Author: Simon Blondel – OGS Trieste
Hello everybody, this week it is my turn to share my research with you. My name is Simon, I am French, I studied geology at the UniLaSalle engineering shool in Beauvais, before joining the IDEA LEAGUE Applied Geophysics Programme (TU Delft, ETH Zürich, RWTH Aachen) in 2015. After a year working as a GIS & data coordinator for TOTAL in Pau, I have joined the SALTGIANT ETN Fellowship in February, so I am starting a bit late compared to my fellows. In a nutshell, my research involves processing and interpreting seismic data acquired in the Mediterranean Sea. With the help of the other ESRs, I will try to better understand the paleoenvironments and the deformations associated with the Messinian salt, and assess if we can classify the Messinian salt structures in terms of geological settings based on their seismic attributes. I have just started my project so this week I will just give you the big picture, highlighting why am I doing this and why is it challenging to image the Messinian salt with seismic techniques.
The Mediterranean Sea is a unique enclosed and rich environment on which depends the life of millions of people. Political instabilities, over-fishing, industrial exploitation of the sea and its subsurface, global warming… The stress parameters are multiple, hence the need to regulate them and ensure environmentally friendly activities. It is against this background that SALTGIANT was born to understand the far-reaching implications that the Messinian Salt Giant (MSG) has for fundamental science, industry and society.
The MSG is exploited on land for the gypsum mining industry and plays an important role offshore for the accumulation of hydrocarbons. This is because salt structures are effective traps and seals for petroleum systems, and many hydrocarbons discovered worldwide are trapped in salt-related structures (Farmer et al., 1996). Hydrocarbon discoveries in the last decade in the Eastern Mediterranean have particularly increased investment from the oil and gas industry, with many exploration licenses granted (Figure 1), changing the geo-economic and geo-political balance of the region (Bornstein, 2018). As for the gypsum mining industry, they anticipate an increasing demand for natural rock gypsum with the foreseen decline of coal-burning power production (EUROPEAN COMMISSION, 2017). This is because flue gas desulphurisation (removal of SO2 gas from coal-combustion power plants) was, and still is, a significant source of industrially produced gypsum. Nowadays a large part of the MSG has been imaged with reflection seismic and it there is a good overview of the seismic markers associated with the Messinian compiled by Lofi (2018) and Lofi et al. (2011). However, we lack a mechanistic understanding of how crustal tectonics, global sea-level and climate forced the Mediterranean into a hypersaline mode, leading to the deposition of evaporite salts on such a huge scale (EUROPEAN COMMISSION, 2017). SALTGIANT will help gaining insight into this poorly understood natural event, and to provide the required state-of-the-art knowledge on the circum-Mediterranean geological settings that will help to better access the risks involved in drilling through salt and mining the gypsum.
One of our goals is to provide the oil industry with conceptual models that can be used to evaluate risks of drilling through thick salt deposits. Salt is a viscous, low density, high seismic velocity and impermeable material that creeps under differential stress, as a function of both temperature and stress difference (Dusseault et al., 2004b). Around and below salt, a wide range of stress and pore pressure conditions can be encountered, including zones with open fractures, overpressured fluids or unusual horizontal stress regimes (Dusseault et al., 2004a). Drillers have to be cautious and address factors that can cause borehole closure through creep within the salt, borehole instability when exiting salt or in cases of sharp stress gradients or lithology contrasts, lost circulation, or sudden transition to overpressure beneath salt that could lead to a blowout at the well-head (Dusseault et al., 2004a, b; Farmer et al., 1996). In addition to this, the deep biosphere of the MSG can produce H2S (“sour gas”) through microbial sulfate reduction. This is a dangerous gas which needs to be removed from producednatural gas (Machel H.G. and Foght J., 2000), or monitored when gypsum is mined. Models that capture the critical aspects associated with salt such as: creep behaviour and high solubility, heat flux, stress orientations and magnitudes, general tectonic regime, and the displacement history of the sediments around the structures are used to establish the strategies for risk reduction in drilling through and around salt. These models are based on the geological history of the salt structure emplacement, estimations of the pressure and temperature gradients in depth, fracture pressures in non-salt rocks or overpressure predictions (Dusseault et al., 2004a, b). These can be based on seismic images and borehole data. The latter one is the most reliable measurement of the subsurface in depth, but it provides us only a local 1-D measurement of the subsurface and when exploring new areas, it is generally acquired at a later stage based on analysis of seismic data. Therefore, seismic data plays a crucial role in the drilling program by providing an accurate image of those salt bodies. Not only the location the position of the drilling is based on these images, but also it is used to build the models used for guiding and assessing risks associated with the drilling.
