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Does the upward propagation of faults, fractures and sub surface features create an expression on the surface of the earth? Faults, fractures and sub surface features buried in many cases thousands of feet below the surface of the earth are very important to the petroleum geologist. Faulting can create porosity and confine petroleum; sub surface structures also contain oil and natural gas. LiDAR imagery can accurately scan the earth’s surface and is readily available to the petroleum geologist. Can the petroleum geologist use LiDAR derived data to locate the upward propagation of these expressions on the surface in drift covered areas? and can the petroleum geologist use the information to improve the odds of successfully drilling oil and natural gas wells?
The question of does the sub surface reflect on the surface has been asked and debated since the birth of modern petroleum geology. W.S. Blatchley, State Geologist of Indiana in the early 1900’s, noticed that most of the new drilling for oil in the Selma-Parker oil field was in the low lands and stream valleys. Blatchley was one of the fathers of modern petroleum geology and he was very skeptical and even dismissive of the drillers actions; he noted that there is no known correlation between the surface and the subsurface, questioning their choices on locations. The drillers were practical men who could care less about the emerging science of petroleum geology (much less Blatchleys opinion) they saw a correlation between surface conditions and oil production and they exploited that correlation.
Aerial photography was used as a tool during WWII to detect enemy troop and armament movements. After the war the technology was successfully used by geologists as a method of remote sensing. Out of this came the first group of photogeologists, these were bright and talented scientists who saw that there was a correlation between the surface and the subsurface. Photographs of the earth’s surface could now be examined with stereoscopic lenses. Using this new technology, the photogeologist could easily see radiating stream patterns over sub surface structural highs and long lineaments over faults and fractures. A great body of work was compiled by this group from the late 1940’s to the 1970’s. With the advent of the computer age and the rise of seismic surveys the information photogeologists provided in the exploration of oil and natural gas diminished. Digital data was becoming increasingly valuable, the observations of the photogeologist was an interpretation of the earth’s surface by an individual which was not easily reduced into digital form. By the early 1980’s photogeology was used very little as an oil and gas exploration tool.
That will change with the advent of the modern LiDAR survey. The surface of the earth can now be scanned and the data placed into digital form. We can use the principals designed and developed by the photogeologists of the 1940’s and 1950’s and apply these principals to the modern LiDAR derived data. No longer is the information a result of an interpretation of an individual it is digital data that can be used for visualization or layered into queries. We can now digitally see what the drillers of the Selma-Parker field saw in 1903; there is a correlation between the surface and the sub surface.
Geomorphology is the study of the earth’s surface and here in the upper Midwest the surface of the earth was sculpted by numerous advances and retreats of the great continental glaciers. Thick layers of drift were left behind as the glaciers melted. Sub surface structures along with faults and fractures did influence the conditions controlling the melting of the glaciers which affected the resulting topography. Fractures in the ice allowed meltwater to drain to the glaciers base and create streams and channels under the glacier. Kames and Eskers developed under the ice. It is these features that we see today; erosional forces have muted their appearance but they are still there and we can model them with LiDAR. What we are finding is the surface features created by a retreating glacier can and often do correlate to sub surface structures that have produced oil and natural gas.
Case studies
The Illinois Basin has proved to be an excellent area to correlate LiDAR derived surface features with oil production. The surface is drift covered from the Wisconsin and Illinois glaciations and oil development has continued since the early 1900’s. The Illinois Height Modernization Program (ILHMP) has done a nice job of making the available LiDAR data easily downloadable. Many of the Illinois counties have LiDAR surveys but regrettably some important counties do not. Using LiDAR as an exploration tool is in its infancy, but we do have a basic set of principals available which were developed by the photogeologists in the 1950’s and 1960’s. Most of the original photogeology principals applicable to LiDAR data use stream flow. LiDAR derived digital elevation models and flow accumulation maps are much more precise than the aerial photographs used by the photogeologist of the past era.
Preliminary work with Indiana LiDAR data has shown that sub surface faults and fractures often manifest themselves as areas of higher slope angles on the surface. Measuring slope angles is specific to modern LiDAR surveys and was not used previously. After study of the Illinois data the same correlation does exist, higher slope angles are evident over many areas of Silurian oil production. The Kincaid and Cooks Mills oil fields are a good example of the correlation between higher slope angles and oil production. Preliminary work on sub surface structures in Indiana has shown that differential compaction over reef structures can and often does extend to the surface and can be modeled with digital elevation models and flow accumulation models. The Nashville and Elbridge oil fields are good examples of these structures.
