integration of sedimentological and spectral data …

N. Asadu et al., IJSIT, 2015, 4(3), 263-274

IJSIT (www.ijsit.com), Volume 4, Issue 3, May-June 2015

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INTEGRATION OF SEDIMENTOLOGICAL AND SPECTRAL DATA FOR

DELINEATION AND CHARACTERIZATION OF LITHOFACIES UNITS IN

WELL X, OFFSHORE NIGER DELTA

N. Asadu1, O.O Omo-Irabor2 and W.N Ofuyah3

1,2,3 Department of Earth Sciences, Federal University of Petroleum Resources, Effurun, Nigeria.

ABSTRACT

Delineating depositional discontinuities from borehole data using the conventional wireline log

analysis is a complex and non-linear problem owing to the noisy nature of these signals. These noise results

from several factors such as pore fluid, effective pressure, fluid saturation, and pore shape etc. It is therefore,

necessary to search for a suitable non-linear method, which could evade these problems. This necessitated

the transformation of adopted recorded lithologic log (gamma ray log) in time domain(amplitude) to

frequency domain and the associated frequency attributes such as response phase and response frequency

which were computed and plotted to obtain low noise signals in order to delineate lithofacies boundaries

with precision. These plotted pseudo-well log attribute sections delineated six lithofacies units named with

the dominant lithology and the response phase attribute was particularly used for this segmentation.

Integration of sedimentological information from cutting rock samples; petrophysical information from

gamma ray and resistivity log in time domain and the response frequency attribute show that the rock

succession is characterized by the alternation of sand and shale of variable thicknesses with a lateral

gradation in particle size typical of a prograding delta recognizing the three lithostratigraphic units in Niger

delta.The lithofacies association include shales, sandy shales, shaly sand, and sands with grain sizes ranging

from fine to medium, coarse sand and conglomerates at the top.

Keywords: lithofacies, Agbada formation, sedimentology, gamma ray, frequency domain, phase spectra



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INTRODUCTION

This research involved the sedimentological analysis of well rock samples and the spectral

decomposition of the lithologic log (gamma ray log) of a hydrocarbon exploratory well from offshore Niger

delta. The of this research work is to map the lithofacies units using spectral decomposition of gamma ray log

in time domain to frequency domain and compute the associated frequency attributes such as magnitude,

response frequency and phase using fast Fourier transform method in order to delineate lithofacies units and

to study the lithologic characteristics and integrate with the petrophysical information from gamma ray,

resistivity and computed pseudo-logs in frequency domain for the characterization of the lithofacies units

delineated. The location of the study Area is offshore Niger Delta, Nigeria. Figure 3

Niger delta stratigraphy: The Tertiary Niger delta complex is divided into three formations, representing

prograding depositional facies that are distinguished mostly on the basis of sand-shale ratios. They are the

Akata, Agbada and Benin Formations. The type sections of these formations have been reviewed as described

in Short and Stäuble (1967) and summarized in a variety of papers (Avbobvo, 1978; Doust and Omatola,

1990; Kulke, 1995 etc).

The Akata Formation which underlies the entire delta is composed of thick shale sequences

(potential source rock), turbidite sand (potential reservoirs in deep water), and minor amounts of clay and

silt. It is of marine origin and formed during Lowstand when terrestrial organic matter and clays were

transported to deep water areas characterized by low energy conditions and oxygen deficiency. Turbidity

currents likely deposited deep sea fan sands within the upper Akata Formation during development of the

delta. This formation is characteristically over pressured and range in age from the Paleocene to Recent.

The overlying Agbada Formation consists of paralic siliciclastic sequences over 3700 meters thick

and represents the actual deltaic portion of the sequence. The clastics accumulated in delta-front, delta-

topsets, and fluvial-deltaic environments. In the lower Agbada Formation, shale and sandstone beds were

deposited in equal proportions, however, the upper portion is mostly sand with only minor shale interbeds.

This formation is the major petroleum bearing unit and deposition started in the Eocene and continues into

the recent.

The Benin Formation, a continental latest Eocene to Recent deposit of alluvial and upper coastal plain

sands that are up to 2000 m thick. This is the freshwater bearing formation in the Niger delta (figure1).



