Structural analysis is a method used to identify and characterize changes in originally flat-lying sedimentary rocks. The structure of these rocks is crucial as it serves as a primary control on the flow paths and rates of subsurface fluids, such as oil, natural gas, and water. Borehole images provide a high-resolution way to examine the three-dimensional geometry of geological structures. A comprehensive understanding of rock structure can be derived from changing measurements of bedding dip observed in a single borehole.
Structure refers to three-dimensional changes in the rock by deformation, which occurs through three processes: displacement, rotation, and distortion. Displacement is a change in the rock’s location, rotation is a change in its spatial orientation, and distortion is a change in its shape. Deformation is apparent when it is observed: rocks may appear bent, broken, stretched, compressed, or sheared apart. Deformation, however, is not easily observed when rocks are deeply buried in a sedimentary basin.
Borehole images allow geologists to view, measure, and analyze fine-scale features of sedimentary rocks. A key characteristic of sedimentary rocks is that they are typically found in flat, horizontal layers. One foundational assumption that geologists use to assess how rocks have been structurally altered is the “Law of Original Horizontality,” first proposed by the 17th-century Danish scientist Nicholas Steno. Fine-grained sedimentary rocks, such as mudstones and shales, are almost always deposited in horizontal layers, as the settling of tiny sediment grains requires calm, non-turbulent water.
Measuring bedding from mudstones and shales in image logs helps to determine the degree of structural alteration from the original horizontal state. Field geologists study outcrops and make strike and dip measurements of bedding (and other features). Image interpreters analyze data from deep within the subsurface, well below rock outcrops. One significant advantage of measuring structural features from borehole image logs is that they allow the collection of thousands of data points very quickly (days). Compiling this many data points from field outcrops would take weeks.
Structural analysis involves understanding how faulting affects the migration and trapping mechanisms of reservoir and seal rocks. It is crucial to determine the spatial geometry of faults, as they offset strata, bend and break rock, thereby creating barriers or pathways for fluid flow. Borehole images identify faults that are too small to detect in seismic data. By integrating this information with seismic data, a more comprehensive understanding of subsurface complexity is achieved. The following material demonstrates the effectiveness of borehole image data in identifying faults and characterizing their spatial geometry.
The diagram illustrates the mapping of 1,225 mudstone bedding planes in the Herrmann well located in northeast Kansas. This well is situated near the Nemaha Ridge, which was deformed during the Carboniferous period due to pre-Cambrian basement uplift. The Nemaha Uplift is a ~400-mile north-northeast to south-southwest tear in the crust, extending from Omaha to Oklahoma City. In northeast Kansas, the east side of the ridge features the vertical Humboldt fault, which drops the pre-Cambrian surface by 2,500 feet on the east side. Dextral strike-slip movement has created northeast-southwest wrench faults, resulting in numerous small hydrocarbon traps formed by the small up- and down-warps in the Paleozoic strata.
These diagrams are commonly used to represent and summarize borehole image data. The circular display is a lower-hemisphere, equal-area (Schmidt) stereonet. It features contoured poles-to-planes with superimposed rose petals that indicate the dip direction of mudstone bedding. Below the stereonet, the histogram is scaled from 0° to 90° in 10° increments, illustrating the frequency distribution of dip angles.
On the right, the tadpole plot shows the depth and geometry of the mudstone bedding picks in relation to the gamma curve and stratigraphy. In this plot, tadpole symbols represent dip angles on vertical lines, which are also scaled from 0° (on the left) to 90° (on the right) in 10° increments. The tails of the tadpole symbols point in the direction of the bedding dip.
This display features a tadpole plot alongside a bedding dip direction versus depth diagram. The dip direction versus depth diagram permits the examination of large-scale changes in bedding dip character with depth. In this diagram, bedding tadpoles are arranged head-to-tail from the bottom to the top, with dip steepness indicated by color coding. Dashed green lines connect points of equal depth between the two diagrams.
The direction versus depth diagram illustrates that the fault zone has caused a complex rotation of the rock mass, first to the northeast (NE) and then to the southwest (SW). These rotations, marked by red arrows, occur over a gentle northwest dip (01°). Together, the tadpole plot and the dip direction versus depth diagram offer a glimpse into the three-dimensional complexity of the deformation generated by this wrench fault.
Borehole image data can be integrated into the broader geological context. In our example, the trends of bedding dips, illustrated in the dip direction versus depth diagram, closely resemble the fault strikes mapped by Gerhard (2004) along the Nemaha Ridge in the vicinity of the well. The dip trends (shown as red arrows) have been superimposed over the fault map to highlight this similarity. The direction of the bedding dips indicates a compartmentalized tilting of the strata, aligned with a NE-SW and NW-SE fault-bounded framework. The location of the Herrmann well is marked by a red dot just east 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
Faulting in horizontal well image data can be easily observed and characterized. In this case, a high-angle fault (represented by the thick purple line in the middle) offsets bedding packages that are inclined in different directions. This variation is indicated by the direction of the gray tadpoles on either side of the fault. On the left side of the fault, the bedding dips between 15° and 20° toward the southeast (SSE). In contrast, the bedding on the right side of the fault exhibits a steeper dip, ranging from approximately 25° to 65° toward the north (N) and north-northwest (NNW). (Refer to the compass directions in the Tadpole track header for clarification).
Borehole images from horizontal wells have a distinctly different appearance than images from vertical wells. In horizontal wells, the wellbore is oriented parallel to (usually low-angle) bedding, causing contacts to appear as elongated, steep sinusoids. In contrast, high-angle features, such as fractures, show up as lines with low sinuosity.