The Kincaid oil field is in Christian County, Illinois. Christian County is located on the southern flank of the Sangamon arch and has had recent oil development. Wells are mostly Silurian in age with some Devonian production. A LiDAR survey was flown between 12/2014 and 03/2015 by Digital Aerial Solutions using a Leica ALS70. Slope angle maps using ArcGis 10.3 were created from a terrain data set. 12 divisions were used with quantile classification and a green to red color ramp. Green is lower slope and red is higher slope. Quantile places an equal amount of data in each division and results in a good signature of the higher slope angles in relatively low slope areas. Natural breaks or "Jenks" classification is more appropriate for areas of high slope angles. As evident in Figure 3 much of the Silurian oil wells are in areas of red (higher slope).
The Cooks Mills oil field, Figure 4 is another good example where oil production and higher slope angles correlate. The Cooks Mills oil field is in Douglas County, Illinois. The LiDAR survey was flown by AeroMetric Inc. using an Optec Gemini and Leica ALS70 and was available 2012. A terrain data set using ArcGis 10.3 was also used for the Cooks Mills study with 10 divisions and the same green to red color ramp and quantile classification as the Kincaid study. Terrain datasets are very useful due to their scalability and ease of visualization of the Earth’s surface. The triangulated surface of the slope angle map is easy to interpret.
The Nashville oil field, Figure 5 is in Washington County, Illinois. Surdex Corp. flew this survey in 2015 using a Leica ALS70-HP. Oil is produced from Devonian sediments that are draped over a Silurian reef. Differential compaction has allowed sediments to drape over the hard reef core. Sediments on the flank of the reef are lower in elevation than the same aged sediments over the core of the reef. This elevation change extends upwards through the geologic column with structural changes having a positive impact on the surface. The Nashville oil pool is one of these reefs that evidence of the differential compaction can be modeled on the surface. Over the history of the discovery of the reefs in the Illinois reef trend a few were discovered by locating drainage radiating away from a central point. By using LiDAR derived digital elevation models and digitally created flow accumulation maps we can locate areas with structural highs and radiating drainage patterns. This DEM was created using a green to red classified color ramp. The raster is 65% transparent and overlies a hill shaded raster giving a 3-dimensional model. There is clearly an area of higher elevation over the reef and the radial drainage away from the reef is modeled nicely by the flow accumulation map.
The Elbridge oil field is also a Silurian reef with oil production from Devonian sediments draped over the reef core. The Digital elevation model and flow accumulation on the surface correlate nicely with the oil production. The Elbridge oil field, Figure 6 is in Edgar County, Illinois. The LiDAR survey was flown by AeroMetric Inc in 2012 using an Optec Gemini and Leica ALS70. The Elbridge is the eastern most reef in the Illinois reef trend and located under the Westfield moraine. The Westfield moraine is the terminal moraine of the Wisconsin glaciation. To the south west is Illinoisan glaciation and to the northeast is Wisconsin glaciation. When looking at the DEM of the Elbridge you can see the influence it has on the moraine, the Westfield moraine expands southward and oil production lies below this area of expansion. As in the Nashville, flow accumulation data shows stream flow radiating away from the area above the reef core.
Conclusions
To correlate the surface and the sub surface an intimate knowledge of the sub surface is needed. Gathering this information is much easier in areas that have been developed by the oil and gas industry. LiDAR derived surface models when compared to known sub surface structures will be powerful tools for any oil and natural gas exploration program. Large areas of the country can be looked at quickly and easily and if one knows what LiDAR models are more likely to produce oil, much acreage can be excluded. On a more localized program the LiDAR surface models over a known oil producing region can be used to expand into areas with the right criteria that have not previously been drilled.
LiDAR is already being used in the oil and gas industry to locate locations suitable for placing pipelines and drilling locations. Most digital elevation models are using LiDAR data and thus by default the industry is using LiDAR. Using LiDAR as an exploration tool is new to the oil industry.
One of the basics of petroleum geology is to accurately locate the depth of sub surface geologic formations. Measurements derive from the surface and an accurate survey derived from LiDAR data will become the standard for the industry. LiDAR derived surface elevations will become the foundation for all the petroleum geologists mapping. The use of LiDAR derived slope angles, digital elevation models and flow accumulation models will become a valuable tool in the exploration for oil and natural gas.
The more we learn about the correlations of the geomorphic conditions on the surface of the Earth and the geological conditions of the sub surface the more valuable LiDAR data will become as a tool in the exploration of oil and natural gas. The photogeologists of the 1950’s and 1960’s built a substantial understanding of the correlation of surface and sub surface. Modern digital LiDAR data will allow us to take the next step in understanding the processes that shaped the surface of the Earth and in turn use that understanding for the continuing hunt for oil and natural gas.
Chuck Knox is the owner of Knox Geological LLC. He is a graduate of Western Illinois University and has spent most of his adult life studying geology and the history of petroleum exploration. He has been working with LiDAR data sets and their relationships to known oil and gas fields since 2011.
A 2.629Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE