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Xiaodong Cheng · Leyuan Fan ·
Weikang Gu
Comprehensive
Practice of Exploration
and Evaluation
Techniques in
Complex Reservoirs
็ᆳ߾ᄽӲมᆶ၌ࠅິ
PETROLEUM INDUSTRY PRESS
Comprehensive Practice of Exploration
and Evaluation Techniques
in Complex Reservoirs
Xiaodong Cheng Leyuan Fan
• •
Weikang Gu
Comprehensive Practice
of Exploration
and Evaluation Techniques
in Complex Reservoirs
Weikang Gu
International Logging Company of CNPC
GreatWall Drilling Company
Beijing, China
© Petroleum Industry Press and Springer Nature Singapore Pte Ltd. 2019
This work is subject to copyright. All rights are reserved by the Publishers, whether the whole or part
of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations,
recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission
or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar
methodology now known or hereafter developed.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this
publication does not imply, even in the absence of a specific statement, that such names are exempt from
the relevant protective laws and regulations and therefore free for general use.
The publishers, the authors, and the editors are safe to assume that the advice and information in this
book are believed to be true and accurate at the date of publication. Neither the publishers nor the
authors or the editors give a warranty, express or implied, with respect to the material contained herein or
for any errors or omissions that may have been made. The publishers remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd.
The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721,
Singapore
Foreword
With the progress of global oil fields E&P, exploration and evaluation trend and
targets already turned to complex reservoirs and subtle traps. These kinds of reser-
voirs, mainly dominated by lithology, are usually characterized of low-amplitude
structures, complex pore-throat textures, thin thickness, and strong heterogeneity.
Due to the difficulties mentioned above, lots of challenges need to be solved
including petrophysical evaluation, reservoir identification and classification, reser-
voir prediction, and subtle trap identification and evaluation.
For the purpose of implementation of internationalization development strategy
of China National Petroleum Corporation (CNPC), GreatWall Drilling Company
(GWDC) of CNPC established its development orientation as a general contractor
of internationalization petrol engineering technology. During the past 15 years,
complex hydrocarbon reservoirs exploration and evaluation studies of more than
ten countries have been carried out by GWDC and China National Logging
Corporation (CNLC, which was reorganized to GWDC in 2009) in African, Central
Asian, Middle East, and other areas, which not only provides strong technical
support for major global oil regains but also forms a comprehensive technology
system for complex reservoirs exploration and evaluation. For better supporting and
serving the demand to global oil fields’ E&P, and engineering technology services
in global market, GWDC summarized and refined these techniques and practice
results, and compiled this book for formal publishing, so as to share and commu-
nicate with counterparts.
This book Comprehensive Practice of Exploration and Evaluation Techniques
in Complex Reservoirs enhances and improves the practice results of exploration
and evaluation techniques by GWDC International Logging Company. In several
technology fields including sequence stratigraphy, structure analysis, sedimentary
facies study, reservoir prediction, and subtle reservoir evaluation, this book sys-
tematically describes the technical application and practice achievements derived
from all kinds of complex reservoirs within clastic rocks, carbonate rocks and
metamorphic rocks, respectively. It is worthy to note that related results contained
in this book have been taken into practical application and generalization in the
Muglad Basin and Melut Basin, Bongor Basin, Agadem Basin, South Turgay
v
vi Foreword
The following organizations and oil companies are thanked for their support and
help during process of the studies implementation mentioned in this book:
China National Petroleum Corporation (CNPC)
Oil Exploration and Production General Administration (OEPA)
Ministry of Petroleum and Mining (MPM)
Greater Nile Petroleum Operating Company (GNPOC)
Petrodar Operating Company (PDOC)
Dar Petroleum Operating Company (DPOC)
China National Petroleum Corporation in Chad (CNPCIC)
China National Petroleum Corporation Niger Petroleum S. A. (CNPCNP)
China National Petroleum Corporation Halfaya (CNPC-Hafaya)
China National Petroleum Corporation Aidanmunai (CNPCADM)
China National Petroleum Corporation Aktobemunaigaz (CNPCAMG)
Thanks are given to the parent company of GWDC and CNLC, i.e., CNPC.