However, imaging the salt and the underlying sediments is not such an easy task. Seismic images are produced from records of the time an emitted source wavefield takes to travel down and back through subsurface where it is reflected by rock interfaces. The recorded signal is then processed using various mathematical operators and filters in order to retain only the compressional waves (P-waves) and produce an image of the subsurface. Before the 2000s, conventional data were mostly collected from just one direction (or azimuth) along a 2D-line applying the Narrow-Azimuth Towed Streamer (NATS) method (Maghoub et al., 2017). The amount of emitted energy reflected at the interfaces is proportional to the difference in the physical properties of rocks. The high velocity and density contrast between the salt and the surrounding sediments makes it a strong reflector, and in fact the transmitted signal below the salt is considerably weakened and it is hard to see what lies beneath it. Moreover, the complex shape of the diapirs and the abundant faults in their vicinity give rise to a complex seismic wave field. The recorded data not only contain the energy from the signal that reflected at the interfaces but also interference generated by refraction, scattering or P- and S-wave mode conversion of the downgoing wavefield, referred to as noise. This noise is particularly hard to get rid of.
The signal-to-noise ratio describes how strong the noise is compared with the reflected signal, and is a good indicator of the quality of the data. If the noise is not dealt with correctly during the processing stage, the resulting seismic data exhibit a low signal-to-noise ratio, a low resolution, a low imaging accuracy and a low amplitude-preservation, making interpretation and velocity model building more difficult and less reliable. For a long time, salt-related noise was not correctly removed and dealt as if it was reflected waves (Jones, 2014). However, many processing operations are based on the assumption of a noise-free horizontally layered isotropic medium, and provide poorer results when these assumptions are violated. Consequently, the salt bodies appeared as a white bottomless structure on seismic images, often referred as “salt mask”, below which interfaces were hardly visible. Only on the 1980s new computer-based seismic processing techniques began to correctly image these structures, but the true technical innovation appeared in the 1990s with the application of pre-stack migration techniques, following the emergence of massive parallel processor computers (Farmer et al., 1996).
These led to a boom in the oil and gas industry, with several discoveries associated with salt-structures, and accelerating the research in this domain. In the 2000s, Wide and Full Azimuth Survey (WAZ and FAZ) methods of acquisition, where data is acquired simultaneously by several vessels, gave a much clearer and more accurate picture of the salt bodies and what lies beneath them (Whaley, 2006). With these methods, the seafloor is illuminated from many different angles (or azimuths), providing a better coverage of the subsurface, a wider frequency content, which results in broadband seismic data (Maghoub et al., 2017). This enabled novel techniques to measure and quantify the amplitude-variation-with-offset and anisotropic effects within the data and improve the signal-to-noise ratio (Singleton et al., 2014). In addition, high-performance computing allowed the use of imaging algorithms that were not performing well with low-quality seismic data in the past. In the 2010s, advanced techniques such as reverse time migration (RTM), Tilted Transverse Isotropy (TTI) anisotropic migration or Full Waveform Inversion (FWI) became the standard technologies for building high resolution velocity model and proper images of complex subsalt geology (Maghoub et al., 2017). One remaining drawback of these techniques is their sensitivity to the velocity model they use. In fact, it is still difficult and time-consuming to build reliable velocity models that correctly represent the subsurface. We often oversimplify the salt cap structures, the composition of the salt bodies, stress-effects and anisotropy because we don’t have sufficient information to adequately describe them, which distorts the final image (Jones and Davison, 2014).
The 2000s also saw the development of joint interpretation/inversion of different geophysical measurements with seismic notably with the rise of 3D marine Controlled Source ElectroMagnetic (CSEM) surveying, a geophysical technique that maps subsurface resistivity that became of interest at that moment for hydrocarbon exploration. The idea is to provide additional information with the measurement of other geophysical parameters that have a better penetration or a different sensitivity. Common multigeophysical surveying includes CSEM, magnetotelluric or gravity gradiometry methods. They can notably be used to diminish uncertainties unresolved by vintage NAZ seismic data and constrain velocity models or structural frameworks (Sun et al., 2015). The so -called “hybrid” interpretation can be an effective alternative to expensive advanced depth migration or wide/full azimuth seismic data acquisition. as shown per example by Zerilli et al. (2014) or Colombo et al. (2013) for complex salt-model imaging.
Nowadays, the latest acquisition technologies such as multicomponent or ocean bottom cable acquisition allows converted waves to be recorded as well as standard compressional waves (Singleton et al., 2014). This opens a new field in seismic processing to remove converted waves and improve the signal-to-noise ratio of salt images, but these technologies are still environment-limited and expensive (Whaley, 2006).
