At Borehole Image Specialists, we also investigate subsurface structures by utilizing archival data. The dipmeter, developed by Schlumberger in the late 1920s, is considered the precursor to the borehole imager. It was specifically invented to determine subsurface structures.
“As long as oil migrates updip, it would seem there is nothing more fundamental in oil exploration than determining which way is up.” (Goetz, 1992).
Dipmeters were commonly used until the mid-1980s to assess the orientation of subsurface structures. The dipmeter measured three points from a dipping bed within a borehole. While this method was effective in determining structural orientation, it sometimes encountered issues if one of the measurement arms malfunctioned. To enhance accuracy, a fourth arm was added. Over time, this technology evolved into modern borehole image logs, which provide a much greater level of detail.
Thousands of paper printouts of dipmeter logs are stored in the deep archives of oil and gas exploration and production companies, as well as in data repositories like the Denver Earth Resources Library (DERL). These datasets, largely overlooked and even forgotten, represent a vast potential reservoir of geological knowledge. We have significant experience in extracting valuable information from these old archival records.
To facilitate efficient analysis, paper printout data must be properly digitized. This process is made possible by a new generation of scanners that allow for the correct orientation of curves and tadpole features at high resolution.
The diagram on the left shows SHDT dipmeter data from a four-arm, eight-electrode tool. Although this provides a much clearer signal than a three-arm dipmeter, it remains challenging to analyze and interpret compared to modern image log data. Nonetheless, the resolution is sufficient to identify bedding contacts.
Once digitized, the printed data must be correctly oriented to ensure the spatial geometry of the mapped bedding features is accurate. To achieve this, four pieces of information are required: Pad 1 Azimuth, Relative Bearing, