Be sure to grab your April issue of First Break, and turn to this month’s special topic: EM & Potential Methods, where NEOS discusses the results from resistivity imaging using ZTEM and MT data in the geophysical study of a ~2900 km2 region of the northern Raton Basin in southern Colorado.
Because of the land access and permitting issues, as well as the large amount of terrain needing to be traversed during the geophysical survey, we decided to incorporate a dense airborne ZTEM survey along with the sparse MT stations.
On the Lebanon neoBASIN regional reconnaissance project, we acquired EM resistivity data from roughly 45 ground-based magnetotelluric (MT) stations that were deployed throughout the survey area. Challenging topography and dynamic geo-political conditions on the ground didn’t always let us deploy the stations where we wanted to, but we did get some interesting results nonetheless.
The MT method relies on three primary sources of electromagnetic (EM) energy, all with different frequency ranges and, therefore, depths of penetration. High-frequency signals originate from lightning, in particular ongoing equatorial lightning strikes; intermediate frequency signals come from ionospheric resonances; and low frequency signals are generated by variations in the solar wind.
Driven by advances in sensors, DP and modeling methods, can computing power, MT has become one of the most important tools in deep Earth imaging with a capability to image to subsurface depths of 10,000 meters or more.
For hydrocarbon exploration, MT is mainly used as a complement to seismic imaging. This is especially true for scenarios that can be problematic for seismic, such as sub-basalt and sub-salt plays. In addition, while seismic is able to image subsurface structure, it cannot detect changes in resistivity associated with hydrocarbons and hydrocarbon-bearing formations.
By measuring both electric and magnetic responses simultaneously and processing the data using statistically rigorous mathematics, MT allows resistivity variations to be mapped with depth in the subsurface. Under the right geologic conditions, MT can differentiate between structures bearing hydrocarbons and those that do not.
In the image above, we analyzed the MT data in several different ways, initially in an unconstrained fashion and then in a constrained inversion where we also considered the measurements from, and structrual models being generated using, airborne-acquired gravity and magnetic data.
The multi-measurement integration and interpretation yielded very good results. In the 2-D constrained EM resistivity section shown, you’ll note several interesting features:
First, the fairly clean resistivity delineations among the key stratigraphic horizons;
Second, the insight EM resistivity information can bring to fault mapping, including the displacement along faults;
Lastly, the interesting intra-horizon resistivity increase as one moves up-section within the Cretaceous.
The scale here is logarithmic, so this is roughly a 10x increase in resistivity taking place within a 500-800 meter thick interval. Equally curious is that it’s taking place along and adjacent to a deeply penetrating fault. Simply an aberration? Noise in the data? A change in lithology? Or a change in fluid type at the highs abutting the fault?
Out this week, First Break’s Annual EAGE Conference Issue, “Experience the Energy”. Get your copy today and skip straight to the “Special Topics” section for the piece on NEOS’ approach to interpreting airborne-acquired EM data much deeper below the surface.
Novel approaches to acquire, process and interpret airborne-acquired, passive-source electromagnetic datasets are now being tested in the field and are showing promise in their potential for delivering new insights…
The rush for unconventional oil and gas resources is increasing the level of hazards faced by producers in the field. Hazards and risks could include anything from long-abandoned wellbores to shallow gas pockets, unknown faults and fractures, and the natural seepage of hydrocarbons into groundwater that may be blamed on drilling operations at some future date if baseline information is not gathered first.
NEOS GeoSolutions, Inc., a Houston-based geosciences company known primarily for its airborne neoBASIN surveys, has now adapted its expertise to a specialized survey created to deal with those concerns. Called Eco-Assurance…
The article continues. To read the full article, click here or on the image above.
To access the website for New Technology magazine – the First Word on Oilpatch Innovation – click here.