OF CONTENTS
PAGE
Title page 1
Abstract 2
Certification 4
Dedication 5
Acknowledgement 6
Table of contents 7
List of Figures 12
List of Table 13
CHAPTER ONE
1.0 Introduction…………………………………………………………………………………………...14
1.1 General statement…………………………………………………….........................14‐15
1.2 Aim and Objectives………………………………………………………………………….….....15
1.3 Location of study area…………………………………………………………………………....15
1.4 Scope of work……………………………………………………………………………………..15‐16
1.5 Literature Review……………………………………………………..........................17‐19
1.6 Sequence stratigraphic Concept………………………………………………………….20‐23
1.6.1 Identification of Parasequence…………………………………………………………......23
1.6.2 Parasequence set and Stacking patterns………………………………………..……..23
1.6.2.1 Progradational stacking…………………………………………………………………...23‐24
1.6.2.2 Retrogradational stacking………………………………………………………………….….24
1.6.2.3 Aggradational stacking……………………………………………………………………..24‐25
8
CHAPTER TWO
2.0 Geologic setting…………………………………………………………………………………….….26
2.1 Geology of Niger Delta…………………………………………………………………………..26‐27
2.2 Basin Evolusion……………………………………………………………………………….…………27
2.3 Stratigraphy…………………………………………………………………………………..…….28‐30
2.4 Niger Delta Structure………………………………………………………………………..………31
2.4.1 Depobelts…………………………………………………………………………………………….31‐32
2.4.2 Growth faults tectonic…………………………………………………………………..…….32‐33
2.4.3 Shale Daipirs………………………………………………………………………………………….…36
2.5 Hydrocarbon Occurrence………………………………………………………………….……….36
2.6 Source Rocks………………………………………………………………………………………...….38
2.7 Reservoir Rocks…………………………………………………………………………………........38
2.8 Traps and Seals……………………………………………………….............................38‐39
CHAPTER THREE
3.0 Material and Methodology………………………………………………………………….……..40
3.1 Data Acquisition……………………………………………………………………………..………...40
3.2 Methodology……………………………………………………………………………………….…….40
3.2.1 Well log Analysis………………………………………………...................................40‐41
3.2.2 Recognition of system tract and key surfaces on logs……………………..…….43‐44
3.2.3 Biostratigraphic analysis………………………………………………………………………….….44
3.2.4 Seismic Data Analysis………………………………………………....................................46
3.2.4.1 Seismic sequence analyaia………………………………………..…….…………………………..46
3.2.4.2 Seismic facies analysis…………………………………….…………………………………………...48
3.2.4.3Seismic structural analysis……………………………………………………………………………...48
3.2.5 Tying of Well data to seismic ………………..…………………………………………………...48‐49
9
3.3 Limitation of study …………………………………………………………..............................49
CHAPTER FOUR
4.0 Stratigraphic Analysis and Geological Interpretation………………………..…………..51
4.1 Core Description ………………………………………………………..................................51
4.2 Well Log Interpretation………………………………………….……………………………….…….51
4.2.1 Facies Description………………………………………………..…...............................57‐59
4.2.2 Depositonal Facies…………………………………………………………………………………..60‐61
4.3 Reservoir Geology……………………………………………………………………………………62‐65
4.4 Biostratigraphic Interpretation…………………………………………….........................65
4.5 Seismic Facies ……………..……………………………………………………………………………....65
4.5.1 Seismic Volume……………………………………………………………………………………………...65
4.5.2 Seismic facies Interpretation……………………………………………………………………...67‐70
4.5.3 Seismic Structural Interpretation……………………………………..…………………….……...71
4.5.3.1 Faults………………………………………………………………………………………………………….…71
4.5.3.2 Shale Diapirs…………………………………………………………………………………………………..71
4.5.4 Implication of the structures on hydrocarbon generation and Accumulation...74‐76
4.6 Sequence Stratigraphy……………………………………………………………………………………..77
4.6.1 Maximum Flooding surfaces………………………………………………….........................77
4.6.2 Sequence Boundaries……………………………………………………………………………………..77
4.6.3 Sequence Stratigraphy Correlation……………………………………………………………77‐79
4.7 Map generation: Time /Depth Structural Maps………………………………..............82
CHAPTER FIVE
5.0 Discussion of Result .……………………………………………………………………………………….88
5.1 Depositional Environment …………………………………………………….........................88
5.2 Implication for Hydrocarbon Exploration………………………………………………………….89
10
CHAPTER SIX
6.0 Summary, conclusion and Recommendation………………..………………………………......90