CNPC is always supporting big stages and opportunities for GWDC and CNLC to
develop mainly in the international market, aiming at a general contractor of
internationalization petrol engineering technology. Special thanks are given to our
colleagues in GWDC International Logging Company, who provide considerable
work and effort during the writing of this book, including:
Huaijiang Ran, You Zheng, and Jiapeng Wu participated in the writing of
Chaps. 1, 2, and 4 in this book.
Yang Li and Rutai Duan participated in the writing of Chaps. 1, 3, and 4 in this
book.
Guohui Ni, Xiaoquan Kang, and Haifeng Guo participated in the writing of
Chap. 3 in this book.
Tai Un Mei and Huizi Baomin participated in the writing of Chap. 2 and reference
in this book.
vii
Contents
ix
x Contents
Keywords Sporopollen assemblage Palynological stratigraphy High-resolution
sequence INPEFA method
Sequence stratigraphy theory is evolved from seismic stratigraphy; the theory was
early applied in sedimentary strata of passive continental marginal marine facies
(with simple structures), and then, it was applied in different basins to investigate
and explore the distribution regularity of different sedimentary filling systems. This
considerately enriches and develops sequence stratigraphy theory. Sequence
stratigraphy provides significant theories and methods for the establishment of
isochronous stratigraphic framework and the analysis of depositional systems tract
within a sequence stratigraphic framework and further provides important theo-
retical guide for efficient petroleum–gas prediction.
Sequence of strata, which was initially used as a stratigraphic unit with
unconformity interfaces boundary, has not been really developed until the
© Petroleum Industry Press and Springer Nature Singapore Pte Ltd. 2019 1
X. Cheng et al., Comprehensive Practice of Exploration and Evaluation
Techniques in Complex Reservoirs, https://doi.org/10.1007/978-981-13-6431-0_1
2 1 Comprehensive Practice of Sequence Stratigraphy Techniques …
exploration areas of lithostratigraphic oil and gas reservoirs (Van Wagoner et al.
1990; Weimer and Posamentier 1994; Wei et al. 1996; Lin et al. 2000; Li et al.
2002; Jiang et al. 2008). In terms of methodology, conventional outcrop, core,
logging, and high-precision seismic data can be applied; moreover, 3D seismic data
visualization technology, paleontology, geochemistry, numeric analysis, and com-
puter analog technology are available. The diversity of methods makes sequence
stratigraphy study more flexible and correct (Jiang 2010).
In recent years, high-resolution sequence stratigraphy theory has gotten wide
attention and made great progress. High-resolution sequence stratigraphic unit
mainly refers to fourth-and fifth-order (below third-order) sequence and systems
tract; fourth- and fifth-order sequences represent fourth- and fifth-order sedimentary
cycles within a third-order sequence (Lin et al. 2000). High-resolution sequence
stratigraphy study aims to establish finer isochronous sequence stratigraphy and
lithofacies frameworks, to finally predict reservoir distribution, reservoir–cap
combination, and others. High-resolution sequence stratigraphy is based on analysis
of combining outcrop, core, logging with high-resolution seismic data. In terms of
development trend, on the one hand, in-deep analysis and study will be conducted
on high-resolution sequence stratigraphy principles and methods, and controlling
factors of high-precision sequence stratigraphic units and sedimentary systems will
be positively explored; on the other hand, high-resolution computer analog tech-
nology and geophysical technology must be developed, so as to improve the pre-
diction of underground complex geological conditions (Lin et al. 2002).
High-resolution sequence stratigraphy study should take fourth-order sequences
division and the interior parasequence architecture style analysis as main objectives;
the relatively low-order regression and transgression interfaces present in different
systems tracts within a third-order sequence are the basis of division of
high-precision sequence stratigraphic units (Jiang 2010). Although different
scholars and “schools” have some different understandings on high-precision
sequence stratigraphy, their basic idea is to seek lower-order isochronal horizons.
Posamentier and Wagoner’s schemes of high-precision sequence unit division
contain lowstand, transgressive and highstand systems tracts, sequences are
bounded by relatively transgressive interfaces, and each fourth-order sequence
contains several parasequences. Theoretical study and practical exploration indicate
that Exxon’s sequence stratigraphic system division is more favorable for estab-
lishment of a sequence stratigraphy framework and prediction of advantageous
reservoir facies.
