The Geophysical Techniques Essay

The Geophysical techniques are important exploration techniques used for the exploration of sub surface deposits of minerals and hydrocarbons. This technique is similar to remote sensing which is used to infer sub surface information while taking measurements from surface. The seismic reflection technique works simply by creating a seismic wave from a source that will travel down in to the earth. The acoustic impedance contrast due to different lithologies cause the acoustic wave to reflect or refract. The reflected or refracted waves are recorded on the surface by different array of receivers (Bakker, 2002). 3D seismic data acquisition and interpretation has now become a tool of vital importance in petroleum industry for the exploration of hydrocarbons. It gives detailed information of the sub surface not in vertical cross section but entire volume of the earth. The seismic wave interacts and signal is altered by hydrocarbon bearing strata and this altered signal is used to identify oil and gas bearing structures and to map reservoir quality (Bacon, 2007).

The Petrophysical interpretation helps to differentiate between reservoir and non-reservoir. It helps to understand the interaction among fluids and rocks or the hydrocarbons and the reservoir containing them. Petrophysical properties such as volume of shale, porosity, fluid saturation depends upon the depositional environment of reservoir rocks. The network of interconnected pores is important for the storage and transmission of fluids (Donaldson and Tiab, 2004). The efficiency of a reservoir depends upon permeability of the reservoir. On the behalf of these techniques, hydrocarbon leads can be identified (Shamim et al., 2014). Facies analysis deals with the specific characteristics of rocks or strata which reflect the depositional environment, appearance, fossils, Sedimentary structures and composition (Walker, 1992). The well log cross plots are helpful for identifying lithologies, lithology variations on regional scale, Porosity variations. The reservoir quality facies can also be easily identified by cross plot analysis (Gray and Andersen, 2000).

Spectral decomposition technique transforms seismic data into time versus frequency domain. Spectral decomposition technique decomposes the seismic signal into its constituent frequencies. The data is decomposed into its spectral components by Fourier transform. The interpreter can not only observe delicate thickness variations and discontinuities, but can also predict bedding thickness quantitatively (e.g. Partyka 1999). The high-frequency response of a reflector can be attenuated by the presence of compressible fluids by means of potential reservoir. Spectral decomposition can also assist in the direct detection of hydrocarbons (Castagna et al. 2003). Attenuation is only observable for reservoirs of considerable thickness that can trap Seismic energy (Castagna and Shengjie, 2002).

Spectral analysis allows the explorer to observe the variations of amplitude with frequency, and understand the stratigraphic units, faults and fractures, and hydrocarbons. Spectral decomposition analysis is an important reservoir imaging tool. Channels filled with porous lithologies and bounded in a nonporous media shows more significant stratigraphic exploration plays (Naseer and Asim, 2017b; Naseer and Asim 2017d). Low frequency appears in oil and gas layer while water layer shows high frequency. The relationship appears as low energy at high frequency and high energy at low frequency (Guoping zou et al., 2012). Attribute analysis indicates the characteristics of subsurface stratigraphy such as Instantaneous Q shows energy absorption, Phase indicates event continuity, Trace envelop shows reflection strength. Attributes help to identify reservoir zone (Taner, 1992).

Introduction

The Sawan area lies in Khairpur District, Sindh. It is Located in Southern part of the Lower Indus basin bounded by Jacobabad High in the north, Indian Shield in the east, Suleman thrust and Fold Belt in the west and Karachi Embayment zone to the south. The geographic map of Study area is shown in figure 1. Southern Indus basin is separated from Central Indus basin by Jacobabad high. Sawan area is well-known for its Gas production, so it is called as Sawan Gas Field. (Nasir et al, 2002)

The whole Sawan block is divided into three compartments south, centre and north by two major strike slip faults (Rahman and Ibrahim, 2009). 3D seismic cube (10×10 km²) lies in the southern compartment. Fourteen wells have been drilled in the Sawan block, majority of these lies in the Central compartment which is the main gas tank (Ibrahim, 2007).

Sawan gas field was discovered in 1998 and its production started in 2003. The reservoir of
Sawan gas field is “C” sands of the Early Cretaceous, Lower Goru Formation. 3D seismic data (274 km²) was acquired over the whole block in 1998. The operator of the Sawan Gas Field is OMV Pakistan. A total of 15 wells have been bored in Sawan Gas Field and 14 are providing gas to Sui Northern Gas Pipelines and Sui Southern Gas Company. It was and still considered as one of the major discoveries of gas reserves in Pakistan.

