Study of fault configuration related mysteries through multi seismic attribute analysis technique in Zamzama gas field area, southern Indus Basin, Pakistan

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Study of fault configuration related mysteries

through multi seismic attribute analysis technique

in Zamzama gas field area, southern Indus

Basin, Pakistan

Shabeer Ahmed Abbasi

a,b,*

, Shazia Asim

c

, Sarfraz Hussain Solangi

a

,

Fareed Khan

c

a_{Centre for Pure and Applied Geology, University of Sindh, Jamshoro, Pakistan} b

Oil and Gas Development Company Limited, Islamabad, Pakistan

c_{Department of Earth Sciences, Quaid-e-Azam University, Islamabad, Pakistan}

a r t i c l e i n f o

Article history:

Received 3 February 2016 Accepted 14 March 2016 Available online 26 April 2016 Keywords:

Seismic attribute Zamzama gas field Fault configuration

a b s t r a c t

Seismic attribute analysis approach has been applied for the interpretation and identifi-cation of fault geometry of Zamzama Gas Field. Zamzama gas field area, which lies in the vicinity of Kirthar fold and thrust belt, Southern Indus Basin of Pakistan. The Zamzama fault and its related structure have been predicted by applying the Average Energy Attri-bute, Instantaneous Frequency AttriAttri-bute, relative Acoustic Impedance Attribute and Chaotic Reflection Attribute on the seismic line GHPK98A.34. The results have been confirmed by applying the spectral decomposition attribute on the same seismic line that reveal the geometric configuration of Zamzama structure. The fault is reverse and started from 0 s and ended at the depth of 2.5 s on the vertical seismic section. Hanging wall moves up along the fault plane under the action of eastward oriented stress, which formed a large northesouth oriented and eastward verging thrusted anticline.

©2016, Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

How to cite this article:Ahmed Abbasi S, et al., Study of fault configuration related mysteries through multi seismic attribute analysis technique in Zamzama gas field area, southern Indus Basin, Pakistan, Geodesy and Geodynamics (2016), 7, 132e142, http://dx.doi.org/10.1016/j.geog.2016.04.002.

*Corresponding author.

E-mail address:shabeer.ahmed@ogdcl.com(S. Ahmed Abbasi).

Peer review under responsibility of Institute of Seismology, China Earthquake Administration.

Production and Hosting by Elsevier on behalf of KeAi

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g e o d e s y a n d g e o d y n a m i c s 2 0 1 6 , v o l 7 n o 2 , 1 3 2e1 4 2

http://dx.doi.org/10.1016/j.geog.2016.04.002

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1. Introduction

Fault configuration related mysteries are one of major challenging and important job in the investigation of hydro-carbons. Faults have a great role in the production capability of the reservoir as small faults which compartmentalize the reservoir can have a major impact on reservoir performance

[1]. Interpreting faults is always a challenge even with the advent of new geophysical attributes and display. Fault interpretation is often seriously hindered by data quality[2].

According to Hardage[3]all instantaneous seismic attributes (amplitude, phase, and frequency) can be used in interpretation. In practice, most interpreters use instantaneous amplitude, or some variation of an amplitude attribute, as their primary diagnostic tool. Amplitude is related to reflectivity, which in turn is related to subsurface impedance contrasts. Recently, multi-attribute analysis approach has been recognized as a successful interpretation approach for the identification of the faulted area on the seismic data. The amplitude content in seismic data is the principal parameter which is used to describe the absorption, reflection coefficient, acoustic impedance and velocity parameters. Seismic attribute analysis approach is also familiar as qualitative interpretation

[4]. Sampson[5]also testified seismic attributes to asses well productivity in the Ninilchik field, Cook Inlet Basin, Alsaka. According to Taner[6], Chopra and Marfurt[7], Liu et al.[8],

Anees [9], Aqrawi et al. [10], the accuracy of mapping in faulted areas is critical for the correct definition of hydrocarbon fields. For decades the interpreter relied on the seismic visualizing techniques to investigate the geometry of the fault. The advancement in seismic visualizing techniques enables us to reduce the risk in interpretation and identification of the faulted area. Seismic attribute analysis technique has been testified by many workers as a significant tool for the study of faults and field structures therefore this approach has been chosen for the study of fault configuration of Zamzama field, Pakistan for the first time.

