Neuquén Basin: Spartan MT Survey Application in Oil & Gas Exploration

Spartan MT Deep Resistivity Imaging represents a powerful and flexible method to obtain deep electrical resistivity data over a variety of terrains. High quality Spartan MT data acquired long recording periods over a large frequency range provides accurate imaging of resistivity to depths greatly in excess of the 2000m shown in this sample. The advanced system design incorporates 24-bit dynamic resolution in lightweight and robust survey modules. The Spartan MT is relatively light and non intrusive as it uses natural sources of energy including EM signal generated by lightening and solar wind.

Advanced Magnetotelluric data acquisition and modeling techniques provided by Spartan MT allow imaging of lithologic units, porosity variations and structures at all depths in the earth’s crust. Spartan MT surveys can be scaled to regional reconnaissance or detailed site studies. 

Spartan MT Features

  • Greater accuracy of MT time-series data with 24-bit signal processing, simultaneous field measurements, and a broad band AMT/MT at a frequency range of 300 Hz to 0.001Hz
  • Sophisticated digital signal processing employing remote MT-site reference for removal of environmental noise.
  • Rugged, portable instrument design
  • Multiple field units in operation signify rapid production rates.

Oil & Gas Applications

Traditionally, MT technology has been utilized in petroleum applications to control the thickness of volcanic rock overlying deposit formations, where the crystalline layer typically degrades or prevents the acquisition of quality seismic data.

Advanced 2D and 3D Spartan MT provides a fast, reliable and flexible means to image porous formations, or unproductive low porosity units. High angle structures can be detected and delineated through MT imaging of marker horizons.

Analysis of reservoir resistivity prior to drilling could provide better target prioritization and decreased drilling cost. Periodic MT monitoring of reservoir resistivity patterns might be employed to verify flow and could provide production gains. 

Spartan MT Survey Neuquén Oil and Gas Project, Argentina

Advanced deep imaging employing Spartan MT resistivity data was used to enhance the interpretation of seismic data for oil exploration in the Northeast Platform portion of the Neuquén Basin in western Argentina. Among the petroleum basins of Argentina, the Neuquén Basin is the leading producer of hydrocarbons. The basin holds 35% of the country’s oil reserves and 47% of its gas reserves. The 137,000 km² basin, situated entirely onshore, is part of the Sub-Andean trend which extends the entire length of South America.

In the survey area a porous host formation with three paleo-marine sand bars forming traps, was detected by 3D seismic surveys at depths of 600 to 650m.  Drilling encountered oil in the central trap and water of differing salinities in the two flanking traps.

Spartan MT Survey Objectives

The Spartan MT survey was designed to:

  • Detect and delineate the porous formations with MT-resistivity imaging.
  • Detect and delineate the petroleum-saline water interface
  • Detect and delineate lithologic variations, structural displacements, and variations in porosity within the sedimentary units and basement.

Survey Specifications for the Neuquen MT Project
          

Equipment: 6 proprietary Spartan MT acquisition units

Acquisition Mode: Full Tensor (highest quality data with best information content for 3D modeling)

Profile Length: 5000 meters;

MT sounding separation: 500 meters;

Frequency Range: 300 Hz to 0.001 Hz;

Productivity: 5 sites per day, 15 hrs recording per site, 2500 meters of line registered per day

One remote reference site: to differentiate near and far signal sources so that noise impulses could be removed from the time-series record prior to stack-averaging, greatly increasing data quality;

Depth of Investigation: actual data depth exceeds 10 km. 2 km depth sections are presented here;

Modeling Software: Geotools - PW1D and PW2D


Figure 1. Seismic time section from a 3D model, plotted with drill-hole traces

A 5 km seismic time section is presented in Figure 1. Drill-hole positions and profiles of Spontaneous Potential (blue) and Resistivity (purple). Note that a major seismic reflector is observed dipping slightly toward the left.

Figure 2.  1D Inverse Models ‘stitched’ together in a section format

‘Stitched’ 1D inverse models create a cross section with good sensitivity to variations in horizontal stratigraphy. Electrical resistivities are denoted by a graduated colour scale. 

 

In the MT sections, porous units are portrayed by the zones of low electrical resistivity (purple-red)  Low porosity and crystalline rock units are identified by elevated resistivity values (green-blue-white).

The modelled MT presentation is scaled in meters, thus depths to formation and interfaces can be accurately predicted. The resulting presentation clearly shows: 150m thickness of upper resistive material, 250m+ thickness of the porous Neuquen and Centernario formations, (flat-lying stratigraphy of varying porosity and irregular extents are clearly evident within these formations). An underlying ‘basement’ that clearly has varied composition and complex structural controls.

2D modeling allows an improved sense of geometry and increased interpretability at depth below laterally discontinuous units. Some of the formations and structural controls have illustrated in Figure 3 for reference.


Figure 3. 2D inverse model cross section with structural interpretation

The down-drop block of pre-Cuyo sediments was not detected in the 3D seismic survey. As evident in the Figure 4 composite image, interpretation of the rock units, electrical resistivities and lithologic setting is clearly enhanced by the MT survey.

Conclusions

  • The results of Seismic 3D survey and the results of the MT survey correlate well with those obtained from the drill-hole surveys.
  • Although the MT survey could not resolve resistivity contrasts related to the fluids contained in the thin trap features, variations in formation resistivity are clearly evident.
  • Lithologic determination and structural interpretation is greatly enhanced with the MT results.
  • Scaled depths of this data format provide an aid to interpretation of seismic time sections.
  • MT can detect basement depth, which may allow for the correction of static effects of seismic studies, using the basement as a common reference.

Spartan MT Advantages

  • The application of MT in settings where other exploration methods (seismic, gravity, magnetic) encounter problems, are cost prohibitive, or yield ambiguous results.
  • MT technology provides easily interpretable results, plotted in the resistivity parameter used in well logging.

MT data provide complementary information to that derived from seismic exploration, even in adverse settings. For example, the diffusive signal propagation used in MT can be an advantage in a region of intense fracturing. While seismic signals will be scattered, the MT signals diffuse and give a reliable estimate of bulk properties such as porosity