Editing Imaging coals with seismic reflection data for improved detection of sandstone bodies

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Carboniferous formations have an extensive subcrop beneath the North Sea. The sequence is up to 9000 m thick, with rapidly changing facies and pronounced local changes in thickness (Besly 1998). It is economically important for two main reasons:* The coals and carbonaceous shales are source rocks, particularly for gas, in the overlying Permian and Triassic reservoirs of the southern North Sea (SNS).
 
Carboniferous formations have an extensive subcrop beneath the North Sea. The sequence is up to 9000 m thick, with rapidly changing facies and pronounced local changes in thickness (Besly 1998). It is economically important for two main reasons:* The coals and carbonaceous shales are source rocks, particularly for gas, in the overlying Permian and Triassic reservoirs of the southern North Sea (SNS).
 
* The Carboniferous sequence itself contains important sandstone bodies that act as gas reservoirs, notably within the Westphalian of the northern part of the SNS.
 
* The Carboniferous sequence itself contains important sandstone bodies that act as gas reservoirs, notably within the Westphalian of the northern part of the SNS.
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Between 1983 and 1993 some 20 gas discoveries were made within the Carboniferous of this region, including several major finds, and commercial development of the Murdoch, Caister and Ketch fields began. However, the Westphalian reservoirs are difficult to locate, because, although the ratio of net reservoir sandstones to gross interval (net-to-gross) can be relatively high, the seismic resolution is low. Westphalian strata lie beneath up to 4000 m of overburden, including Zechstein salts, which rapidly attenuates the seismic signal. Within the Westphalian itself, the highly reflective coals dominate the seismic response. The sandstone bodies forming the gas reservoirs are the coarse-grained fluvial channel features, which are inherently complex in both their geometry and internal architecture. They are generally less than 100 m thick and close to or below seismic resolution. Additionally, late Carboniferous uplift and erosion has reduced the area within which the Carboniferous strata are preserved, and subsequent tectonic activity has broken them into a series of northwest-trending fold and fault blocks, all of which makes their identification and correlation on seismic data particularly difficult.
 
Between 1983 and 1993 some 20 gas discoveries were made within the Carboniferous of this region, including several major finds, and commercial development of the Murdoch, Caister and Ketch fields began. However, the Westphalian reservoirs are difficult to locate, because, although the ratio of net reservoir sandstones to gross interval (net-to-gross) can be relatively high, the seismic resolution is low. Westphalian strata lie beneath up to 4000 m of overburden, including Zechstein salts, which rapidly attenuates the seismic signal. Within the Westphalian itself, the highly reflective coals dominate the seismic response. The sandstone bodies forming the gas reservoirs are the coarse-grained fluvial channel features, which are inherently complex in both their geometry and internal architecture. They are generally less than 100 m thick and close to or below seismic resolution. Additionally, late Carboniferous uplift and erosion has reduced the area within which the Carboniferous strata are preserved, and subsequent tectonic activity has broken them into a series of northwest-trending fold and fault blocks, all of which makes their identification and correlation on seismic data particularly difficult.
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=== 2.1 Previous work ===
 