6.1 Summary …………………………………………………………………………………………………..………90
6.2 Conclusion ………………………………………………………………………………………………….…90‐91
6.3 Recommendation…………………………………………………………………………………………..……91
REFERENCES………………………………………………………………………………………………....92‐97
APPENDICES…………………………………………………………………………………………………….98‐100
11
LIST OF ILLUSTRATION
Figures Page
1.1 Map of Niger Delta region showing the study area……………………………………...….16
1.2 Base map of the study area showing locations of the wells……………………………..17
1.3 The Schematic representation of the sequence and system tracts……………….....21
1.4 Parasequence stacking pattern…………………………………………………………………………25
2.1 Stratigraphic column showing the three formations of the Niger Delta………...29
2.2 Growth faults and seals in Niger Delta…………………………………………………………..34
2.3 Various macrostructures and mega structure type in Niger Delta………………….35
2.4 Location of the lobe of early Niger Delta……………………………………………………...37
3.1 General gamma ray response to variations in grain size………………………………..42
3.2 Niger Delta chronostratigraphic chart ……………………………………………………..….45
3.3 Generalized stratigraphic section of a sequence…………………………………………...47
3.4 Terminations that defines boundaries in depositional sequence……….…………..47
3.5 Time‐ depth (T‐Z) graph for Fabi‐001……………………………………………………….……..50
4.1 Core pictures of Fabi field……………………………………………………………………………...53
4.2 Sedimentary structures identified on core picture………………………………………….54
4.3 Well correlation with core pictures……………………………………………………..…………55
4.4 Well correlation in Fabi field…………………………………………………………………….…….56
4.5 Well log of Fabi‐001 ‐003 for facies description………………………………………………58
4.6 Depositional facies type based on log motif……………………………………………..……..61
4.7 Well log showing the sandstone unit in Fabi field…………………………………………….63
4.8 Correlation of well data with biofacies………………………………………......................66
12
4.9 Seismic section showing the reflection pattern on trace 1169……………………..…..68
4.10 Seismic facies and reflection pattern for trace 1201………………………………………...69
4.11 Seismic facies and structural interpretation for trace 1169……………………………..73
4.12 Seismic facies and structural interpretation for trace 1201…..……………..………..75
4.13 Identification of maximum flooding surface and sequence boundary……………....78
4.14 Arrows indicating the parasequence stacking pattern on Fabi‐003 well………….…81
4.15 Seismic section showing horizons of interest……………………………………………………83
4.16 G60 horizon time map………………………………………………………………………………….…….84
4.17 H80 horizon time map…………………………………………………………………………………….…..85
4.18 G60 horizon depth map…………………………………………………………..............................86
4.19 H80 horizon depth map…………………………………………………………..............................87
13
LIST OF TABLES
Table Page
4.1 Description of physical properties of Fabi field on core picture………………………………..…52
2 Fabi Field biostratigraphic report…………………………………………………………………………….……98
3 Checkshot data for Fabi Field……………………………………………………………………………………....99
4 Sandstone unit with gross and net pay thickness……………………………………………………….100
14
CHAPTER 1
1.0 INTRODUCTION
1.1 GENERAL STATEMENT
Rapid increasing demand for oil and gas, worldwide, has caused an increase in exploration and
development in pre‐explored areas around the world such as the Niger Delta. Consequently, more
detailed methods apart from the structural approach are being developed and as the emphasis
therefore shifts from structural traps to stratigraphic prospects, more accurate techniques of
stratigraphic analysis are needed. One of such techniques is seismic‐sequence stratigraphy. Together
with the structural methods, it can help locate some of the world’s largest known oil reservoirs and even
remain one of the major frontier plays of the immediate future.
Seismic stratigraphy, which was developed in 1960’s, entails interpreting unconformities based on
tying together global and local sea level changes, seismic reflection patterns, seismic structures and
horizon mapping. These unconformities were found to be controlled by changes in the relative sea level
which could be recognized on well logs and outcrops as well as on seismic reflections.
