Geophysical techniques are important exploration techniques used to explore subsurface deposits of minerals and hydrocarbons. This technique is similar to remote sensing, which infers subsurface information while taking measurements from the surface. The seismic reflection technique works simply by creating a seismic wave from a source that travels down into the earth.
The acoustic impedance contrast due to different lithologies causes the acoustic wave to reflect or refract. The reflected or refracted waves are recorded on the surface by different arrays of receivers (Bakker, 2002). 3D seismic data acquisition and interpretation has now become a tool of vital importance in the petroleum industry for exploring hydrocarbons. It gives detailed information about the subsurface, not in vertical cross-section but the entire volume of the earth. The seismic wave interacts and the signal is altered by hydrocarbon-bearing strata; 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 between fluids, rocks, hydrocarbons, and the reservoir containing them. Petrophysical properties such as volume of shale, porosity, and fluid saturation depend upon the depositional environment of reservoir rocks. The network of interconnected pores is important for storing and transmitting fluids (Donaldson and Tiab, 2004). The efficiency of a reservoir depends upon its permeability. Hydrocarbon leads can be identified using these techniques (Shamim et al., 2014). Facies analysis deals with the specific characteristics of rocks or strata that 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 a regional scale, and Porosity variations. Cross-plot analysis can also easily identify The reservoir quality facies (Gray and Andersen, 2000).
The spectral decomposition technique transforms seismic data into a time versus frequency domain. The 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 observe delicate thickness variations and discontinuities and 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 the potential reservoir. Spectral decomposition can also directly detect 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 amplitude variations 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 show more significant stratigraphic exploration plays (Naseer and Asim, 2017b; Naseer and Asim, 2017d). Low frequency appears in the oil and gas layer, while the 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 stratigraphies, such as Instantaneous Q showing energy absorption, Phase indicating event continuity, and Trace envelops showing reflection strength. Attributes help to identify reservoir zones (Taner, 1992).
Introduction
The Sawan area lies in Khairpur District, Sindh. It is located in the 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 the Karachi Embayment zone to the south. The geographic map of the Study area is shown in Figure 1. The southern Indus Basin is separated from the Central Indus Basin by Jacobabad High. Sawan area is well-known for its Gas production, so it is called Sawan Gas Field. (Nasir et al, 2002)
The whole Sawan block is divided into three compartments, south, center, and north, by two major strike-slip faults (Rahman and Ibrahim, 2009). A 3D seismic cube (10×10 km2) lies in the southern compartment. Fourteen wells have been drilled in the Sawan block; the 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. 1998 3D seismic data (274 km2) was acquired over the whole block. OMV Pakistan operates the Sawan Gas Field. A total of 15 wells have been bored in Sawan Gas Field, and 14 provide gas to Sui Northern Gas Pipelines and Sui Southern Gas Company. It was and is still considered one of Pakistan’s major discoveries of gas reserves.
The latitude of the area is 26° 59′ 39.4″ N to 26° 98′ 30.5″ N
The longitude of the area is 68° 32′ 28.9″ E to 68° 58′ 25.1″ E
The study data is acquired by Landmark Resources (LMKR) with the permission of the Directorate General of Petroleum Concessions (DGPC). The Wells and Seismic data used for the study are:
- Navigation Files
- 3D Seismic Cube
- Seismic Header Files
- Well Logs (LAS Files)
- Formation tops
The 3D seismic section (10×10 km2) has been utilized to study the area, mark horizons, and identify structures or stratigraphic geometries. The 3D seismic data is provided by the 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 range from 700 to 860. The Cross lines range from 874 to 1019
Wells used for
Three wells, Sawan-0, Sawan-0, and Sawan-0, have been used for research purposes in the Sawan area. Sawan lies in the middle of a 3D cube and also in the middle of Sawan and Sawan.
Sawan-0 is used to create a Synthetic seismogram and for a well-to-seismic tie. After the successful well-to-seismic tie, the accurate position of the interested horizons, Lower Goru top, and C-interval was identified. Sawan-01, Sawan-07, and Sawan-08 are used for Petrophysical analysis of the reservoir rock. The details of coordinates, well depth, and Top of Lower Goru in each well are 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 the thesis work is to demarcate the C-interval of the Lower Goru Formation using seismic data and petrophysical analysis.
- Structural interpretation by 3D seismicity is used to identify the most probable zones for hydrocarbon accumulations or reservoir geometry and to acquire information about the configuration of subsurface structures.
- Petrophysical 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 then helps to identify the possible hydrocarbon bearing zones by evaluating reservoir properties using well log data.
- Spectral Decomposition of the available data set for analyzing the behavior of different Spectral components at the gas-bearing reservoir level.
- Identification of Channels containing reservoir quality sands with the variation of amplitudes or energy entrapment.
- Confirmation of Spectral Decomposition interpretation by calculating the Sweetness attribute.
Base Map
The 3D seismic and well data is loaded to (SMT) Kingdom software to generate a base map showing the seismic lines and well locations. Variations of in-lines occur along the Y-axis, and crossline variations 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. A Base map is prepared using the available data set, which shows the orientation of the 3D seismic data cube and well locations inside the cube. A synthetic seismogram is generated using well logs Sawan-07, which then helps in the identification and marking of Horizons, with their Time and Depth contouring of marked Horizons.
Petrophysical studies were conducted to determine the reservoir parameters and mark the favorable zones for hydrocarbon accumulation within the reservoir interval. The general structural and Petro-physical interpretation workflow is given in (Figure 3).
Spectrum analysis of 3D seismic data is performed to determine the frequency range and dominant frequency of the data. Spectral decomposition is very useful in seismic interpretation, especially in 3D seismic interpretation. Conventional broadband seismic data is decomposed into its spectral components, which reveal stratigraphic and structural details that are often shadowed in conventional seismic data.
Spectral decomposition is applied to the available data set to view the behavior of gas-bearing reservoirs at different frequencies and see the amplitude variation. Spectral decomposition best reveals three hydrocarbon indicators: 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 thickness.
