Our objective is to provide clients with advanced magnetic and gravity data analysis, visualization, and interpretation to complement seismic and enhancing geology knowledge.
Basement Structures, Faults
Magnetic and gravity data are relatively inexpensive to acquire. An approach integrating magnetic and gravity data with seismic and geological knowledge offers the best chances to reveal the structure of the crystalline basement and to unravel its influence on the basin above. “If you want to understand your basin, begin with the basement” (Source: New Approach to Basement Studies for Oil and Gas Explorers by Granham Chandler).
Within sedimentary basins, the high resolution magnetic or gravity surveys allow a clear distinction between basement anomalies and near-surface rocks and between basement faults and intra-sedimentary faults (some faults affecting the sedimentary rocks can be basement controlled). These faults might have propagated into the overlying sedimentary and influenced fluid flow and distribution of hydrocarbon traps and mineralization zones as well as salt dilution and carbonate alteration**. Magnetic and gravity data provide vital information abut the basement structure, the location of faults in the basement and their propagation. Often, basement structures, fault block and grabens, steep. brittle, and seismically sub-resolution faults become evident and the basement fabric is revealed.
The map below shows aeromagnetic data for a strip crossing the Albuquerque basin. Geologic contacts (white lines) outline bedrock areas. Intrabasin faults and buried volcanic rocks are clearly imaged. The magnetic expressions of the faults commonly connect isolated exposures, which significantly increase the knowledge of their linear extents, patterns, and density.
There is strength in numbers; the ambiguity of separate interpretations drops dramatically with each constraint from a different type of data interpretation that we include. Magnetic and gravity data continually prove useful in mapping basement faulting as well as understanding the structural grain of the sedimentary section.
When interpreting scattered 2-D seismic lines there can be many choices for fault correlation. By combining the available geology and the magnetic or gravity data with the seismic fault picks, one particular correlation often becomes compelling.
When interpreting small 3-D surveys it is often hard to make a rational fault map without the regional structural perspective that magnetic can provide. Identifying wrench faults on seismic can be especially challenging because the vertical offset can be small and variable, but the fault can be of a major scale. (Source: AAPG John W. Peirce, Search and Discovery Article #40606 (2010).
The following maps below show an example of seismic and magnetic overlay:
TerraNotes also map fault and fracture networks at various depth.
The 1st maps below shows the survey area with overlay of the major known or interpreted faults.
The 2nd map shows the network of structures edges, faults and fracture network at a depth of less than 1400 m below ground level (BGL), the top of the Precambrian basement.
The 3d maps shows the basement fault network at 2700 m BGL.
The 4th map shows details of the fault network on the centre portion of the survey area. The fault network is at a depth of 1400 meters. The lines in black are the major faults identified in the 1st map. Note some discrepancies between the fault attributes from the 1st map and the corresponding faults in the 5th maps. We believe that the location of the faults in the 5th map is more realistic than in the 1st map.
TerraNotes has researched and developed an innovative technology to map the basement relief in 3 D.
Applications of the 3D Basement Toporgraphy technology in regional geology:
Applications in petroleum geology of sedimentary basin (in association with seismic)
• To delineate:
• To closely map sedimentary depression and local structures related to oil and gas
In the Athabasca Fort McMurray region:
The TerraNotes High Density Resistivity™ Surveys system has 120 smart-electrodes channels... the largest in North America.
It allows for high density data acquisition necessary for high resolution imaging of fainter contrasts at depths up to 500 metres below ground level.
The main advantage of this system is that more electrical power can be injected in the ground therefore getting higher resolution. Also, our process scans the field in such a way that it provides much higher density of data points.
Compared to EM31-38 or GEM2 or ERT systems, the TerraNotes High Density surveys go much deeper; provide high resolution; provide imaging in 3D of the anomalies; and has a more advantageous signal to noise ratio.
The TerraNotes system allows for the following performance in the Athabasca Fort McMurray region:
MAPPING SHALLOW OILSANDS
A client with shallow oilsands asked us to assess the validity of the TerraNotes High Density Resistivity system to map the contours of oilsands layers and shallow traps.
For this project, we interpreted the resistivity values in wells and compared them to values that would have been provided by the TerraNotes system.
Based on comprehensive analysis of logs associated with information on bitumen and gas concentration in several wells, we found that the resistivity differences between several wells can be identified and interpreted on the TerraNotes resistivity sections.
This successful outcome shows the effectiveness and usefulness of the TerraNotes system and answered our client's question.
While successful on its own, we recommend to interpret the ground resistivity (GEI) data in integration with other geological information (i.e. shale contents, porosity, and seismic attributes).
The 1st graph below shows that in term of both logs’ shapes and magnitudes, the log resistivity curves of Well 08 are very similar to those of Well 17. Black arrows mark these typically similar layers for convenience to compare. However, the resistivity logs of Well15 are considerably different from those of either Well 17 or Well 08 (see blue arrows marking these layers for comparison). Such difference can clearly be detected by the TerraNotes GEI.
The 2nd graph below shows the GEI resistivity measurements on logs for Well 17 (left) and Well 08 (right).
The 3d graph shows the sections of GEI resistivity measurements (Well 17 left and Well 08 on right).
RESULT: The TerraNotes system can detect even very faint resistivity differences on the sections.
MAPPING WELL SITE LEGACY LEAK
The imaging below shows the hydrocarbon plumes at a well site - anomaly B visible on the panel below.
Subsequently, test drill holes were drilled around that anomaly. 4 holes encountered contamination directly surrounding the anomaly. No contamination was encountered in holes further from the anomaly.
MAPPING HYDROCARBON AND GROUNDWATER FLOW
The TerraNotes High Density Resistivity system can be used to map possible groundwater and hydrocarbon flow in the subsurface.
DIFFERENCE WITH ERT
The TerraNotes High Density Resistivity System versus ERT
This system can be deployed in the field to acquire data over a survey line length of 1,200 meters at one time.
The TerraNotes 120 channels have been developed to measure simultaneously several readings so as to reduce the total acquisition time.
Field applications require one TerraNotes geophysicist. A field crew of 2 to 3 individuals is required to move the electrodes and cables. The field crew can be provided by TerraNotes or by the clients or their sub-contractors.
The High Density system's environmental footprint is minimal. The environmental footprint is not wider than about a couple of meters.
Note that the system can also be deployed in a circle, which is very useful for some types of exploration approaches.
The TerraNotes High Density Surveys result in the following:
High resolution 2-D and 3D geoelectrical cross sections and images; Depth values for the anomalies using TerraNotes' proprietary inversion software specific for this equipment (not a commercial off-the shelf software); and section profiles
Full geotechnical, geological and environmental interpretation of the field’s subsurface and relation to soil, water and geologyThis equipment and method has a greatly enhanced electrical power and signal to noise ratio compared to commercial resistivity systems that provides greater confidence than the conventional techniques.
We observed that 2D and 3D resistivity surveys at the same location produced very different images on the same cross section. In this paper we found that false anomalies are often seen on the 2D images (traditional ERT). Therefore, 3D resistivity imaging methods are the better technology for subsurface imaging (Source Publication: Comparison of 2D and 3D Electrical Resistivity Imaging Methods for Environmental and Engineering Hazards, Xianjin Yang , Mats Lagmanson, 19th Annual SAGEEP, 2006.)
Since 2007, TerraNotes has combined technology with expertise and teamwork to deliver 3D site characterizations to its customers.
below is a letter written by a client. The last line says it all. We contributed to save millions of dollars to the oil company client.
Geophysics Analysis & Interpretation