The aim of my PhD is to contribute to the assessment of drilling hazards by identifying and classifying salt structures in different geological environments of the Mediterranean Basin. We need to improve our images of MSG units, pre-salt reflectors, polygonal fault systems and other indicators of fluids migration in the sedimentary sequence. To do that we will apply an amplitude-preserving processing sequence and compare MSG units among different margins and basins of Mediterranean Sea. I will reprocess and interpret available offshore multi-channel seismic reflection data including regional vintage public data, academic data, and industry data. Salt deformation structures will be identified and classified through depth imaging in order to reconstruct geometries in detail. Seismic data and attributes will be used to identify evidence of overpressure below the salt and to link the deformation phases to gravitational spreading and gliding. Finally, the drilling hazard working group will contribute to the Integrated Ocean Discovery Project (IODP) uncovering a Salt Giant Multiplatform Drilling (DREAM) Project accepted in January 2015, by contributing to the choice of the drilling trajectory based of a new understanding of the depth, the temperature and the stress within and around the MSG.
Bornstein, R., 2018. Eastern Mediterranean Regional Dynamics: Conflicts and Opportunities for Conflict Resolution Support, Policy Brief. EuroMeSCo, p. 7.
Colombo, D., McNeice, G., Curiel, E.S., Fox, A., 2013. Full tensor CSEM and MT for subsalt structural imaging in the Red Sea: Implications for seismic and electromagnetic integration. The Leading Edge 32, 436-449.
Dusseault, M.B., Maury, V., Sanfilippo, F., Santarelli, F.J., 2004a. Drilling Around Salt: Risks, Stresses, And Uncertainties, Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS). American Rock Mechanics Association, Houston, Texas, p. 12.
Dusseault, M.B., Maury, V., Sanfilippo, F., Santarelli, F.J., 2004b. Drilling Through Salt: Constitutive Behavior And Drilling Strategies, Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS). American Rock Mechanics Association, Houston, Texas, p. 13.
EUROPEAN COMMISSION, 2017. Grant Agreement number: 765256 — SALTGIANT — H2020-MSCA-ITN-2017, In: Agency, R.E. (Ed.).
Farmer, P., Miller, D., Pieprzak, A., Rutledge, J., Woods, R., 1996. Exploring the subsalt.
Jones, I.F., 2014. The Seismic Response to Strong Vertical Velocity Change, 76th EAGE Conference and Exhibition 2014 EAGE.
Jones, I.F., Davison, I., 2014. Seismic imaging in and around salt bodies. Interpretation 2, SL1-SL20.
Lofi, J., 2018. Seismic atlas of the Messinian salinity crisis markers in the Mediterranean sea. Volume 2. Commission for the Geological Map of the World (CGMW) / Mémoires de la Société Géologique de France 181, 72.
Lofi, J., Déverchère, J., Gaullier, V., Gillet, H., Gorini, C., Guennoc, P., Loncke, L., Maillard, A., Sage, F., Thinon, I., 2011. Seismic atlas of the “Messinian Salinity Crisis” markers in the Mediterranean and Black seas. Commission for the Geological Map of the World (CGMW) / Mémoires de la Société Géologique de France 179, 72.
Machel H.G., Foght J., 2000. Products and Depth Limits of Microbial Activity in Petroliferous Subsurface Settings, In: Riding R.E., S.M., A. (Eds.), Microbial Sediments. Springer, Berlin, Heidelberg.
Maghoub, M.A.G., Elias, B., Santos, M., Lundungo, W., 2017. Cost-effective Seismic Data Reprocessing for Subsalt Imaging Enhancement, First EAGE/ASGA Petroleum Exploration Workshop EAGE, Luanda.
Singleton, S., Gillooly, J., Ridyard, D., Horn, B., 2014. Technological Innovations Drive Trends In Onshore, Offshore Seismic Acquisition, The American Oil & Gas Reporter, Houston.
Sun, L., Fang, C., Sa, L., Yang, P., Sun, Z., 2015. Innovation and prospect of geophysical technology in the exploration of deep oil and gas. Petroleum Exploration and Development 42, 454-465.
Whaley, J., 2006. The sub-salt imaging challenge.
Zerilli, A., Labruzzo, T., Zanzi, M., Brgc, S., Buonora, M., Crepaldi, J., Menezes, P., p, P., Geof, Indonesia, M., 2014. Broadband marine CSEM: New benefits for subsalt and around salt exploration.