The application of sequence stratigraphic method in off shore sedimentary basin with little or no
geographic control often enhances, correlation of locally recognized depositional sequence with the
world‐ wide pattern of sea level change (Payton 1977). Several authors includes (Bowen 1994, Stacher
1994 ,Ozumba 1999 ) among others have worked in Niger Delta using this method of sequence
stratigraphy. This method also facilitates the identification of major progradational sedimentary
sequences which offer the main potential for hydrocarbon generation and accumulation. Today it has
been accepted by oil company for predicting reservoir sand bodies and their corresponding environment
of deposition.
The present study is located in FABI field, onshore Niger Delta. The aim of this study is to improve the
understanding of the structural development, sequence stratigraphic history and predict the
hydrocarbon potential of Fabi field, located in the coastal swamp region of the Niger Delta.
Using the seismic‐sequence stratigraphic approach as described by Vail et al.,1977, the determination
of the seismic‐ sequence stratigraphy of the field is done, by delineating the sequence boundaries (SB),
maximum flooding surface (MFS), trangressive surface of erosion, the system tracts and infer the
15
environment of deposition of the sand bodies within the field, through the integration of seismic data,
well logs data sets and bio‐data.
1.2 AIM AND OBJECTIVES
The aim of this study is to provide a sequence stratigraphic interpretation using well logs, seismic, core
and bio‐fancies data sets covering an area in the coastal swamp Field of the Niger Delta. This is to aid
further exploration activities within the field of study.
This study has the following objectives:
(i) Predict the reservoir characteristics of the sand bodies.
(ii) Deduce the environment of deposition of the sand bodies.
(iii) Correlation of the reservoir sand bodies within the field.
(iv) Identification of major structures within the field.
1.3 LOCATION OF STUDY AREA
The study area is situated in OML‐X belonging to Shell Development Company of Nigeria in the
coastal swamp region of the Niger Delta in Nigeria,( Fig1.1 & Fig.1.2). The Fabi field structure is a large
collapsed crest rollover anticline treading east‐west and It is bounded to the north by the major
bounding fault. The field is operated by Shell Petroleum Development Company and it was first
discovered in 1975.
1.4 SCOPE OF WORK
The scope of this project includes the following:
(1) Integration of the available data sets namely, 3D seismic data along with well log data from the
wells, core data and biostratigraphic data, so as to get a detailed vertical resolution of the
sedimentary section, determine the continuity of the stratigraphic frame work and age of
sediments.
(2) Identification and interpretation of various facies using lithology sensitive logs such as gamma ray.
(3) Recognition of well logs responses that characterize sequence boundaries and the maximum
flooding surfaces as well as the different system tracts.
(4) Identification of seismic facies pattern, faults and mappable horizons identified in the well log.
(5) Construction of time and depth map for the reservoir sands.
16
Fig.1.1. Map of Niger Delta region showing the study area (Corredor et. al,2005)
17
5
3
Fig.1.2: Base map of Fabi field showing distribution of the wells
18
1.5 LITERATURE REVIEW
Stoneley (1966) and Burke et al. (1970, 1972) analyzed and discussed the mega tectonic setting of
the Niger Delta. The syn‐sedimentary tectonics of the Tertiary delta was extensively described by Merki
(1972) and Evamy et al. (1978).
Short and Stauble (1967) and Weber and Daukoru (1975) outlined the three major depositional cycles
in the coastal sedimentary basins of Nigeria. They first began with an Albian marine incursion and
terminated during the Santonian time; the proto‐Niger Delta started during the second cycle, the
growth of the Niger Delta continued from Eocene to Recent time. At several stages during the late
Quaternary, sedimentation was interrupted by uplift and erosion, where by several cycles of channels
were cut and filled which resulted to submarine canyons (Evamy et al, 1978).
Burke et al (1972) correlated these late Quaternary canyons to the drowning of river mouths, which
were incised on the continental shelf during the Wisconsin fall in sea level, which probably resulted in
the formation of the Afam canyon and the Qua lboe clay fill, during the Miocene, in the south‐east delta.
According to Short and Stauble (1967) and Doust and Omatsola (1990), the Niger Delta comprises of a
regressive sequence of deltaic and marine clastics, defined by three major lithofacies at the base of
marine shale, made up of Akata Formation, followed by paralic sequence of Agbada Formation and
topmost non‐marine alluvial (continental) sands of the Benin Formation. Oomken (1974) examined the
sediments in the terrestrial and submarine parts of the modern delta and grouped them into five major
lithofacies, using lithological characteristics and other sedimentary features. These lithofacies are
grouped into sandstone, heteroliths and mudstone.