=== 2.1 Previous work ===
  
The encoding of variations in seam thickness within the amplitude of the reflection follows the Widess (1973) “thin bed” theory. Widess considered a layer embedded in an infinite homogeneous medium and defined a thin bed as one that produces a composite reflection because of the interference of the reflections from the top and bottom of the layer. The point at which the composite reflection no longer changed with the dimensions of the bed was the maximum thickness of the “thin bed”. In practical terms he concluded that a bed is defined as seismically thin where the layer thickness is less than one eighth of the dominant wavelength. He also demonstrated that the amplitude of the thin-bed reflection is approximately proportional to the thickness of the bed and inversely proportional to the dominant wavelength. Widess assumed that the wavelet complex reflected by the layer is obtained by addition of the wavelet reflected from the top of the layer and the one reflected from the bottom, the latter being phase inverted and time shifted relative to the former. The tuning thickness is the bed thickness at which constructive interference from the top and bottom of the bed produces the strongest reflection. Tuning occurs at a thickness of one quarter of the wavelength and is therefore a function of the dominant temporal frequency and interval velocity. Widess’s theory assumes transmission loss and internal multiple reflections to be negligible; both are untrue in cyclic coal sequences.
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The encoding of variations in seam thickness within the amplitude of the reflection follows the Widess (1973) “thin bed” theory. Widess considered a layer embedded in an infinite homogeneous medium and defined a thin bed as one that produces a composite reflection because of the interference of the reflections from the top and bottom of the layer. The point at which the composite reflection no longer changed with the dimensions of the bed was the maximum thickness of the “thin
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bed”. In practical terms he concluded that a bed is defined as seismically thin where the layer thickness is less than one eighth of the dominant wavelength. He also demonstrated that the amplitude of the thin-bed reflection is approximately proportional to the thickness of the bed and inversely proportional to the dominant wavelength. Widess assumed that the wavelet complex reflected by the layer is obtained by addition of the wavelet reflected from the top of the layer and the one reflected from the bottom, the latter being phase inverted and time shifted relative to the former. The tuning thickness is the bed thickness at which constructive interference from the top and bottom of the bed produces the strongest reflection. Tuning occurs at a thickness of one quarter of the wavelength and is therefore a function of the dominant temporal frequency and interval velocity. Widess’s theory assumes transmission loss and internal multiple reflections to be negligible; both are untrue in cyclic coal sequences.
  
 
An important factor in a multi-seam environment is that not only do the large reflection coefficients cause high amplitude primary reflections, but strong surface multiples and interbed multiples are also set up within and between seams (Greenhalgh et al. 1986). The generation of a train of short-path multiples broadens the transmitted seismic pulse with a consequent lowering of its frequency content. Ri ter & Schepers (1978) calculated synthetic seismograms from the Coal Measures of the Ruhr and concluded that a single seam can give a distinct reflection even at a thickness of one fiftieth of the wavelength. However, in a sequence of layers containing many coal seams, they noted that individual reflections may become obscured by constructive interference from short-lag multiples.
 
An important factor in a multi-seam environment is that not only do the large reflection coefficients cause high amplitude primary reflections, but strong surface multiples and interbed multiples are also set up within and between seams (Greenhalgh et al. 1986). The generation of a train of short-path multiples broadens the transmitted seismic pulse with a consequent lowering of its frequency content. Ri ter & Schepers (1978) calculated synthetic seismograms from the Coal Measures of the Ruhr and concluded that a single seam can give a distinct reflection even at a thickness of one fiftieth of the wavelength. However, in a sequence of layers containing many coal seams, they noted that individual reflections may become obscured by constructive interference from short-lag multiples.
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The geological succession in the study area consists of Triassic mudstones with dolomitic sandstones and waterstones and irregular sandstone beds, overlying a Permian sequence of marls and limestones. The Upper Magnesian and the Lower Magnesian Limestones are important reflectors, generating distinctive seismic reflections in the upper part of the seismic data. The Basal Permian Sand is difficult to distinguish on seismic sections, but in this area lies just below the Lower Magnesian Limestone. The base of Permian overlies the Westphalian C strata with a slight angular unconformity. The Cambriense Marine Band (Bolsovian–Westphalian C) lies near the top of the Westphalian succession and there are approximately 145 m of Bolsovian strata consisting of a sequence of mudstones with frequent seat earths and thin coal seams, and occasional sandstones. The main coal development lies in approximately 240 m of Duckmantian strata with the Main Bright, Kent’s Thick, Top Hard and Dunsil seams being the thickest of many coal seams. Below the Vanderbeckei Marine Band, the top part of the Westphalian A (Langsettian) succession is similar to the Duckmantian, with the Deep Soft, Parkgate and Blackshale coal seams forming significant reflectors above increasingly sand-rich lower Westphalian A and Namurian sequences. The top Dinantian limestone forms the next widespread and correlatable reflector. The Westphalian strata lie in a shallow syncline plunging gently to the northwest. The surface elevations range from 29 m to 90 m above ordnance datum.
 
