Exploration Insights September 2020 - Page 26

14 | Halliburton Landmark

New Frontiers in Plate Boundaries

by : Jean-Christophe Wrobel-Daveau , Bruce Eglington and Graeme Nicoll

Crustal dynamics showing earthquakes , plate boundaries , and volcanoes . Image from https :// svs . gsfc . nasa . gov / 155 with credit to NASA / Goddard Space Flight Ce Research Project ( GCRP ), National Oceanic and Atmosphere Administration ( NOAA ), United States Geological Survey , National Science Foundation ( NSF ), Defen ( DMA ), New York Film and Animation Company , Silicon Graphics , Inc . ( SGI ), Hughes STX Corporation .

Given a plate tectonic model is meant to be a singular representation of what the Earth looked like back through geological time , one might ask , why are so many different models available , and which one should I use ? In order to answer these questions , it is useful to understand the genealogy of the different plate tectonic models , what data underpin them , what methods and techniques are used to construct and update them , and what uncertainties are involved .

We will go on to consider how the scope and complexity of these models went hand-in-hand with growing computational power over the last 60 years . In order to compare and contrast the variety of plate tectonic models that exist today , we need to set out the main difference between plate tectonics and continental drift models , and consider the importance of plate tectonic boundaries .

Wrobel-Daveau and Nicoll , 2019 ), and how in the modern age , plate tectonic models are an important tool for use in natural resource exploration workflows ( Lang et al ., 2020 ).

PLATE TECTONIC MODEL GENEALOGY

The first attempts at reconstructing the paleoposition of continental land masses were hand-drawn as far back as the 17 ^th century by Dutch map makers , and later in the 20 ^th century by Alfred Wegner and Boris Choubert ( Kornprobst et al ., 2018 ). However , these were really just singular snapshots of often poorly constrained geological times , and are more akin to paleogeographic maps ( Figure 1 ). Indeed , they lacked the understanding of geodynamic processes , such as the absolute and relative motion of plates on a sphere , the driving mechanisms behind plate motion , and the existence of plate limits .

Exploration Handbook | 27 circulation models consist of a three-dimensional representation of the atmosphere coupled to the land surface. As atmospheric models are unable to model ocean processes they need to be provided with these data. summer and cold winter. This also applies to wind and ocean currents, which tend to follow a distinct seasonal pattern. Of course, in some years, summers may be cooler than normal and winters warmer, but climate is the 30-year average of weather and, as such, is more predictable. There are also ocean model counterparts to atmospheric models. The most complex models couple both atmospheric and ocean models, so that the whole system is considered. These represent the “flagship” models used in paleoclimate studies, and many are also able to simulate additional features, such as vegetation coverage and type. Even the most sophisticated models do not incorporate all aspects of the Earth’s systems. These other features, such as tides, need to be modeled independently. Like all models, paleoclimate models are a simplification of reality. Model resolution may result in an over-simplification of certain aspects of the Earth system. For instance, the width of ocean upwelling zones, important for source rock formation, may be broader than they were in reality, or rain shadows may be missed due to an averaging of the topography. Some processes may not be able to be resolved by a model, such as the rate of precipitation from clouds. In these instances, a value is assigned within the model, which may be an over- simplification as in reality this value may change spatially. There are also uncertainties in the boundary conditions, such as the paleogeography, topography, and bathymetry, and the concentration of CO 2 in the atmosphere. This is why it is important to run several simulations to explore the range of possible scenarios, rather than relying on a single simulation. The choice of model depends on how the simulations will be used and is normally a pragmatic decision that reflects the best trade-off between resolution and sophistication, and computational cost and the time taken to run the simulation. The most sophisticated climate models have geographic cells sizes of several kilometers and contain over a hundred vertical levels. Some models have a cell size of many hundreds of kilometers and contain a small number of vertical layers. The most sophisticated models may only be able to simulate 0.5 years in 24 hours, even using supercomputing facilities, while other models can simulate several thousand years in the same time. Why is this important? Climate is defined as the 30-year average of the weather. Therefore, at least 30 years of simulation are needed to model climate. However, it is not as simple as this. For a climate simulation to be reliable, it is imperative that the climate has reached equilibrium. For instance, if CO 2 levels are changed in a model, the climate is perturbed and becomes unbalanced. The climate must warm or cool to reach a new equilibrium consistent with the new CO 2 level. The true test of the ability of a climate model to predict past conditions is to compare simulations with data. Unfortunately, it is impossible to measure many of the simulated parameters, such as sea temperature, directly in the geological past and we must, therefore, rely on a variety of paleoclimate proxies. These proxies range from sophisticated geochemical paleotemperature estimates to the simple occurrence of climatically controlled facies, such as coral reefs. In general, there is very good correspondence between climate simulations and climatically controlled facies (Figure 3), but this is not always the case for more complex geochemical proxies. These proxies may possess uncertainties from a range of sources, including diagenesis, calibration, and seasonal bias (e.g. Davies et al., 2019), so they must be used with care. When paleoclimate simulations are generated, it is not only atmospheric CO 2 levels that are changed, but also many other boundary conditions, such as the land elevation and the bathymetry of the ocean. It is, therefore, necessary to simulate thousands of years for robust results. To ensure the most sophisticated models are providing robust simulations, they would need to be left running for several years! This is not practical for most applications, least of all hydrocarbon exploration. This is further compounded, as it is good practice to run several simulations with different boundary conditions, due to uncertainties in the boundary conditions (such CO 2 levels). CAN WE REALLY MODEL CLIMATE? Is it possible to model past climates accurately? After all, meteorologists are often unable to predict the weather a few days in advance! It is important not to confuse weather with climate. Weather is the phenomenon that happens every day. It is extremely dynamic (often chaotic) and is, therefore, often hard to predict. However, it is possible to predict, with a high degree of certainty, that it will be warm in Photozoan carbonate (high confidence) B A Photozoan carbonate (low confidence) Large benthic foraminifera Mixed photozoan and heterozoan carbonate Heterozoan carbonate 18 ºC Cold month mean sea temperature 14 ºC Cold month mean sea temperature © t o u r l l i b H a 0 2 0 2 n © 0 2 0 2 l l i H a n r t o b u Figure 3> A) Distribution of modern-day tropical carbonates versus cold month mean sea surface temperature. Most corals are bound by the 18°C cold month temperatures; whereas, larger benthic foraminifera can tolerate conditions as low as 14°C. B) Comparison of an early Eocene simulation against carbonate distribution. Notice the high level of correspondence. “Unfortunately, it is impossible to measure many of the simulated parameters, such as sea temperature, directly in the geological past and we must, therefore, rely on a variety of paleoclimate proxies. These proxies range from sophisticated geochemical paleotemperature estimates to the simple occurrence of climatically controlled facies, such as coral reefs.” 26 | Halliburton Landmark