Journal of Sciences Well Logs – 3 D Seismic Sequence ... · PDF fileVolume 4 No. 2,...

Volume 4 No. 2, February 2014 ISSN 2224-3577

International Journal of Science and Technology

©2014 IJST. All rights reserved

http://www.ejournalofsciences.org

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Well Logs – 3 D Seismic Sequence Stratigraphy Evaluation of “Holu” Field,

Niger Delta, Nigeria

John O. Amigun1, Olumide Adewoye1, Temitope Olowolafe1 and Emmanuel Okwoli2 1Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria.

2Department of Physics, Kogi State University, Anyigba, Nigeria.

ABSTRACT

Three Wells, check-shot and 3-D seismic data were used to evaluate the lithology, lithofacies, sequence stratigraphy, seismic facies and

depositional environments of “Holu” field, Niger delta. Its well logs sequence stratigraphic analysis revealed five depositional sequences

with associated six sequence boundaries (SB1, SB2, SB3, SB4, SB5 and SB6 ) occur at respective depths of (1905 m, 1680 m, 1550 m,

1385 m, 1345 m, and 1275 m), five maximum flooding surfaces (MFS1, MFS2, MFS3, MFS4 and MFS5) each associated with depositional

sequences at respective depths of (1755 m, 1575 m, 1405 m, 1360 m and 1305 m) and five system tracts. The 1 st depositional sequence

consist of two system tracts (TST and HST), 2nd depositional sequence is made up of three system tracts (LST, TST and HST), 3rd

depositional sequence consist of two system tracts (TST and HST), 4th depositional sequence is made up of three system tracts (LST, TST

and HST) and the 5th depositional sequence also possessed three system tracts (LST, TST and HST). The well logs also revealed dominant

lithologies (sand and shale). Log facies shows heamipelagic shale, marine shale, fluvial channel fill sands, transgressive sands and crevasse

splay sands. Characteristics seismic facies involving amplitude, continuity, frequency and reflection configuration deduced “Holu” field

environment of deposition to be fluvial systems to marine environments.

Keywords: seismic sequence stratigraphy; system tracts; lithofacies; Niger delta; well logs and seismic facies.

1. INTRODUCTION

It has been observed over the years that hydrocarbon exploration

and exploitation attention has been on structural traps. At present

most of the identified structural closures on the shelf and upper

slope have been drilled and the search for hydrocarbon is

becoming increasingly more difficult and expensive [1]. In a

country like Nigeria where oil has been the backbone of her

economy, combined geophysical well logs and 3-D seismic

stratigraphy approach has not been a doubt an effective

exploration tools to delineate lithology, lithofacies, sequence

stratigraphy, depositional environment and hydrocarbon

reservoirs.

Sequence stratigraphy tremendous ability to decipher the earth’s

geological record of local to global changes has help greatly to

improve the predictive aspect of hydrocarbon economic

exploration and production. It analyses the sedimentary response

to changes in sea level and the depositional trends that emerge

from the interplay of accommodation i.e. space available for

sediments to fill as well as sedimentation [2].

The success and recognition of sequence stratigraphy stems from

its applicability in both mature and frontier hydrocarbon

exploration basins, where data-driven and model-driven

predictions of lateral and vertical facies changes can be

formulated, respectively.

Therefore in this study, existing well logs and 3-D seismic data

were analyzed and interpreted using the predictive models

capability of sequence stratigraphy which has proven to be

effective in reservoir characterization and in reducing lithology-

prediction risk for hydrocarbon exploration in order to evaluate

the “Holu” field.

2. LOCATION AND GEOLOGY OF THE

STUDY AREA

“Holu” field is located within onshore Niger Delta, Nigeria

(Figure 1). The Niger Delta is situated in the Gulf of Guinea and

extends throughout the Niger Delta Province. It is located in the

southern part of Nigeria between the longitude 40– 90 East and

latitude 40– 60 North. From the Eocene to the present, the delta

has prograded southwestward, forming depobelts that represent

the most active portion of the delta at each stage of its

development. These depobelts form one of the largest regressive

deltas in the world with an area of some 300,000km2, a sediment

volume of 500,000 km3 and a sediment thickness of over 10 km

in the basin depocenter.

Niger Delta is divided into three formations, (Figure 2)

representing prograding depositional facies that are distinguished

mostly on the basis of sand-shale ratios [3] [4].

The Benin formation is the uppermost unit, it consists of massive

freshwater bearing continental sands and gravel deposited in an

upper deltaic plain environment and extends from the west across

the whole Niger Delta area and southward beyond the existing

coastline. The thickness of the formation ranges from 305 m in

the offshore to 3050 m onshore.

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The Agbada formation forms the hydrocarbon-prospective

sequence in the Niger Delta [5]. It is composed of sands, silts and

shales in various proportions and thicknesses, representing cyclic

sequences of off-lap units. It reaches a maximum thickness of

more than 3050 m.