With the development of sequence stratigraphy theory and process of sedi-
mentation being discussed in the time–space framework of geological evolution, a
set of new methods will be gradually formed in the study of process of sedimen-
tation in an isochronal stratigraphic framework in combination with multi-cycle
evolution research, so as to provide significant tools for oil–gas exploration and
prediction (Lin 2009). Sequence stratigraphy will develop toward standardization of
research methods and specialization of application areas; sedimentary system study
will develop from macroscopic analysis toward microscopic depiction, from static
description toward dynamic simulation; moreover, sedimentation process
4 1 Comprehensive Practice of Sequence Stratigraphy Techniques …
Sequence is a genetic stratigraphic unit with unconformity interfaces and the cor-
respondent conformity interfaces as its boundaries; the key of sequence stratigraphy
analysis is to identify sequence boundary and to establish the isochronal strati-
graphic framework by tracing and correlating boundaries. As the upper and lower
boundaries of sequences commonly experienced cessation of deposition, sedi-
mentary facies transformation, etc., sequence boundaries show response charac-
teristics on core data, drilling logging curves, and 2D and 3D seismic profiles.
These characteristics from the data as mentioned above can be applied to identify
sequence boundaries. Besides, with deepening of sequence stratigraphy study,
technological methods that are applied will be continuously innovated; paleonto-
logical, geochemical, and other methods have played a great role in sequence
stratigraphy research.
1.1 Sequence Stratigraphy Study Overview 5
As seismic data has a large lateral identification range, high lateral resolution and
low longitudinal resolution, sequence boundaries can be identified more clearly, so
seismic data is mostly applied to identify high-order sequence boundaries.
Particularly, 3D seismic data can reflect stratigraphic structure and palogeomor-
phology in 3D space. Different termination types of seismic reflection events reflect
different termination and wedge-out types of strata. The denudation and wedge-out
information of strata reflected by seismic reflection events are rightly the main
indicators to identification of sequence boundaries on seismic profiles. Typical
identification indicators of seismic sequence boundary include onlap, truncation,
downlap, toplap, and other termination types of seismic reflection events. Certainly,
a major sedimentary transformation occurred, and thus, obvious difference in
seismic reflection characteristics exists between the upper and lower sequence
boundaries; this is also an indicator to identify seismic sequence boundaries. If an
intense tectonic movement occurred in the late stage of sedimentation and resulted
in a great variation of attitude of original strata, it would be more difficult to
discriminate some termination characteristics of seismic reflection characteristics,
e.g., onlap and downlap. Thus, it is required to analyze the location of provenance
and subsidence center as well as the tectonic movement of a basin and reconstruct
paleogeomorphologic characteristics during stratigraphic deposition. Seismic data
is characterized by large coverage and can reflect mutual contact relationship of
strata and macroscopic 3D shape of sedimentary bodies. Although vertical reso-
lution is lower in seismic data than that in outcrop and drilling logging data,
continuous seismic reflection of seismic data has stratigraphic significance in rel-
ative age, which provides good basis for establishing the chronostratigraphic
framework within a basin range.
Outcrops are the most direct, real and detailed data in sequence stratigraphy study
and have high-resolution characteristics that drilling and seismic data do not have;
and a series of characteristics that reflect sequences and sequence boundaries can be
directly observed on outcrops, and thus, some subjectivities produced from drill
hole, logging, and seismic data interpretation can be avoided. Considering cover-
age, discontinuity, and deformation by tectonic movement within the study area,
outcrops that contain complete strata can be continuously traced, are easily
observed, and are selected for field outcrop observation, layering, and measure-
ment; sequence boundaries, systems tract boundaries, and sedimentary facies
markers are collected for high-resolution sequence stratigraphic interpretation.
Fossil distribution and conservation conditions in strata are closely related to the
location and key boundaries in sequences. Some flora and specific sporopollen
assemblages were formed in some certain geological time and specific natural
conditions. Therefore, sporopollen assemblages can indicate the lithofacies paleo-
geography environment at that time. Loutit et al. (1992) believed that: detailed
biostratigraphic research on key sequence boundaries, key systems tracts, and key
positions not only can attain the result with half effort in correct identification,
division and correlation of sequences, and establishment of a sequence chrono-
logical framework, but also can extend key sequence boundaries outwards till to the
correspondent horizons/strata that contain very rare and even no fossils.