2. Tectonic background of the study area

Zamzama gas field lies in the vicinity of Kirthar fold and thrust belt, Southern Indus Basin of Pakistan. The location of Zamzama gas field is shown in Fig. 1. The southern Indus Basin is surrounded by the Indian Shield to the east and the marginal zone of Indian plate to the west, and it also bounded by Sukkur rift from north to offshore Indus in the south [11]. The southern Indus basin is comprised of five units, Thar Platform, Kirther Foldbelt, Kirther Foredeep, Karachi trough, and Offshore Indus. Most eastern part of the southern Indus Basin is comprised of Thar Platform, and its western part is comprised of Kirther Foldbelt. Kirther Foredeep area lies between the Kirther Foldbelt and Thar

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Platform. The most southern part of the southern Indus Basin is comprises of offshore Indus which is the passive margin of the Indus offshore. The portion of the southern Indus Basin that lies between Kirther Foredeep offshore Indus is known as Karachi trough[12].

3. Structural style and geology of area

The Zamzama structure is a large northesouth orien-tated, eastward verging thrusted anticline (Fig. 2). Pab sandstone of Late Cretaceous forms the primary hydrocarbon reservoir in the Zamzama gas field. The thickness of Pab sandstones is uniform across the entire Zamzama structure [13]. Top seal for the Pab sandstone reservoir is provided by marine shale of the Girdo (Ranikot) Formation. SembareGoru Shales are proved as main source rocks of the southern Indus Basin [14e16]. Generalized stratigraphy of area is shown in Fig. 3. Khan et al. [17] carried out work on hydrocarbon estimation using geological and petrophysical evaluation of Zamzama gas field, and obtained average pay zone thickness of 257.48 feet, average water saturation of 40%, average porosity calculated from neutron and density porosity of 18%, and proved hydrocarbon reserves of Zamzama gas field. The total estimated proven plus probable recoverable reserves for the core area of the Zamzama field to be developed are 1.7 Tcf of gas (gross), of which BHP Billiton's equity share is around 650 Bcf.

Zamzama gas is sweet and dry, with a low condensate to gas ratio of 6.5 barrels/MMcf[18].

4. Data and methodology

The seismic data used for this study was provided by the Land Mark Resources (LMKR) with the permission of Direc-torate General of Petroleum Concessions (DGPC), Ministry of Petroleum and Natural Resources, Islamabad, Pakistan in the form of“SEGY”. Base map of few seismic lines lying in Zam-zama gas field area is shown inFig. 4. Ranking fourth in terms of Pakistan's discovered gas reserves which adds approximately 15% to Pakistan's daily gas production[18].

Seismic attribute is described as physical attribute which mostly depends on the nature of the material through which the wave is propagated. Physical attributes are clas-sified as the pre-stack and post-stack attributes. Post-stack attributes are further classified as instantaneous attribute and wavelet attribute. Computation of seismic properties sample by sample deals with instantaneous attributes continuously represent the change of seismic properties along time and space. In this study, seismic trace envelope is computed and all minima envelopes are picked. The distance is defined between two consecutives minima as a wavelet. The instantaneous attribute which are computed at maximum of the trace envelope and between two envelope minima are known as wavelet attribute, and this attribute is also called the response attribute. Wavelet attribute is preferred in this study as it is related to the characteristics of wavelet[19].

The spatial and temporal relationship of the other attri-butes is illustrated through geometrical attriattri-butes in which lateral continuity has been found through resemblance, and also is a good detector of discontinuity and similarity of beds.

Fig. 2eGeological structural cross-section on basis of interpretation.

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For the interpretation of stratigraphy, the geometrical attri-butes were used by the investigation of the curvatures along the bedding dips, which is more informative in describing the depositional environments[20].

However, the detailed investigation through geometrical and instantaneous attribute exhibits an idea about faulted area. For the marking of faulted area, the instantaneous fre-quency, relative acoustic impedance attribute, average energy and chaotic reflection attribute have been selected. The comparison has been made between the instantaneous fre-quency, relative acoustic impedance attribute, average en-ergy, and chaotic reflection attribute with maximum curvature attribute. The practical applications of all attributes are shown inFig. 5.

Kingdom SMT 8.4 software has been used throughout the interpretation and attribute analysis on the final PSTM seismic Line GHPK98A.34 in SEGY format.

5. Results and discussion

5.1. Average energy attribute

As seismic energy travels in the form of wave energy, and when it passes through a rock body within the subsurface,

some seismic energy may converts into heat energy. This conversion of seismic energy into heat energy shows a net loss of the wave energy. The decay of seismic energy shows the presence of fault, but energy decay also occurs with the change in crystal phase and presence of hydrocarbons, thin beds with low acoustic impedance, compaction of porous medium, viscous loss of fluid flowing in the porous medium, and temperature loss due to compression but in case of presence of the fault, this low energy area form the certain angle with the reflector on the seismic data. Numerous mechanisms which described the seismic energy attenuation are based on the principle that seismic wave induces an interaction between rock matrix and pore fluid. Physical properties of rock matrix and the composition of fluids within the rock matrix determine the degree to which seismic energy is attenuated. Energy attenuation is also varying in response due to changes in water saturation, clay content, porosity, pore geometry, permeability, micro-fracturing and pressure. The strong evidence of presence of the fault increases the attenuation, and also the presence of fluid content makes it more pronounced on the seismic data.