The Akata formation composed of shales and silts at the base of

the known delta sequence. They contain a few streaks of sand,

possibly of turbiditic origin and were deposited in holomarine

environment. The thickness of this sequence is not accurately

known; but may reach 7000 m in the central part of the delta.

Figure 1: Location and Base Map of the Study Area showing Seismic Lines and Wells

Figure 2: Niger Delta Stratigraphy. (Modified from [3] [4]).






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3. METHODOLOGY

Wireline logs (gamma ray, resistivity, density and neutron), 3D

seismic data and check-shot data were examined, analysed and

interpreted following a procedure in line with the specific

objectives of this study.

3.1 Well logs Analysis

Well logs which represent the geophysical recordings of various

rock properties in boreholes were employed in this study for

facies analyses i.e. lithostratigraphy Identification and sequence

stratigraphy.

Lithostratigraphy Identification

Lithostratigraphy involves the correlation of similar lithology

that are commonly diachronous. Gamma ray (GR) log was used

to delineate the lithology in the study area (Sand and shale

bodies). The sand bodies were identified by the deflection to the

left of the GR log due to the low concentration of radioactive

minerals in sand while deflection to the right signifies shale

which is as a result of high concentration of radioactive minerals

in it. Conventionally, GR log is set to a scale of 0-150 API,

central cut off of 65 API units in which less than 65 API is

interpreted as sand while greater than 65 API is interpreted as

shale.

Well log Sequence Stratigraphy

Well logs allow the identification of stratigraphic sequences by

analysing the stacking patterns of the genetic units. The strata

patterns in the sedimentary record of an area are the results of

tectonics, eustasy and climate hence, stratigraphic surfaces

which signify depositional changes become the key to

establishing sequence stratigraphic units of such study area.

A depositional sequence is the basic unit of sequence stratigraphy

which can be explained as a relatively conformable succession of

genetically related strata bounded by unconformities or their

correlative conformities. According to [6] [7], in vertical

succession all depositional sequences are composed of the

following elements in this order: sequence boundary; lowstand

systems tract; transgressive surface; transgressive systems tract;

maximum flooding surface; highstand systems tract.

The concept of systems tract as applied in this study; define a

linkage of contemporaneous depositional systems forming the

subdivision of a sequence [8]. The interpretation is based on

strata stacking patterns, position within the sequence and types

of bounding surfaces. The timing of systems tracts is inferred

relative to a curve that describes the base-level fluctuations at the

shoreline. In a summary, Table 1 describes the various forms of

depositional sequence elements of system tract.

From the explanation in Table 1, log patterns are therefore

diverse and generally indicative of changing energy regimes

through time. According to [9] [10], the ranges of log motifs

related to different environment of deposition are shown in (Fig.

3).

Hence for “Holu” field well log analysis carried out in this study,

sequence boundaries, maximum flooding surfaces and the

system tracts were delineated based on the stacking patterns and

log motif and as well as lithologic correlation within the three

wells i.e. holu 1, holu 2 and holu 3 shown in Figure 4 and 5.

Table 1 : The Depositional Sequence Elements of System Tract and their Description






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3.2 3 D Seismic Interpretation

To ensure the continuity of events both on the seismic section

and well log, a well to seismic tie was done using check shot data.

In Figure 6, wells holu 2 and 3 with the sequence boundaries

were tied and displayed on the seismic section. The sequence

boundaries identified on the wells were mapped directly on the

seismic section i.e. where surface is identified by distinctive

reflection pattern observed over a layer with relatively large

extent (Figure 7). Also, three major faults observed from the

seismic section were mapped as well.

Seismic Stratigraphy Interpretation

The concept of seismic stratigraphy is the deduction of

stratigraphy and depositional facies from seismic data. In its

application, seismic reflection terminations and configurations

are interpreted as stratification patterns, and are used for

recognition and correlation of depositional sequences,

interpretation of depositional environment and estimation of

lithofacies. Seismic sequence analysis subdivides the seismic

section into enclosures of concordant reflections, which are

separated by surfaces of discontinuity defined by systematic

reflection terminations. These enclosures of concordant

reflection (seismic sequences) are interpreted as depositional

sequences consisting of genetically related strata and bounded at

their top and base by unconformities or their correlative

conformities. As described in Figure 8(ia), reflection

terminations interpreted as strata terminations include erosional

truncation, toplap, onlap, and downlap [7]. Afterward the

definition of seismic sequences, the environment and lithofacies

within the sequences are interpreted from both seismic and

geologic data of the study area.

Seismic facies analysis which is a geologic interpretation of

seismic reflection parameters i.e. configuration, continuity,

amplitude, frequency, and interval velocity was further carried

out. It involved the recognition of the seismic facies units,

definition of their limits and mapping of their areal associations.

