Broadband seismic to support hydrocarbon exploration on the UK Continental Shelf Gregor Duval 1 1 CGGVeritas Services UK Ltd, Crompton Way, Manor Royal Estate, Crawley, RH10 9QN, UK Variable-depth streamer acquisition (BroadSeis TM ) is emerging as an effective technique for providing wide-bandwidth seismic data (Soubaras and Dowle, 2010). This technology combines elements of improved equipment, acquisition and processing to deliver broadband data. The use of solid streamers, reducing acquisition noise, allows the recording of signal at the very low frequency range, down to 2Hz (Dowle 2006). The proprietary curved, variable-depth geometry typically uses cable depths down to 50m, such that the data acquired tends to be less noisy due to the quieter recording environment. The optimised variation in the receiver depth introduces receiver ghost diversity over different offsets, which enables the receiver ghost to be fully removed by using a joint-deconvolution method (Soubaras 2010). Combining all of these aspects together results in truly broadband data, giving the interpreter additional information in the form of stunning, high resolution images of the subsurface. Details are enhanced thanks to the sharper wavelets giving better resolution of important features such as thin beds and stratigraphic traps. The improved low frequencies provide better penetration for deep targets, as well as better stability for seismic inversion. In a mature basin such as the North Sea, such data provide significant enhancements in a range of environments: exploring for new stratigraphic traps and subtle structural closures, and enhancing hydrocarbon recovery with more information about local facies variations and reservoir compartmentalization. This paper takes a step-by-step approach through the stratigraphy of the North Sea, demonstrating the benefits of broadband data from shallow to deep, illustrated by examples from two 3D surveys from the North Sea, one in the region of UKCS Q20 and the other around UKCS Q29. Need for a broad bandwidth It is easy for seismic interpreters to understand the need for higher frequencies in seismic data since they provide more detail about the geology, such as thin stratigraphic features and subtle rock structures. However, low frequencies are just as important. They contribute to better imaging of deep targets and large scale facies variations as well as providing better stability for pre- and post-stack seismic inversions. Ideally, the seismic interpreter would like to see a seismic wavelet with as sharp a peak as possible and no side-lobes. Adding in more high frequencies narrows down the wavelet central peak while introducing more low frequencies reduces the amplitude of the side-lobes, making the wavelet look more like a single spike. Broadening the frequency spectrum at both the high and low ends has a combined effect, thereby giving the genuine seismic signature of formation interfaces. For instance, Figure 1 displays a comparison focused around the top of the Dornoch Formation, which includes several thin and highly reflective coal layers, in the Outer Moray Firth area. Broadband data are less subject to tuning effects and allow interpretation of pinch-outs (Zone 1 in Figure 1) and thin beds (Zone 2). Furthermore, the absence of side-lobe interferences gives the genuine seismic signature of the Dornoch coal layer (Zone 3). Conventional data fails to resolve any of these details.
Shallow interpretation Figure 1: Wavelet signature comparison at top Dornoch Formation level. Due to the steep slope of the near offsets when using curved variable-depth streamers, the benefits of broad bandwidth are achieved even at very shallow depths, giving stunning images of recent deposits and formations lying directly below the sea floor. Figure 2 shows a time slice comparison at 216ms TWT. The high frequency content is key in defining the sharp edges of the channels and imaging the narrowest ones (less than 50m width). On the other hand, the low frequency content helps differentiate the bed rock from channel fill deposits which appear darker gray in Figure 2. Such high resolution seismic data are of great assistance for preliminary site surveys and identification of shallow hazards. Also, these shallow channels produce rapid velocity variations that affect seismic ray path modelling. If not modelled properly, this can result in severe artefacts on tomographic velocity models generated for Pre-Stack Depth Migration (PSDM) work. The use of broadband data then becomes very important to carefully model these channels and get the best out of the PSDM processing sequence. Figure 2: Time slice comparison at 216ms showing shallow channels