Latitude of the area is 26° 59′ 39.4″ N to 26° 98′ 30.5″ N

Longitude of the area is 68° 32′ 28.9″ E to 68° 58′ 25.1″ E 

The study data is acquired by Land Mark Resources (LMKR) by the permission of Directorate General of Petroleum Concessions (DGPC). The Wells and Seismic data used for study are:

• Navigation Files
• 3D Seismic Cube
• Seismic Header Files
• Well Logs (LAS Files)
• Formation tops

S

The 3D seismic section (10×10 km²) has been utilized for studying the area, marking horizons and identification of structure or stratigraphic geometries. The 3D seismic data is provided by Directorate General of Petroleum Concessions (DGPC) and land Mark Resources (LMKR). The 3D cube is formed of In-lines and Cross lines. The In-lines ranges from 700 to 860. The Cross lines ranges from 874 to 1019

Wells used for

Three wells namely Sawan-0, Sawan-0, Sawan-0, have been used for research purpose in Sawan area. Sawan  lies in the middle of 3D cube and also in the middle of Sawan ,  and Sawan .

Sawan-0 is used to create Synthetic seismogram and for well to seismic tie. The accurate position of the interested horizons Lower Goru top, C-interval was identified after successful well to seismic tie. Sawan-01, Sawan-07, and Sawan-08 are used for Petro-physical analysis of the reservoir rock. The details of coordinates, well depth, and Top of Lower Goru in each well is shown in Table 1.

Well Name	Latitude-N	Longitude-E	Well Depth(m)	Formation Tops(m) TLG
Sawan	26.991828	68.906992	3587	2696
Sawan	26.999283	68.923317	3400	2691
Sawan	27.009156	68.933394	3430	2697

 

Table 1 Basic information of the Wells

 Objectives of the Study

The objective of thesis work is the demarcation of C-interval of Lower Goru Formation integrated with seismic data and petrophysical analysis.

• Structural interpretation by 3D seismic to identify the most probable zones for hydrocarbons accumulations or reservoir geometry and to acquire the information of the configuration of subsurface structure.
• Petro physical analysis to calculate the reservoir properties such as volume of shale (Vsh), Porosity (ø), Resistivity of formation water (Rw), Saturation of water (Sw), Saturation of hydrocarbons (Sh) which than helps to identify the possible hydrocarbon bearing zones by evaluating reservoir properties using well log data.
• Spectral Decomposition of the available data set for analysing the behaviour of different Spectral components at gas bearing reservoir level.
• Identification of Channels containing sands of reservoir quality with the variation of amplitudes or energy entrapment.
• Confirmation of Spectral Decomposition interpretation by calculating Sweetness attribute.

Base Map

The 3D seismic and wells data is loaded to (SMT) Kingdom software to generate base map is which shows the seismic lines and well locations. Variation of in-lines occur along the Y-axis and variation of crosslines occur along the X-axis. The base map is shown in Figure 2.

Methodology

To attain the objective, the study is carried out in the following steps:

Geological understanding of an area is vital for seismic interpretation, Base map is prepared by available data set which shows the orientation of 3D seismic data cube and well locations inside the cube. Synthetic seismogram is generated by using well logs Sawan-07 which then helped in the identification and marking of Horizons, with their Time and Depth contouring of marked Horizons.

Petro physical studies was carried out to determine the reservoir parameters and to mark the favourable zones for hydrocarbon accumulation within the reservoir interval. General workflow for structural and Petro-physical interpretation is given in (Figure 3).

Spectrum analysis of 3D seismic data is performed to find out the frequency range in the data and to determine dominant frequency of the data. Spectral decomposition is very useful in seismic interpretation especially in 3D seismic interpretation and conventional broadband seismic data decomposed into its spectral components which reveals stratigraphic and structural details which are often shadowed in conventional seismic data.

Spectral Decomposition is applied to the available data set in order to view the behaviour of gas bearing reservoir at different frequencies in order to see the variation of amplitude. Three hydrocarbon indicators are best revealed by spectral decomposition which are abnormal seismic attenuation, low frequency shadows associated with hydrocarbon related bright spots, and differences in tuning frequency between gas and brine sands. Attenuation is observed in reservoirs of considerable thinkness.