Compared to seismic data and well logging data, core data is reliable, direct and can
be easily identified, and has a higher longitudinal resolution than logging data,
which is a good tool to identify sequence boundaries. However, due to high cost of
1.1 Sequence Stratigraphy Study Overview 7
coring, small quantities of core, and other factors, core has a lower lateral resolution
in identifying sequence boundaries and basically can only reflect characteristics of
sequences at drilling location. In terms of core, sequence boundaries are identified
mainly according to exposure markers, abrupt changes of sedimentary facies and
lithofacies.
As sequence boundaries mostly are unconformity planes, the underlying for-
mations mostly exposed and suffered from long-term weathering denudation,
forming paleosol layers, paleoweathering crust and plant roots and stems, and other
weathering and exposure markers. However, above the sequence boundaries,
subsidence of deposition base level resulted in rivers rejuvenation, weathering
formations which were exposed formerly received deposition again, and a set of
coarse grained, highly mature lag deposits, which were in eroded and scoured
contact with the underlying strata, were formed. On the other hand, rapid change of
deposition base level of upper and lower sequence boundaries led to the abrupt
change of sedimentary environment of upper and lower strata, which is reflected by
core, namely abrupt change of lithofacies or sedimentary facies characteristics.
In recent years, new methods have been applied in sequence stratigraphy study,
including paleontology high-resolution sequence stratigraphy study, sample anal-
ysis and test and organic geochemistry research, 3D visualization, seismic intel-
lectual analysis, geostatistics, numerical stimulation and mode recognition and
others; these methods enrich and promote the development of sequence stratigraphy
theory and research methods. Besides, as modern sediments are the most direct
places for observation of geological phenomena, strengthening modern sediments
geological survey and research is of great significance to the promotion of sequence
stratigraphy research and development.
conservation status, etc. of strata very different, which also leads to the difficulty in
sequence boundary identification, sequence division, and correlation by conven-
tional technological methods. In order to resolve this problem, many new experi-
ments and technological methods have been gradually applied to sequence
stratigraphy study, e.g., sequence stratigraphic simulation, shelf-edge trajectory
quantitative analysis, and geochemical parameter analysis. As species, enrichment
degree, and combination characteristics of paleontology are different in different
development periods and are commonly closely related to the position and key
boundaries in sequence, paleontology provides good reference for determination of
stratigraphic age. In addition, with expanding of application range of sequence
stratigraphy, sequence stratigraphy study continuously deepens from basin-scale
sequence and sedimentary systems tract analysis to sedimentary microfacies and
reservoir-scale high-resolution sequence stratigraphic analysis; fine sedimentary
system, sedimentary facies analysis, and sands distribution prediction require a
high-resolution effective stratigraphic correlation framework.
On the basis of Exxon’s sequence stratigraphy theory and in combination with
sequence analysis cases of multiple basins, Lin et al. (2002) proposed a classifi-
cation scheme and analysis methodology for high-resolution sequence stratigraphic
framework. The high-resolution sequence stratigraphic framework refers to the
isochronal stratigraphic framework that is established by taking fourth- and
fifth-order sequences and systems tracts within a third-order sequence as strati-
graphic units. Fourth-order sequence is a basic stratigraphic unit of the
high-resolution sequence stratigraphic framework, whose sequence boundary can
be fourth-order sea (lake) level or transgression surface or regression surface of a
deposition base-level cycle. Sedimentary sequence research of Chinese continental
facies lake basins and shore-shallow sea basins indicates that the transgression
surface of a fourth-order sedimentary cycle can be traced and correlated within
basin range or most part of a basin; the identification is a key to establishment of the
high-resolution sequence stratigraphic framework. The fourth-order sequence can
be generally subdivided into several fifth-order sequences or parasequences with
sea or lake flooding planes as boundaries, and the general superimposition pattern
displays a sequence structure from progradation to retrogradation. In terrigenous
detrital basin filling, the fourth-order sequence represents one relatively obvious
sedimentary episode from advancing to recession then to transgression. This sed-
imentary episode or sedimentary cycle is controlled by synchronous variation of
regional sea level or climate, etc., which is of “other cycle” that is not directly
related to sedimentary process itself. The fifth-order sequence represents a single
sedimentation from progradation to retrogradation; these sedimentary cycles might
be direct products of sedimentary “autogenetic cycles” like river migration or delta
abandoning. Establishment of the high-resolution sequence stratigraphic framework
can provide significant reference for analysis and prediction of reservoir–cap
combination; sandstone sedimentary bodies within the fourth-order sequence and
argillaceous sediments during transgression period constitute a reservoir unit and
trap cover.