Weber (1971) reported the cyclic nature of sedimentation of the Tertiary paralic deposits. According to
him, a complete cycle consists of thin, fossiliferous transgressive marine sands followed by an offlap
sequence which commences with marine sediments and another transgression may terminate the cycle.
Doust and Omatsola (1990) recognized six depobelts in the Niger Delta, which are distinguished
primarily by their age. They are: Northern delta (late Eocene‐ Early Miocene), Great Ughelli (Oligocene‐
Early Miocene), Central swamp I(Early‐Middle Miocene),Central Swamp11(Middle Miocene), Coastal
swamp I and II (Middle Miocene) and Offshore mega structures (Late Miocene).
Posamentier and Kolla (2003) analyzed 3‐D seismic data in predominantly basin –floor settings
offshore Niger Delta. These revealed the extensive presence of gravity‐flow depositional elements. Five
19
key elements were observed and are; turbidity‐flow levees channel, channel‐over bank sediment waves
and levees, frontal splays and distributary channel complexes, crevasse‐spray complexes, debris – flow
channels, lobes and sheets. The reservoir architecture of each of these depositional elements is a
function of the interaction between sedimentary process, seafloor morphology, and sediment grain‐size
distribution.
Adeogba et al (2005) have interpreted a near surface, 3‐D seismic data set from the Niger Delta
continental slope, offshore Nigeria and revealed important stratigraphy and architectural features.
Architectural features and sediment deposits interpreted from seismic character and seismic
stratigraphy in the absence of borehole data include mass‐transport complexes, distributary channels,
submarine fans and hemi pelagic drape complex.
Heinio and Davies (2006) have interpreted three‐ dimensional seismic of the toe‐ slope region in deep
water Niger Delta. These reveal a range of erosion and depositional features that are the result of the
degradation of thrust‐propagation folds. The dominant style of degradation of these folds occurs as
retro gradational, small volume failures that form thin deposits at or below seismic resolution. Slope
morphology, sedimentology, and the presence of anisotropies affect the type of failure that occurs.
Sequence stratigraphic concepts are increasing finding new and unique applications in the regressive
siliciclastic deposits of Niger Delta. The lithostratigraphic interpretation of the Niger Delta sediments, cut
across time lines and their lateral associations suggest that the sedimentary deposits were strongly
influenced by eustacy and tectonics.
Sequence stratigraphy thus facilitates the subdivision of the Niger Delta into packages of sediments
that are essentially bounded together by chronostratigraphically significant surfaces. Various works have
been carried out, in relation to the sequence stratigraphy of different parts of Niger Delta. These include
Bowen (1994), who established an integrated geologic framework of the Niger Delta slope, by applying
established sequence stratigraphic concepts, on the newly acquired seismic data set of the Niger Delta,
coupled with biostratigraphic data, from 26 key wells.
Stacher (1994), produced a delta wide framework of creataceous chronostratigraphic surfaces, and a
sequence stratigraphic chart for the Niger Delta, using digitally stored biostratigraphic data, obtained
from over 850 wells.
20
Krusi and Idiagbor (1994) linked some types of stratigraphic traps to incised valley fills and low stand
fans. They were thus able to improve the identification of stratigraphic plays in eastern Niger delta.
Ozumba (1999), developed a sequence stratigraphic framework of the western Niger Delta, using
foraminifera and wire line log data obtained from four wells drilled in the coastal and central swamp
depobelts. He concluded that the late Miocene sequences were thicker than the middle Miocene
sequences.
Poston et al. (1983) presented the geology and reservoir characteristics at Meren field. They noted
evidence for syn‐depositional displacement on growth faults across the field. They also suggested
combining well‐log interpretations and laboratory analyses of sidewall cores to aid in the determination
of the spatial variation of porosity and permeability within particular reservoir intervals.
1.6 SEQUENCE STRATIGRAPHY CONCEPT
Sequence stratigraphy is the study of rock relationships with time‐ stratigraphic framework of
repetitive, genetically related strata bounded by surfaces of erosion or non deposition, or their
correlative conformities (Posamentier et., 1988., Van Wagoner, 1995).
Galloway (1989) defined sequence stratigraphy as the analysis of repetitive genetically related
depositional units, bounded in part by surface of nondeposition or erosion. The fundamental unit of
sequence statigraphy is the sequence, which is a relatively conformable succession of genetically related
strata bounded by unconformities or their correlative conformities (Mitchum, 1977). Each sequence is
composed of a succession of system tracts (Fig.1.3). Each system tract is composed of a linkage of
contemporaneous depositional systems (Brown and Fisher, 1977).