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METHODOLOGY

(i) Mapping of Depositional Sequences: This was achieved by the spectral decomposition of the selected

lithologic log (gamma ray) in time domain (amplitude) to frequency domain and associated frequency

attributes such as response phase and response frequency using fast Fourier transform (FFT) method to

achieve the discrete Fourier transform (DFT) for computational savings within MATLAB software. The FFT is

an efficient algorithm useful in signal and image processing for filtering, convolution and frequency analysis

to power spectrum estimation. This method is in accordance with Oppenheim et al., 1999, Ligges et al., 2002

and Soliman et al., 2003. The theory behind spectral decomposition is that a reflection from a thin bed has

characteristic expression in the frequency domain that is indicative of temporal bed thickness (Amplitude

Spectra) Partyka (1999).

This amplitude spectrum interference pattern from a tuned reflection defines the relationship

between acoustic properties of the individual beds that comprise the reflection. Amplitude spectra delineate

thin bed variability via spectra notching patterns, which are related to local rock mass variability. Likewise

phase spectra respond to lateral discontinuities (lithofacies boundaries) via local phase instability (Partyka et

al, 1999). The representative pseudo-well log attribute sections (magnitude, frequency and phase) were

computed and plotted with surfer 8 software. These pseudo-sections give distinct lithofacies compartments

with each of the boundaries defining the limits of change in the depositional trend. The plot of the computed

phase attribute values was particularly used for the purpose of the segmentation of the sedimentary fills with

each segment corresponding with a lithofacies unit.

Fourier transform procedure:

Given a function f (t) of a single variable, t, the discrete Fourier Transform (DFT) is the digital

equivalent of the continuous Fourier transform and is expressed as

f (w ) =

w

t

f(t) exp (-iwt) (1)

F (w) =Fr (w) + iFi (w) (2)

A (w) = [Fr2 (w) +Fi

2 (w)] 1/2 (3)

)(

)(tan)(

1

wF

wFw

r

i (4) (Yilmaz, 2001)

Where A (w) and (w) are the amplitude and phase spectra respectively.

(ii) Sedimentological Analysis of the Rock Samples: A Total Of Three hundred and twenty-four (324)



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ditch cutting rock samples made available for this study were analyzed texturally and lithologically.

Variations in the gamma ray log signatures were used in differentiating the lithologic units with high

gamma ray log values depicting shale units, while low gamma ray values corresponds with sandy units. The

textural analysis was made by viewing these samples under the microscope with a grain size

comparator in order to identify the different rock types penetrated by the wells and its variability within

succession. Also the cross-plots of resistivity versus gamma ray log were used to show the facies

associations present in the lithofacies units delineated.

RESULTS AND DISCUSSIONS

The results of the spectral decomposition of the gamma ray log yielded frequency (pseudo) logs that

reveal six lithofacies units (Figure 1).The phase attribute was used for the delineation of the lithofacies units.

Integration with the lithologic log recognized the seventh lithofacies unit (figure 8). Both the sedimentological

results and petrophysical information from both well logs and pseudo- logs were utilized for the

characterization of the lithofacies units. The low frequency bandwidth of (0 – 65Hz) is associated with sandy

units while the high frequency bandwidth of (65 – 100Hz) represents the shaly intervals (Sheriff, 1973). The

cross plots of resistivity versus gamma ray log reveal facies associations ranging from shales to sandy shales,

shaly sand and sands (Figures 4 to 7). The lithofacies units are characterized as follows:

The Litholog of the well is presented in Figure 1. The rock succession is characterized by the

alternation of sand and shale of variable thicknesses with a lateral gradation in particle size (coarsening

upward sequence) typical of a prograding delta.The lithofacies association include shales, sandy shales, shaly

sand, and sands with grain sizes ranging from fine to medium, coarse sand and conglomerates at the top

(Figure 1).

(a) Lithofacies 1: The black shale and sand facies: This facies is characterized by thick black shales with

alternation of medium grained, fairly well sorted sands with appreciable thickness of 720ft (ranging from the

base of the well, 12920ft to 12200ft (Figure 8). The gamma ray value ranges from 31 to 128API; frequency

ranges from 85to 95Hz, while resistivity ranges from 2 to as high as 45 Ohm-m. The plot of resistivity versus

gamma ray values also shows that the facies in association include clean shale (potential source rock), sandy

shale, shaly sand and clean sand facies (figure 4).

This lithofacies unit is part of the upper Akata formation. The sand at the base is believed to be

turbidite sand (potential reservoir rock). Turbidity currents deposited this deep sea fan sands within the

upper Akata Formation during development of the delta.

(b) Lithofacies, 2: the black shale facies: this facies unit range from 12200ft to 11500ft (Figure 8). The

Gamma ray value ranges from 89 to 127 API; frequency ranges from 75 to 85 Hz, while resistivity ranges from

1 to 3 Ohm-m. Plot of resistivity versus gamma ray log values confirms that the unit is predominantly a shale



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facies (potential source rock. This facies unit is the top of Akata Formation in this well.