1.2 Sequence Stratigraphy Application in Clastic Formation 9
Muglad Basin, the largest rift basin discovered in Sudan, is located in the middle of
African Plate, inside Sudan Republic. Its total area is about 12 104 km2, and
hydrocarbon discovery is also the most among the rift basins of central African
Plate. Oil and gas exploration in Muglad Basin began in the 1970s, Chevron Corp.,
SPC company, GNPOC, etc., carried out oil exploration within the basin and has
discovered oil fields in the northern and southern parts of the basin. The oil dis-
covered in the basin now is more than 14 108 t. The structural belt in strike is
mainly in NW-SE, and the formations are thin in the west and thick in the east. It is
mid-Cenozoic rift basin, bounded by East African fault belt in the northwest, in
WN-SE strike, narrow in the south, and wide in the north. It contains four
depressions, three uplifts, and some small sags (Fig. 1.1). The study area is on the
western slope of B-Z area of Muglad Basin, located on the trend belt of Kaikang
Depression (also called Abyei Slope Belt) (Figs. 1.1 and 1.2). To the east side of
the study area is Kaikang Depression, to the northwest is HS oil field, and the
exploration area is about 5500 km2. Since December 2009, the total recoverable oil
of the district has been proven up to 10.55 billion barrels, HS oil field, D oil field,
and B oil field are as the main producing areas, and the main producing layers are
Bentiu and Aradeiba Formations of Cretaceous.
10 1 Comprehensive Practice of Sequence Stratigraphy Techniques …
The study area is B-Z area on the western slope of Muglad Basin, located on the
trend belt of Kaikang Depression (also called Abyei Slope Belt). The basement of
the basin is Precambrian granite and granodiorite. In early Cretaceous period,
Muglad Basin began to depress and receive deposition. The remnants of basin
filling strata is about 5000 m, thinner than the formation thickness of the abdomen
of Muglad Basin which is more than ten thousand meters thick. The basin is located
high due to its location, so in each structural conversion period, it was eroded more
seriously.
1.2 Sequence Stratigraphy Application in Clastic Formation 11
Fig. 1.2 Location map of the western slope in B-Z area of Muglad Basin, Sudan
The research strata in this study is from Cretaceous to Paleogene, which are Abu
Gabra Formation, Bentiu Formation and Darfur Group (Aradeiba Formation, Zarqa
Formation, Ghazal Formation, and Baraka Formation) of Cretaceous, and Amal
Formation, Nayil Formation, and Tendi Formation of Paleogene from bottom-up.
The deposition period of Abu Gabra Formation corresponds to the movement
period of initial rift structure, and the deposition period of Bentiu Formation cor-
responds to thermal subsidence stage after integration of the basin; the deposition
period of Darfur Group corresponds to the second rift movement stage; the depo-
sition period of Amal Formation corresponds to the second thermal subsidence
stage; the deposition period of Nayil Formation corresponds to the third rift stage,
and the deposition period of Tendi–Adok Formation corresponds to the third
thermal subsidence stage. Tectonic background of different periods has close
relationship with the sedimentary facies types of each period.
The Abu Gabra Formation of Cretaceous is the first depositional layer in the
study area, developed at the faulting stage of the first rift valley period, overlying
the Precambrian basement as angular unconformity. Due to the effect of fracturing
motion, the depositional thickness in different areas varies greatly; this formation
had structural reversal at the later depositional stage, and its remaining thickness in
some regions is very small, even absent at the edge of the basin. The Abu Gabra
Formation in the east of this study area and the thrown side is up to 2000 m thick;
the west part and upthrown block is only about 500 m due to severe denudation.