The measurement of attenuation from seismic data has great potential for reservoir characterization and direct detection of faulted area. Loss of high frequency of seismic data led to high energy attenuation zone, and further led to

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fractured area. Amplitude attribute has been applied on the seismic section which resulted inFig. 6. After applying the average energy attribute on the seismic line GHPK98A-34, loss of high energy led to fracture area which starts from the shot point 1175e1300 between the times 0e2.5 s as shown in

Fig. 7.

5.2. Instantaneous frequency attribute

Instantaneous frequency has been computed as the time derivative of phase which responds to both depositional characteristics and also the effects of wave propagation. As that instantaneous frequency can be used as an effective discriminator of hydrocarbons, fractured area and unconsol-idated sand with oil in pores by low frequency anomaly, which

can also be used as a thickness indicator as higher frequencies show a sharp interfaces in thin laminated shales and lower frequencies show massive bedding geometries, e.g. sand-prone lithologies. But at some locations, the instantaneous frequencies have a negative sign because of closely arrived reflected wavelets.

Instantaneous frequency attribute is the physical attribute, and due to lower frequency zone it can be used as indicator of fractured area. Instantaneous frequency attribute demon-strates the fault surface when applied on the seismic data. Taner [19] described that the actual loss of compaction is produced where the faulting occurs. When the seismic wave passes through less compacted area, it increases in wavelength and decreases in frequency. So the zone where the loss of frequency occurs, is the indicator of the faulted

Fig. 4eThe base map of the study area.

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surface. The loss of frequency occurs because of presence of hydrocarbons and low acoustic impedance within thin beds, but in case of fault this low frequency zone form the certain angle with the reflector on the seismic data.

After applying the instantaneous frequency attribute on the seismic line GHPK98A-34, loss of high frequency led to fractured area which starts from the shot point 1175e1300 between the times 0e2.5 s as shown inFig. 8.

5.3. Relative acoustic impedance

Seismic data has been processed to minimize the signal to noise ratio, and finally processed data have a zero phase and have a wide band wavelet. The seismic traces in the final processed data have a series of band-limited reflectivity which can be expressed by equation(1).

fðtÞ ¼1 2

Drn

rn (1)

This equation can also be expressed by equation(2).

fðtÞ ¼1

2DlnðrvÞ (2)

Acoustic impedance has no absolute value because of the band-limited integral is applied to zero phase natural log of the acoustic impedance which mathematically is expressed by the equation(3).

lnðrvÞ ¼2 Zt¼T t¼8

fðTÞdt (3)

Relative acoustic impedance which is calculated by taking integration will produce the arbitrary long wavelength, because seismic data has long wavelength and very low

frequencies which result the noise on seismic data and imperfect spectral content. Therefore, this information cannot be utilized. This effect can be removed by applying the low cut filter or band pass filter during the processing of the seismic data.

Relative acoustic impedance attribute demonstrates the fault surface after applying on the seismic data. The loss of compaction and free space within the rock body occurred in the presence of faults. So, when the seismic wave passes through less compacted zone or faulted zone, it decreases in velocity, and the loss of velocity causes the decrease in acoustic impedance. So the loss of acoustic impedance is the indicator of fault surface. The loss of acoustic impedance also occurs as change of crystal phase, compaction of porous me-dium, viscous loss of fluid flowing in the porous meme-dium, and temperature loss due to compression but due to presence of fault, the low acoustic impedance zone form the certain angle with the reflector.

Relative acoustic impedance attribute has been applied on the seismic line GHPK98A-34, low value led to fractured area which starts from the shot point 1175e1300 between the times 0e2.5 s as shown inFig. 9.

5.4. Chaotic reflection attribute

Chaotic reflection attribute is a hybrid attribute that is more specifically used to detect the chaotic beds among the most organized events. The area with high lateral bedding continuity and varying dips is known as chaotic zone. Vari-ance and similarity attribute is used to find the lateral conti-nuity and running standard deviation of dips weighted by the lateral semblance in the chaotic reflection attribute[4].

Important applications of the chaotic reflection attribute are:

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(1) The zones of parallel dipping reflectors and the lateral dips will be consistent so they will have a low standard deviation of dip.