They are interpreted to express certain stratification, lithologic

and depositional features of the deposits that generated the

reflections within the units. Major units of reflection

configurations and that of prograding configurations are

described in Figure 8(ib) and 8(ic) respectively [7].

Figure 3: The Different Types of Log Motif as related to the Environments of Deposition






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Figure 4: Well Log Sequence Stratigraphy showing the Delineated Lithology, Systems tract and Lithofacies.






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Figure 6: seismic-well tie using check shot data at Inline 11754

Figure 5: Well Log Sequence Statigraphy showing the Stacking Patterns within holu 1 and 3






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Figure 8: (ia) Seismic stratigraphic reflection terminations within idealized seismic sequence.

(ib) Various seismic reflection configurations and modifications. (ic) Seismic reflection patterns interpreted

as prograding clinoforms [7].

(ii) seismic facies units based on amplitude, frequency, continuity and reflection geometry [13].

Figure 7: Seismic Section showing the Sequences and the associate Sequence Boundaries .

Agbada formation

Akata formation

Benin formation






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4. RESULTS AND DISCUSSION

The lithostratigraphic interpretation of well logs shows that the

lithology within the area of study is mainly sands and shale. In

Figure 4, the well logs sequence stratigraphy analysis revealed

five depositional sequences with associated six sequence

boundaries namely; SB1, SB2, SB3, SB4, SB5 and SB6 which

occur at respective depths of 1905 m, 1680 m, 1550 m, 1385

m, 1345 m, and 1275 m.

Also in Figure 4, five maximum flooding surfaces (MFS1,

MFS2, MFS3, MFS4 and MFS5) each associated with

depositional sequences were delineated at their respective depths

of (1755 m, 1575 m, 1405 m, 1360 m and 1305 m). The first

depositional sequence consist of two system tracts (TST and

HST), second depositional sequence is made up of three system

tracts (LST, TST and HST), third depositional sequence consist

of two system tracts (TST and HST), fourth depositional

sequence had three system tracts (LST, TST and HST) and the

fifth depositional sequence also possessed three system tracts

(LST, TST and HST). Figure 5 shows the stacking patterns

within the wells, this is interpreted as the depositional patterns of

the sediments where MFS represents the turning point from

finning upward into coarsening upward.

Lithofacies analysis was done across the three wells (Figure 4);

log motifs described in Figure 3 were used to classify the facies.

The following lithofacies namely; fluvial channel fill sands,

marine shale, heamipelagic shale, transgressive sands and

crevasse splay sands were revealed. Its detail across the three

wells is summarised in Table 2. The depositional environments

were mapped across the wells using log signatures and stacking

patterns the results were summarised also in Table 2. The

environment of deposition varies from one depositional sequence

to another i.e. from the top is fluvial / shore face to fluvial /

estuarine at the bottom.

In seismic sequence stratigraphy, seismic reflection is inferred to

represent an isochronous surface except where the reflection

surface is an unconformity identified by toplap, baselap, onlap or

truncation. In this study, the depositional sequences mapped

from the wells were also mapped on the seismic section (Figure

9). These depositional sequences are surfaces of principal

unconformities within the basin and the boundaries associated

with them are erosional boundaries. The Seismic facies analysis

involves critical recognition of reflection characteristics such as

continuity, amplitude, frequency and configuration (Figure 10).

The Figures 9 and 10 show seismic facies analysis where seismic

reflection configurations reveal gross stratification patterns from

which lithology, depositional processes and environments were

interpreted. Table 3 summarized the result of the facies analysis

carried out.

Figure 9: seismic interpretation showing depositional sequences and associated seismic facies

.

Benin formation

Agbada formation

Akata formation






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Table 2 : Sequence Stratigraphy, Depositionan Environments and Lithofacies Analysis from Wells

Figure 10: seismic facies characteristics for seismic facies interpretation






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Table 3: Summary of Seismic Facies Analysis

5. CONCLUSION

Five depositional sequences within “Holu” field Niger Delta

have been mapped both on well logs and 3D seismic data using

sequence stratigraphy approach. Well logs revealed dominant

lithologies (sand and shale). Log facies shows heamipelagic

shale, marine shale, fluvial channel fill sands, transgressive sands

and crevasse splay sands. Majority of the sand bodies are those

of the channel-fills. These are sand deposits occurring

underneath the paralic sandy sediments of Agbada formation.

These same sands corresponded to the lowstand system tract.

Similarly majority of shale body are those of marine shale /

heamipelagic shale and they fall within late to early HST and late

TST of Agbada formation. Characteristics seismic facies

involving amplitude, continuity, frequency and reflection

configuration deduced the environment of deposition to be

fluvial systems to marine environments

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