The geological succession in the study area consists of Triassic mudstones with dolomitic sandstones and waterstones and irregular sandstone beds, overlying a Permian sequence of marls and limestones. The Upper Magnesian and the Lower Magnesian Limestones are important reflectors, generating distinctive seismic reflections in the upper part of the seismic data. The Basal Permian Sand is difficult to distinguish on seismic sections, but in this area lies just below the Lower Magnesian Limestone. The base of Permian overlies the Westphalian C strata with a slight angular unconformity. The Cambriense Marine Band (Bolsovian–Westphalian C) lies near the top of the Westphalian succession and there are approximately 145 m of Bolsovian strata consisting of a sequence of mudstones with frequent seat earths and thin coal seams, and occasional sandstones. The main coal development lies in approximately 240 m of Duckmantian strata with the Main Bright, Kent’s Thick, Top Hard and Dunsil seams being the thickest of many coal seams. Below the Vanderbeckei Marine Band, the top part of the Westphalian A (Langsettian) succession is similar to the Duckmantian, with the Deep Soft, Parkgate and Blackshale coal seams forming significant reflectors above increasingly sand-rich lower Westphalian A and Namurian sequences. The top Dinantian limestone forms the next widespread and correlatable reflector. The Westphalian strata lie in a shallow syncline plunging gently to the northwest. The surface elevations range from 29 m to 90 m above ordnance datum.
  
The 3-D seismic dataset was acquired using 1lb (454g) dynamite charges in 40ft (12.2 m) shot-holes. The receiver group interval was 12 m and the receiver lines were placed 120 m apart. The shot lines were approximately 168 m apart and were at right angles to the receiver lines. The shot interval was 12 m. Each 40 ft shot hole was drilled twice and shot into a 144-channel patch. The total area of common mid-point coverage was 1.9km<sup>2</sup> and the nominal fold of cover was 6. The common mid-point (CMP) bins were 6x6 m. The acquisition parameters are summarized in [[:File:YGS_CHR_08_IMAG_TAB_01.jpg|Table 1]].
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The 3-D seismic dataset was acquired using 1lb (454g) dynamite charges in 40ft (12.2 m) shot-holes. The receiver group interval was 12 m and the receiver lines were placed 120 m apart. The shot lines were approximately 168 m apart and were at right angles to the receiver lines. The shot interval was 12 m. Each 40 ft shot hole was drilled twice and shot into a 144-channel patch. The total area of common mid-point coverage was 1.9km<sup>2</sup> and the nominal fold of cover was 6. The common mid-point (CMP) bins were 6x6 m. The acquisition parameters are summarized in Table 1.
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'''Table 1 Acquisition parameters.'''
  
'''[[:File:YGS_CHR_08_IMAG_TAB_01.jpg|Table 1]] Acquisition parameters'''
 
  
 
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* The short offsets limit the usefulness of some seismic attributes such as AVO.
 
* The short offsets limit the usefulness of some seismic attributes such as AVO.
  