21
Type 1 sequence
Type 2 sequence
Fig 1.3: The schematic representation of the sequence and systems tracts (Van Wagoner et al., 1990)
22
Four systems tracts are recognized (Posamentier et al.1988 and Vail, 1988): low stand, transgressive,
high stand and shelf margin.
Sequence stratigraphy is an approach that is used to interpret depositional origin of sedimentary
strata and assumes an implicit connection to base level change. It does this by establishing how the
sequences of strata accumulated in order in the sedimentary section over a subdividing framework of
surfaces.
Behind the general definition of sequence stratigraphy lie a number of assumptions and general
concepts (Van Wagoner et al,1990). These are:
Marine sedimentation patterns are controlled by changes in relative sea level.
Relative sea level is controlled by eustasy, subsidence, tectonics, and sedimentation rate. On
trailing‐edge continental shelf environments, eustasy is of primary importance. In epeiric basins,
tectonics may overshadow the role of eustasy. Subsidence and sedimentation rate are
commonly of secondary importance and are assumed to be processes operating at constant
rates.
Sedimentation patterns controlled by sea level have distinct geometries (systems tracts) that are
easily recognized on seismic lines, well logs, well log cross sections, outcrops, and cores.
On passive margin shelves, as these geometries are eustatically‐controlled, they are similar
worldwide. Once the geometry has been calibrated in a known area, it can be used as a
correlation tool to identify and date seismic strata elsewhere.
The building blocks of a depositional sequence are laminae and laminae sets, beds and bed sets,
parasequences and parasequence sets, systems tracts, sequences and sequence sets. Sequences
are bounded above and below by unconformities (also termed sequence boundaries), which
record a fall in relative sea level.
Sequence stratigraphy may be applied at several scales and in this sense, it is fractal in nature
(meaning that at any scale sequences have the same characteristics). Phanerozoic history is
comprised of first‐order eustatic sequences. First‐order sequences are called mega sequences
by ( Haq et al.1988) and are equivalent to the cratonic sequences of (Sloss,1963). Eras are
comprised of second‐order eustatic sequences (super sequences of Haq et al.1988). Seismic
stratigraphy normally is concerned with third‐order sequences (1‐5 MY duration). Geologic
23
studies of well log cross sections, outcrops, and cores deal with third, fourth (105 years
duration) and fifth‐order (104 years duration) sequences (Van Wagoner et al. 1990).
1.6.1 Identification of Parasequencs
In clastics the second step in the interpretation of well logs is the use of parasequence stacking
patterns (the vertical occurrence of repeated cycles of coarsening or fining upwards sediment) to
identify the low stand system tracts (LST), transgressive system tracts (TST) and high stand system tracts
(HST) that are enveloped by the MFS, TS and SB. These parasequence cyclic stacking patterns are
commonly identified on the basis of variations in grain size and when these fines upwards are indicated
by triangles whose apex is up while those that coarsen upwards are indicated by inverted triangles
whose apex is down.
1.6.2 Para sequence sets and stacking patterns
In most cases, there will not be simply one parasequence but a series of them. Sets of
successive parasequences may display consistent trends in thickness and facies composition and
these sets may be progradational, aggradational, or retro-gradational (Fig1.4) (Van Wagoner et
al.1990). These patterns are dependent on the ratio of the rate of deposition to that of
accommodation.
Some other differentiating factors include interplay between the ratio and the kind of material
deposited that is sandstone or mudstone, the environment of deposition (Coastal/shallow marine to
deep marine) and the ratio of the thicknesses of the different parasequences and parasequence sets.
1.6.2.1 Progradational Stacking
In a progradational set of parasequences, each parasequence builds out or advances somewhat farther
seaward than the parasequence before it. Arising from this, each parasequence contains a somewhat
shallower set of facies than the preceeding parasequence . This produces an overall shallowing‐upward
trend within the entire parasequence set and the set is referred to as a progradational parasequence set
or is said to display progradational stacking,(Van Wagoner et al.1990).
Progradational stacking results when the prolonged rate of accommodation is exceeded by the
increased rate of sedimentation. In this way, accommodation space is filled more rapidly than it is
created, water depth becomes shallower, and facies increasingly move farther seaward over time. Each
parasequence shallows‐upward and is bounded by a flooding surface, across which water depth