Structural analysis is used to identify and characterize the changes in rock structure that have occurred to an originally flat-lying sedimentary rock. Rock structure is important in controlling the flow path and the flow rate of subsurface fluids such as oil, natural gas and water. Borehole images provide a high-resolution means of examining the 3D geometry of geological structures. Comprehensive elucidation of structure may be distilled from the bedding dip changes measured in a single borehole.
Structure refers to 3-dimensional changes to the rock mass from deformation, which results from 3 processes: displacement, rotation or distortion. Displacement is a change in the location of the rock. Rotation is change in the spatial orientation of the rock. Distortion is a change in the shape of the rock. Any or all of these mechanisms may be imparted to a pile of sedimentary strata, and when such change happens, it is usually readily apparent. Deformed rocks may appear bent, broken, stretched, squished or sheared apart. Deformation is NOT readily apparent when the rocks involved are deeply buried in a sedimentary basin.
Borehole images permit us to view, measure and analyse fine-scale features of sedimentary rocks. One of the fundamental characteristics of sedimentary rocks is that they occur in flat-lying layers. One of the foundational assumptions that geologists use to determine how rocks have been structurally altered is the “Law of Original Horizontality”, first articulated by 17th century Danish scientist Nicholas Steno. Fine-grained sedimentary rocks, mudstones and shales, are almost always deposited in horizontal layers because settling of tiny sediment grains requires tranquil, non-turbulent water.
Measuring bedding of mudstones and shales in image logs establishes the degree to which structural deformation has removed the rocks from a state of original horizontality. Unlike a field geologist, an image interpreter is measuring data from deep in the subsurface, far beyond the limits of outcropping rock. An additional plus is that borehole image logs permit collection of many thousands of data points, a task that is virtually impossible for a field geologist.
Structural analysis includes deciphering the impact of faulting on the migration and trapping mechanisms of reservoir and seal rocks. Determining the spatial geometry of faults is important because faults offset different stratal types and bend and break rock creating barriers or conduits to fluid flow. Borehole images are capable of visualizing faults that may be too small to resolve in seismic data. When this type of information is integrated with seismic, a more complete picture of subsurface complexity emerges. The following material illustrates the sensitivity of borehole image data to identifying faults and characterizing their spatial geometry.
The diagram shows the results of mapping the spatial geometry of 1225 mudstone bedding planes in the Herrmann well in NE Kansas. The well is situated near the Nemaha Ridge, a portion of the mid-continent that was deformed in Carboniferous time as a pre-Cambrian basement uplift. The Nemaha Uplift is a ~400 mile NNE-SSW tear in the crust running from Omaha to Oklahoma City. In NE Kansas, the east side of the ridge is the vertical Humboldt fault that drops the Pre-Cambrian surface 2,500′ down to the east. Dextral strike-slip created NE-SW wrench faults which created numerous small hydrocarbon traps in small up- and down-warps in the Paleozoic strata.
These diagrams are a common way to portray and summarize borehole image data. The circular display is a lower-hemisphere, equal-area (Schmidt) stereonet with contoured poles-to-planes and superimposed rose petals that show the dip direction of mudstone bedding. The histogram below the stereonet is scaled from 0° to 90° (in 10° increments). It shows the frequency distribution of dip angles. The tadpole plot on the right displays the depth and geometry of mudstone bedding picks plotted against the gamma curve and stratigraphy. Tadpole symbols plot the dip angle on the vertical lines, which are scaled from 0° (left) to 90° (right) in 10° increments. The tails on the tadpole symbols point in the direction of bedding dip.
This display shows a tadpole plot compared to a bedding dip direction vs. depth diagram. The dip direction vs. depth diagram is a means of displaying and inspecting large-scale changes in bedding dip character with depth. To create the diagram, bedding tadpoles are arranged head-to-tail from bottom to top. Dip steepness is also color-coded. The dashed green lines connect equal depths between the 2 diagrams. The direction vs depth diagram shows that the fault zone has created a complex rotation of the rock mass, first to the NE, then to the SW. These rotations, indicated by the red arrows, are superimposed over the gentle (01°) NW dip. Together, the tadpole plot and the dip direction vs. depth diagram provide a glimpse of the 3D complexity of this wrench fault generated deformation.
Borehole image data can be integrated into the larger geological context. In our example, the bedding dip trends from the dip direction vs. depth diagram bear a striking geometric similarity to the fault strikes mapped by Gerhard (2004) along the Nemaha Ridge in the region near the well. These dip trends (red arrows) have been superimposed over the fault map to emphasize the similarity. The directionality of the bedding dips reflect compartmentalized tilting of strata according to a NE-SW and NW-SE fault-bounded framework. The location of the Herrmann is indicated by the red dot just E of the upper right corner of the map.
Figure 18 from Gerhard, Lee C., 2004, A New Look at an Old Petroleum Province, Current Research in Earth Sciences Bulletin 250, Part 1, Kansas Geological Survey webpage, http://www.kgs.ku.edu/Current/2004/Gerhard/Gerhardarticle.pdf
Faulting in horizontal well image data is also readily observed and characterized. Here, a high-angle fault (thick purple line in the middle) offsets bedding packages that are dipping in different directions. (This is indicated by the direction of the gray tadpoles on either side of the fault). Bedding on the left of the fault is dipping from 15° to 20° to the SSE. Bedding to the right of the fault is dipping at a steeper angle (~25° to 65°) to the N and NNW. (Note compass directions in the Tadpole track header).
Borehole images from horizontal wells have a distinctly different appearance compared to images from vertical wells. Because the wellbore is subparallel to (usually low-angle) bedding, contacts appear as very elongate “steep” sinusoids. High-angle features like fractures appear as low sinuosity lines.