(2) Chaotic zones having a higher lateral coherency and their dips will change more rapidly with time and space on the seismic data. Chaotic bedding zone will have high attribute values.

(3) Non-reflecting zones having lower average lateral semblance will also have a high standard deviation of dip.

Lower values of chaotic reflection attributes shows a massive carbonated zone with incoherent noise that appear as weak reflections on the seismic data.

Fig. 6e-Final PSTM seismic line GHPK98A.34 displayed in amplitude attribute.

Fig. 7eAverage energy attribute applied on seismic line GHPK98A.34.

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Chaotic reflection attribute have been applied on the seismic line GHPK98A-34, high value led to fracture zone which starts from the shot point 1175e1300 between the times 0e2.5 s. The dips change more rapidly with time and spaceFig. 10.

5.5. Maximum curvature attribute

Curvature is the reciprocal of radius of a circle that is tangent to a given curve at a single point. Curvature has a large value for a curve which bent more and become zero for

Fig. 8eInstantaneous frequency attribute applied on seismic line GHPK98A.34.

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straight line. Mathematically, second-order derivative of a curve is defined as a curvature. When a radius of a circle at the point of contact with a curve is simply replaced by results of normal vectors, then it is possible to assign a sign for curva-ture having different shapes. This normal vector on curve may be diverging vector, converging vector or normal vector. Diverging vector is associated with the anticlines, converging vector is associated with the synclines and parallel vector is associated with the planar surfaces which have normally zero curvature. This concept of curvature can be further extended to three dimensional surfaces by simply considering that three dimensional curved surface being intersected by a plane. If the planes which cut the given curved surface are orthogonal, then curvature is considered as normal curvature

[20].

Curvature attribute is used for the prediction of the frac-tures using surface seismic data. Curvature quantifies the degree by which the surface deviates from being planar and introduced the difference of curvature for surfaces that have a small-scale features which are associated with small-scale faults and also with primary depositional features. In this paper only maximum curvature attribute is used to illustrate the fault surface

By applying the maximum curvature attribute on the seismic line GHPK98A-34, high value led to the fractured area which starts from the shot point 1175e1300 between the times 0e2.5 s. The dips will change more rapidly with time and space on the seismic section which is shown inFig. 11.

6. Conclusions

From the above discussion, we obtain the conclusions of multi-attribute analysis approaches applied on the seismic data of Zamzama gas field area as follows:

(1) The instantaneous frequency, instantaneous phase, chaotic reflection and relative acoustic impedance attribute analysis demonstrate the discontinuity on the seismic section which are further used to elaborate the structural trend of Zamzama gas field area. The obser-vation on results of these attribute put concentration between the shot point 1175e1300 on GHPK98A-34, 1190e1285 on GHPK98A-32, 1170e1275 on GHPK98A-30, 1160e1280 on GHPK98A-30 and 185e300 on GHPK96-07 between 0 and 2.5 s in the left hand side of the fault, there is discontinuity. Attribute analysis results show that the roll over anticlinal structure is associated with eastewest oriented thrust.

(2) Maximum curvature attributes provides an important information which is commonly used for fault detection such as instantaneous frequency, instantaneous phase, chaotic reflection and relative acoustic impedance attribute. But because of second-order derivative mea-sures, the curvature attribute is sensitive and significant to noise.

Maximum curvature attributes enhance the channel fea-tures that improve the definition of subtle faults and fracfea-tures.

Fig. 10eChoatic reflection attribute applied on seismic line GHPK98A.34.

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(3) Combined attributes analysis proved as a principal tool used for the prediction of fault in near future. Because combined attributes translate the fault system more accurately than single attribute analysis which have no tool for further confirmation.

(4) Seismic attributes have made it possible to describe seismic data in a multi-dimensional form, in order to understand the structure of Zamzama gas field which ultimately will contribute comprehensive tool for un-derstanding the complications and doubts in the structure of the Zamzama gas field and will be useful for the development of the field. Also this approach can be applied to any other area for getting better and reliable picture of subsurface structures.

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Shabeer Ahmed Abbasiis a Ph.D, research scholar from Centre for Pure and Applied Geology, University of Sindh, Jamshoro, Pakistan. He is doing research on tectonic evolution of structures and their hydrocar-bon potential in southern Indus Basin, Pakistan. He obtained his master degree from Geophysics from Department of Earth Sciences, Quaid-e-Azam, University, Islamabad. He is an active member of by American Association of Petroleum Geo-scientists (AAPG) and Society of Exploration Geophysicists (SEG). He also contributed many publications as author and co-author.