A multi-physics approach to geoscience allows NEOS to simultaneously interpret several possible geophysical measurements in order to uncover deeper insights into regional prospectivity and well productivity. While there may be potential for the inclusion of a number of measurements within an interpretation, each is equally unique and beneficial on its own accord.
GRAVITY & MAGNETICS – A PART OF MMI
NEOS incorporates gravity and magnetics in all neoBASIN programs, as these measurements are fundamental to the company’s multi-measurement interpretation (MMI) methodology. Gravity and magnetic data is acquired, along with other MMI measurements, using airborne systems simultaneously in a fast, efficient and non-invasive way, which reduces the turnaround time for the data acquisition, integration and interpretation. The data collected gives NEOS insight into the subsurface over large regional, basin-scale areas or at the more detailed, prospect level.
Gravity and magnetic methods are commonly referred to as potential field methods because the measurements involve a function of the potential of the observed field of force (gravity or magnetic). The data is used to help delineate geologic features in the Earth’s upper crust related to lithologic changes caused by natural hazards (faults, volcanoes), natural resources (gas, minerals) and tectonic events such as the formation of mountain belts.
These measurements take advantage of the variation in different lithologic characteristics of subsurface rock in a given area and can quickly and easily map location, extent, depth and structure of sedimentary basins. In addition, these methods can quickly identify faults, mineral deposits, igneous intrusive and extrusive, and depth-to-crystalline basement.
While typically grouped together, it’s still important to understand how each functions, so let’s look at gravity and magnetics individually.
[Left] gravity image and [right] magnetic image of the Gulf of Mexico.
Gravity measures very small variations (anomalies) of the Earth’s gravitational field that are caused by lateral variations in the density of Earth material.
Specifically, gravity data is useful whenever the formation(s) of interest have densities that are different from surrounding formations. There is a wide range of densities within the Earth’s crust, from essentially zero density of air-filled voids in near-surface formations to the highest densities related to iron/magnesium-rich basement rocks and metallic ores. Because of the wide range of densities within all rock types, geoscientists interpret measurements and draw conclusions regarding the distribution of underground rock types that may be commonly favorable to trapping oil or gas. A typical gravity survey will map sedimentary basin configuration, structure and thickness, and faults. It will also map basement configuration where basement rocks are of higher density than sedimentary section, and salt bodies and distribution.
For example, salt has a very low density of 2.15. The identification of a salt dome would help to locate possible oil and gas reservoirs as many oil and gas deposits are located along the structures caused by salt movement within the sedimentary basin. A typical example of this is the large oil and gas reservoirs along the Gulf of Mexico.
The magnetic field of the Earth is generated by electrical currents in the liquid outer core. Rocks that have high magnetite content typically have the property ideal for magnetization, also known as susceptibility. Magnetic data measures the minute changes in a rock’s magnetic response caused by contrast in the susceptibility after an external magnetic field is removed. Field data responds to tectonic fabric, due to the redistribution of magnetite rich rocks and is typically used in mapping basement structure or the thickness of sedimentary section.
Faults and fractures are easily detected by magnetic method due to mineralization along fault planes. This method is widely used in the exploration and distribution of mineral deposits containing magnetite. In oil and gas exploration, magnetic method is used in determination of depth-to-basement, fault detection and distribution, basement rock type leading to recognition of differential thermal conductivity or the thermal maturity of a basin.
In the field of regional exploration, magnetic data is useful in delineating crystalline basement structures and tectonic elements as rift basins and basement involved tectonic elements. It also can delineate the distribution of volcanic material, both intrusive and extrusive. Recently, NEOS has used magnetic data to look for changes in basement lithology and successfully related it to enhanced production in overlying shales.
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