The lower part is mainly sand–mud interbeds, the middle part is mainly large suite
of pure mudstone, and the upper part is severely denudated, and sandstone content
is more than that in the middle part. This formation in the study area is mainly
deltaic deposition, and at the center of the sag, there developed shore-shallow lake.
The Bentiu Formation was developed in late stage of early Cretaceous, overlying
Abu Gabra Formation as regional unconformity; it is the most widely distributed
formation in the whole basin and is divided into upper part and lower part. In the
12 1 Comprehensive Practice of Sequence Stratigraphy Techniques …
northwest of the study area, the thickness of upper remaining formation is 1400 m,
and the remaining thickness in the western edge and southwest is about 700 m. The
lower section of Bentiu is about 700 m thick, and the thickness of some faulted
highs is about 500 m; the northwest and northeast parts of the study area are
evidently controlled by fracture, and the deposition thickness can be up to 1000 m,
mainly interbeds of thin sandstone layers in superimposition. The upper part of
Bentiu Formation is composed of thick layers of sandstone interbedded with silt-
stone and mudstone, reflecting a braided river environment of high energy, instable
channel and large width-to-depth ratio; channel sand body (including channel bar)
is the major reservoirs of Block X/Y/Z. These two parts do not have evident proof
of unconformity or depositional break between them, just that the lake level sub-
sided and braided river deposition advanced to the center of the lake basin, so the
lower braided river delta and lake facies developed on the slope gradually evolved
into the upper braided river–braided river delta deposition.
The Darfur Group was developed in late Cretaceous and created with the control
of the faulting in the second rift period; the tectonic motion of this period was
weaker than that of the previous faulting mobility, so the differences of formation
thickness of footwall and hanging wall are small; the hanging wall formation
thickness is about 800 m, and the footwall thickness is about 1600 m; the thickness
of Darfur Group developed near Kaikang Sag in the east is up to 2800 m. The
Darfur Group Aradeiba Formation locally onlaps above Bentiu top interface, and
the onlap angle is low. In the study area, braided river delta facies and
shore-shallow lake subfacies mainly developed, generally in cycle of sedimentation
becoming thick upward.
Well logging data shows that Aradeiba Formation has high silt content and is a
favorable regional cap rock in this region, forming the best source–reservoir–cap
rock assemblage in the whole block together with Gabra–Bentiu–Aradeiba
Formations. The average thickness of Cretaceous Aradeiba Formation is about
1000 m, overlying the Bentiu Formation as parallel unconformity; the thickness
may be up to 1800 m near the center of Kaikang Depression and local faulted
places.
Zarqa Formation’s bottom is in conformity contact with underlying Aradeiba
Formation. It is about 250–500 m in thickness, mainly braided deltaic sand and
shore-shallow lake subfacies; formation distribution is relatively stable.
A stable mudstone has developed on top of Ghazal Formation; the sandstone
demarcation at the bottom of Baraka Formation is relatively apparent. This for-
mation is mainly interbedded and successive braided river delta medium-coarse
sandstone and sandy mudstone of different thicknesses.
The upper section of Baraka Formation is mainly sandstone, containing small
amount of mudstone; the lower section has high content of silt and forms another
source layer with mudstone at the top of underlying Ghazal Formation. The for-
mation thickness of this formation is small, about 300 m at the northwestern
boundary zone and about 500 m at the southwestern boundary zone, a little bit thick
in the east. This formation changed gradually from large delta front subfacies to
delta plain subfacies.
1.2 Sequence Stratigraphy Application in Clastic Formation 13
river delta and shore-shallow lake, and its plane spread is mainly controlled by
rising and decreasing of lake level. The lithology of formations within Darfur Group
tends to be fine, widely seen within the area, such as Aradeiba Formation, forming
the best cap rock overlying the source–reservoir unit of the first rift stage. Also, a
number of thin layers of sandstone and mudstone assemblages have developed
within Darfur Group, and each formation has oil–gas shows, so a number of
self-generated and self-stored oil–gas reservoirs have been formed.
