Exploration Insights September 2019 - Page 12

12 | Halliburton Landmark D OL E OC Exploration Insights | 13 BASA AN f n o n gio sio Re pres m co SUBS Region of tension LT I C T R AT UM Ascending Currents (Early Stage 1) Island or Swell NEW n ea Oc eep D L AY E SUBS O C EA N R T R AT OL D OC R EA co egi N mp on res of sio n UM Platea u Basalt s Mo u Ra ntain nge ain unt Mo ange R SUBS Ec log T R AT UM Basaltic Magma rises with Ascending Currents (Stage 2) SUBS Oc De ean ep T R AT UM Ec ite log ite Figure 2 > A representation of the mechanism by which Holmes envisaged continental drift might operate, first presented in Holmes’ Principles of Physical Geology in 1944. The World-Wide Standardized Seismograph Network (WWSSN) was established in the 1950s, to monitor the post-WWII nuclear threat via a network of about 120 seismometers. This allowed seismologists to locate a greater number of earthquakes, and understand that the majority align with mid-ocean ridges and deep trenches, effectively mapping out all the plate boundaries. The race for the formulation of a theory of plate tectonics was on. In 1966, Dan McKenzie published his article, The Viscosity of the Lower Mantle (McKenzie, 1966). Together with The North Pacific: An Example of Tectonics on a Sphere (McKenzie and Parker, 1967), it established the mechanisms and the mathematical theory of the motion of tectonic plates at the surface of the Earth, using Euler’s Fixed-Point Theorem, in conjunction with magnetic anomalies and earthquakes. PLATE TECTONICS IN THE HYDROCARBON INDUSTRY Throughout the 1970s, plate tectonic theory became mainstream across the geoscience community and was rapidly adopted by the oil and gas industry. Geologists were able to assign a cause to the formation of great geological structures, such as mountain ranges, rifts, and oceans, all created by horizontal tectonic forces generated through the motion of plates. Plate tectonics, as a unifying theory of geoscience, soon started to support geological predictions. Away from data control, geologists could predict with more confidence the occurrence of structural 4 3 2 1 Simultaneous advances in seismic imaging techniques along the trenches bounding many continental margins, together with many other geophysical (e.g. gravimetric) and geological observations, showed how the ocean crust could be subducted, providing the mechanism to balance the extension of ocean basins with shortening along their margins. 1 2 3 4 Normal magnetic polarity Reversed magnetic polarity Mid-oceanic ridge features and stratigraphic patterns along a margin, or even on conjugate margins across an ocean. Plate tectonics controls eustacy on long-term cycles (10 7 –10 8 years) mainly through changes in ocean spreading rates (Conrad, 2013). This, in turn, has a profound impact on sedimentation patterns, which affects the broad distribution of source rocks, reservoirs, and seals. The dream of geologists is to visualize the Earth as it once was. An understanding of plate tectonics allows us to reconstruct plate motions back in time, enabling the location of data points in their original geographical positions. Consider the North Atlantic; by appreciating that the margins of Nova Scotia and Portugal were once aligned, we can make inferences about the possible extension of known petroleum system elements on one side of the Atlantic to unexplored areas on the other side. Ultimately, we can draw palaeogeographic maps that show the depositional environments on ancient models of the Earth. Such maps are extremely powerful for developing an understanding of the location of reservoir, source, and seal facies. Plate Tectonics 2.0: Into the Digital Age The adoption of plate tectonics and the recognition of its applications for the prediction of petroleum systems elements led to the development of various academic and commercial plate tectonic models, to support the reconstruction and interpretation of geospatial data back through geological time (e.g. Scotese and Baker 1975; Scotese, 1976; Muller et al., 1993; Stampfli and Borel, 2002). These models (see full review in Verard, 2018) are effectively dynamic maps, in which the components move throughout geological time on a spherical representation of the Earth’s surface. The Neftex ® Plate Model originated in the 1990s at the University of Lausanne, under the supervision of Gerard Stampfli. It was originally co-funded by Shell and Neftex, eventually becoming a full part of the Neftex portfolio in 2010. Since then, it has been intensively modified to redefine its kinematics and is now underpinned by a huge array of new supporting information. New delivery mechanisms have also been developed. It has evolved and expanded to become a practical and intuitive cloud-hosted online plate reconstruction micro-service through the addition of our QuickPlates platform, in 2016. Building detailed plate models is not a trivial task. Plate positions are difficult to track back from observational methods alone and one has to rely on a number of proxies. It generally requires the use of absolute and relative plate positioning methods, which can be both quantitative and qualitative. It is possible to determine paleo-latitude directly from a number of proxies such as paleo-magnetic or temperature sensitive biostratigraphic data, but paleo-longitude information cannot be assessed directly (Torsvik, 2019). The Neftex Plate Model is constrained primarily to fit the Age before present (millions of years) Calculated magnetic profile assuming seafloor spreading Observed magnetic profile from oceanographic survey © 2019 Halliburton Zone of magma injection, cooling and “locking in” of magnetic polarity Figure 3 >Magnetic stripes on the ocean seafloor, discovered by Vine and Matthews (1963). (Source: https://www.geolsoc.org. uk/Plate-Tectonics/Chap1-Pioneers-of-Plate-Tectonics/Vine-and- Matthews). Figure 4 >From left to right: palinspastic paleogeographic map, paleo-digital elevation model, paleo-climatic zones, and source rock chance map, all underpinned by plate reconstructions using the Neftex Plate Model.