Interpreting the Tertiary stratigraphy Most of the Tertiary structural closures have been drilled in the North Sea basins. However, part of the remaining potential lies within subtle stratigraphic traps and pinch-outs. This is where broadband technology has the potential to play a major role. The higher frequency content pushes the limits of tuning effects further. It helps in resolving thin beds and pinch-outs that have never been seen before. The low frequencies also play an important role by reducing side-lobe interferences and helping in the interpretation of facies transitions. Focusing on the prospective Tertiary turbidite systems of the North Sea, Figure 3 displays images of a thick, low acoustic impedance shale sequence in the Paleocene Lista formation. The seismic correlates very well with well log data showing top and base of these soft shales. The colour scale uses the following convention: white for a decrease of impedance and black for an increase of impedance. At first glance, the broadband seismic data give the impression of a shaded relief 3D image, but in fact this effect is due to the presence of strong amplitude low frequencies as they bring in the true envelope signature of this formation revealing a major impedance contrast. Away from the well calibration point, in the central part of the section, both the conventional and the broadband seismic data show a prominent relief structure corresponding to a channel compactional feature. The presence of such a feature usually tells the geologist that the channel fill deposits have a different lithology, i.e. that they are composed of less compactable sediments, including the desirable reservoir channel sands. Figure 3 shows that the top of these sands can hardly be identified on conventional data while it appears very clearly on broadband data. We can also see that channel levee sands, over bank deposits and sand body pinch-outs are easier to identify. Even Top Chalk appears as a much more continuous horizon. Auto-picking tests have demonstrated that it is five times faster to interpret this horizon on broadband data. This example emphasizes again the important role that broadband seismic data can play in efficiently and accurately mapping facies distributions and ultimately describing new potential stratigraphic play concepts in the UKCS area. Figure 3: Seismic section comparison focusing on the turbidite complexes of the Paleocene Lista formation
Sub-BCU and sub-basalt imaging Sub-BCU imaging can often be a challenge in the North Sea basins, especially for deep HP/HT targets in the Central Graben or in the Viking Graben. The addition of low frequencies to the spectrum allows easy discrimination of the main sedimentary packages and sequences found below the BCU. These low frequencies give an envelope to the seismic signal that shapes the larger scale impedance variations corresponding to major lithology variations. This helps in correlating seismic interpretation across major Jurassic rift faults, as well as better defining sub-bcu fault planes on broadband data (Figure 4). In the West of Shetland area, imaging of deep sub-basalt features is enhanced thanks to the deep penetration of low frequencies. Deep sub-basalt stratigraphy and volcanic intrusions can be interpreted with more confidence. Figure 4: Deep sub-bcu seismic section comparison Figure 5: BCU amplitude map comparison
The benefits for sub-bcu interpretation are also demonstrated in Figure 5 which displays a comparison of BCU amplitude maps for variable depth streamer and conventional data. This comparison illustrates the fact that broadband data give a much better definition of fault polygons and their associated amplitude discontinuities. In the upper left corner, the broadband amplitude map gives additional details of sub-cropping reflectors which are more difficult to identify on conventional data. It is also worth noting that using the same technique to pick the BCU reflector on both the broadband and conventional datasets (i.e. manual horizon picking every 10 inlines, no crossline interpretation, and 3D auto-tracking using a 10ms correlation window), the auto-tracking was noticeably faster on the broadband dataset and the final 3D BCU horizon was less noisy (i.e. fewer mis-picks and spikes). Conclusions The use of broadband seismic data is key to unlocking the remaining hydrocarbon potential of the UKCS. Variable-depth streamer acquisition technology delivers a much broader frequency range to final seismic images, enabling the interpreter to: Accurately interpret rock stratigraphy, facies distribution and subtle structures benefiting from the high frequency content Produce a clearer interpretation of deep targets: sub-salt, sub-basalt and sub-chalk HP/HT plays benefiting from the low frequency content Extract the true seismic signature of the geological formations by removing the wavelet side-lobes benefiting from both the low and high frequencies Thanks to these benefits, such data save seismic interpreters precious time, a key benefit in the race for finding the remaining hydrocarbon accumulations in the UK Continental Shelf area. References Dechun, L. et al. (2011). Optimizing the processing flow for variable-depth streamer data. First Break vol. 29, September 2011, pp. 89-95. Dowle, R. (2006). Solid streamer noise reduction principles. 75 th SEG Annual Meeting, Expanded Abstracts 25, pp. 85-89. Soubaras, R. (2010). Deghosting by joint deconvolution of a migration and a mirror migration. 80 th SEG Annual Meeting, Expanded Abstracts, pp. 3406-3410. Soubaras, R. and R. Dowle (2010). Variable-depth streamer a broadband marine solution. First Break vol. 28, December 2010, pp. 89-96.