(c) Lithofacies 3: Grey shale facies: this facies unit range from 11500ft to 10,000ft (Figure 8). The gamma

ray value ranges from 91 to 127API; frequency ranges from 55to 75Hz, while resistivity ranges from 1 to 2

Ohm-m. Plot of resistivity versus gamma ray log values shows that the unit is predominantly a clean shale

facies with an intercalation of sandy shale (Figure 6). This represents the base of the Agbada formation in this

well with a notable increase in sandiness from the lower facies unit.

(d) Lithofacies 4: Grey sandy shale facies: this facies unit ranges from 11000ft to 9800ft). The Gamma ray

value ranges from 33 to 137 API; frequency ranges from 35 to 55 Hz, while resistivity ranges from 0.6 to 4

Ohm-m. Plot of resistivity versus gamma ray log values shows that the facies in association are also shales

and sandy shales (Figure 7). Facies 3 and 4 represent the lower Agbada Formation deposited in an outer

neritic environment.



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Figure 1: Litholog of well A.

(e) Lithofacies 5: Sandy shale facies: lithologically, this facies comprise of shaly sand alternating with

medium to coarse grained rounded to sub-rounded sand with siltstones at intervals . This facies unit range



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from 8200 to 7200ft. The Gamma ray values range from 32 to 143 API; frequency ranges from 20 to 35 Hz,

while resistivity ranges from 0.6 to 3.0 Ohm-m. Plot of resistivity versus gamma ray log values shows that the

facies in association are shales, sandy shale, and shaly sand and sand facies. This facies is the beginning of the

main paralic interval of the Agbada Formation (the upper Agbada Formation) with high sand percentage.

(f) Lithofacies 6: the shaly sand facies: The lithologic characteristics include black shales with intercalation

of medium to coarse grained rounded to sub-rounded sandstones and siltstones and ranges from 7200 to

5300 ft. The gamma ray values range from 26 to 14 7API; frequency ranges from 5 to 20 Hz, while resistivity

ranges from 0.2 to 2.6 Ohm-m. Plot of resistivity versus gamma ray log values shows that the facies in

association are shales, sandy shale, and shaly sand and clean sand facies. The top of this facies interval is the

top of Agbada Formation in this well. Facies 5 and 6 represent the upper Agbada Formation, the sandiest

interval of the paralic Agbada Formation which houses most of the hydrocarbon reservoirs in the Niger delta

The Agbada formation has a total thickness of 5600 feet in this well.



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Figure 2: Lithofacies delineation from spectrally transformed data in frequency domain



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Figure 3: Niger Delta oil mining lease (OML) map showing location of the study area block.

STUDY AREA BLOCK



272

Figure 4 (facies 1).

Figure 5 (facies2).



Figures (4-7): XY Plot of Resistivity (Ohm-m) versus Gamma ray (API) showing lithofacies associations in

Facies (1 – 4).



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Facies 7: the sand and conglomerate facies: although the gamma ray log did not penetrate this interval, the

litholog shows that this facies is characteristically, a sandy facies unit with very thin lenses of minor shale

intercalation on thickly bedded, rounded to sub- rounded coarse grained sand with pebble sandstones and

quartz pebble conglomerates of the coastal plain sands of the Benin Formation (Figure 8) and ranges from

5300 to 400ft.

Figure 8: Lithostratigraphy, facies analysis and depositional sequences in Well A



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SUMMARY AND CONCLUSION

The sedimentological results and spectral decomposition of gamma ray log delineated seven

lithofacies units named with their dominant lithology. They are the black shales and sand facies of the deep

water Akata Formation, characterized by thick medium grained, fairly well sorted sand of appreciable

thickness capped with very thick black shale; the grey shales and the grey sandy shale of the lower Agbada

Formation of outer neritic setting; the sandy shale and the shaly sand of the paralic upper Agbada Formation

characterized by black shales intercalating with medium to coarse grained sandstone and siltstone at

intervals (the main paralic interval of the Agbada Formation); and the coarse sand and conglomerate facies of

the coastal plain sands of the Benin Formation characterized by very thin lenses of minor shale intercalation

on thickly bedded rounded to sub- rounded coarse grained sand with pebble sandstones and quartz pebble

conglomerates of the Benin formation dated Upper Miocene to Pliocene age.

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integration of sedimentological and spectral data …

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