Starting from Paleogene, the faulting motion of second rift period of the Muglad
Basin completed, and the faulted basin was transformed to extension depression
basin; affected by regional thermal subsidence, the Amal Formation was deposited.
The basin structures were stable in this period, so large fluvial sandstone was
deposited in Amal Formation, in box-like shape as shown in well logs; this for-
mation becomes another favorable reservoir stratum.
Due to the expansion of Red Sea and volcanic eruption of Mid-African rift, the
second successive extension motion ended, and the basin entered the third exten-
sion depression stage and subsided as a whole. The study area deposited Nayil
Formation mainly composed of dark mudstone of sand–mud interbedding. After
that, the entire structures of the basin were relatively stable, and the third thermal
subsidence stage began, depositing the Tendi Formation mainly composed of
deltaic sandy mudstone of lake–land transitional phase and Adok Formation mainly
composed of fluvial sandstone.
The studied 204 palynological samples (including nine core samples and 195 ditch
cutting samples) were collected from 16 wells. The locations of the wells and the
stratigraphic level and sampling depth of the samples are shown in Figs. 1.3 and
1.4, respectively. Generally, the samples were collected at a 5.0 m interval for
cutting samples.
Standard preparation procedures for palynological study, such as the chemical
solution, heavy fluid flotation, slide fixation, and identification, were carried out
(Fig. 1.5). Palynological fixed slides were made for each sample. A quantitative
method was employed for this study. Using variation of the proportions of the
palynofloral components, evolution trends of the sporopollens were identified, and
the palynozone were established, by which one could define meaningful correlative
units/horizons, identify stratigraphic gaps and environmental changes. Identification
and photography of sporopollens were carried on using a Leica D4000B micro-
scope attached with a SPOT-FLEX digital camera system. At least three to four
slides (size of the cover slides were 15 mm 15 mm) were identified for each
1.2 Sequence Stratigraphy Application in Clastic Formation 15
sample, the first 50–100 palynomorphs were counted and labeled as dominant
(>30%), abundant (11–30%), common (5–10%) or rare (<2%). Thereby, a rough
estimation of the relative frequency of each specie in each sample could be made. In
addition, one slide of each unsieved residue was examined for supplemental
palynofacies identification. Species with well-known stratigraphic ranges (Figs. 1.6
and 1.7) in other contemporaneous basins of West Africa and Egypt were used as
key elements (or index species) for demarcating the palynostratigraphic zones.
16 1 Comprehensive Practice of Sequence Stratigraphy Techniques …
Fig. 1.4 Depth and formation of the samples from the wells
1.2 Sequence Stratigraphy Application in Clastic Formation 17
A total of 204 samples were processed, and among which 129 samples occupied
sporopollens, which can be categorized into five main palynological groups,
including spores of fungi, algae and ferns, and pollens of angiosperms and gym-
nosperms. In total were found 137 genera and 138 known species of spore and
pollen including 59 genera and 49 known species of fern spores, 16 genera and 25
known species of gymnosperms, and most abundantly, 62 genera and 64 known
species of angiosperms, plus spores of three genera of green algae or other
microphytoplanktons.
According to the range and frequency distribution of palynomorph species, ten
palynozones have been differentiated, corresponding to the ten lithostratigraphic
units in the Muglad Basin (Fig. 1.8). Each zone has its characteristic palynological
assemblage and dominated palynomorphs. They range in age is from Neocomian of
early Cretaceous to Miocene. The ages assigned to these palynozones have been
determined by comparing the microfloral assemblages with those known from
previously dated sequences.
(1) Zone I (Abu Graba Formation): Appendicisporites spp.–Exesipollenites
tumulus–Cycadopites spp. Assemblage Zone (Representative wells Sb-1 and
HW-1, depth 3465–3470 m and 3695–3905 m, respectively). Characteristics
of the assemblage:
18 1 Comprehensive Practice of Sequence Stratigraphy Techniques …
Fig. 1.6 Key Paleogene sporopollens from the Muglad Basin of Sudan
Fig. 1.7 Key Cretaceous sporopollens from the Muglad Basin of Sudan
Fig. 1.8 Lithostratigraphy and palynozones in the western slope of B-Z area