A suite of seismic attributes was calculated on the 3-D volume. Of these, the most useful were expected to be reflection strength or amplitude envelope, instantaneous phase and instantaneous frequency. Reflection strength highlights changes in acoustic impedance and is used to identify anomalous impedance horizons such as gas-charged sandstones. As coal seams have a very low impedance compared with the surrounding strata, the reflection-strength plots will show detail in the coal seam that can lead to the recognition of small faults and variations in seam thickness . Instantaneous phase emphasizes the linearity of events and thus highlights breaks such as pinch-outs at unconformities and faults; it is not a useful attribute for detecting variations in seam thickness. The cosine of the instantaneous phase is particularly useful for enhancing sedimentary features ([[:File:YGS_CHR_08_IMAG_FIG_07.jpg|Figure 7]]a). Instantaneous frequency measures the temporal change in instantaneous phase and can be used to identify features that alter the frequency content of the data, such as gas accumulations that attenuate the high frequencies. Theoretically, instantaneous frequency can be used to detect variations in seam thickness, but it is susceptible to noise contamination and can therefore be of limited use when the anomalies are very small. It was not found to be very useful in this study. The absolute trace amplitude plot requires careful processing, but makes the identification of small faults and other low-amplitude zones easier ([[:File:YGS_CHR_08_IMAG_FIG_07.jpg|Figure 7]]b).
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A suite of seismic attributes was calculated on the 3-D volume. Of these, the most useful were expected to be reflection strength or amplitude envelope, instantaneous phase and instantaneous frequency. Reflection strength highlights changes in acoustic impedance and is used to identify anomalous impedance horizons such as gas-charged sandstones. As coal seams have a very low impedance compared with the surrounding strata, the reflection-strength plots will show detail in the coal seam that can lead to the recognition of small faults and variations in seam thickness . Instantaneous phase emphasizes the linearity of events and thus highlights breaks such as pinch-outs at unconformities and faults; it is not a useful attribute for detecting variations in seam thickness. The cosine of the instantaneous phase is particularly useful for enhancing sedimentary features ([[:File:YGS_CHR_08_IMAG_FIG_07.jpg|Figure 7]]a). Instantaneous frequency measures the temporal change in instantaneous phase and can be used to identify features that alter the frequency content of the data, such as gas accumulations that attenuate the high frequencies. Theoretically, instantaneous frequency can be used to detect variations in seam thickness, but it is susceptible to noise contamination and can therefore be of limited use when the anomalies are very small. It was not found to be very useful in this study. The absolute trace amplitude plot requires careful processing, but makes the identification of small faults and other low-amplitude zones easier (Figure 7b).
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'''Table 2 Processing sequence.'''
  
'''[[:File:YGS_CHR_08_IMAG_TAB_02.jpg|Table 2]] Processing sequence'''
 
  
 
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A significant sandbody is known to lie between the Dunsil and Top Hard seams (mid-Westphalian B), these seams forming the effective lower and upper limits, respectively, of the overall channel system containing the sandbody. The sandbody is up to 4km wide, possibly including more than one contemporary channel belt, and crosses the study area from northwest to southeast. The maximum sandstone thickness is probably about 15 m, the overall feature being essentially single storey. The Top Hard is typically a multi-coalbed seam up to 2 m thick, and is locally eroded in the wider area (the result of later Coal Measures fluvial systems). The Dunsil Seam is typically less than 1 m thick itself, but is locally united with the underlying First Waterloo Seam, to give a composite seam of more than 2 m (as at Nickerbush). The Dunsil Seam is locally eroded within the channel belt area.
 
A significant sandbody is known to lie between the Dunsil and Top Hard seams (mid-Westphalian B), these seams forming the effective lower and upper limits, respectively, of the overall channel system containing the sandbody. The sandbody is up to 4km wide, possibly including more than one contemporary channel belt, and crosses the study area from northwest to southeast. The maximum sandstone thickness is probably about 15 m, the overall feature being essentially single storey. The Top Hard is typically a multi-coalbed seam up to 2 m thick, and is locally eroded in the wider area (the result of later Coal Measures fluvial systems). The Dunsil Seam is typically less than 1 m thick itself, but is locally united with the underlying First Waterloo Seam, to give a composite seam of more than 2 m (as at Nickerbush). The Dunsil Seam is locally eroded within the channel belt area.
  
'''[[:File:YGS_CHR_08_IMAG_TAB_03.jpg|Table 3]] Sand bodies, mapped from borehole and mining records, across or adjacent to 3D study area (in descending stratigraphical order)'''
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'''[[:File:YGS_CHR_08_IMAG_TAB_03.jpg|Table 3]] Sand bodies, mapped from borehole and mining records, across or adjacent to 3D study area (in descending stratigraphical order).'''
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==== 3.2.3 Selected sandbodies: above the Parkgate Seam ====
 
==== 3.2.3 Selected sandbodies: above the Parkgate Seam ====
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Widess, M. 1973. How thin is a thin bed? ''Geophysics ''38, 1176–80.
 
Widess, M. 1973. How thin is a thin bed? ''Geophysics ''38, 1176–80.
 
[[category:Carboniferous hydrocarbon resources: the southern North Sea and surrounding onshore areas ]]
 

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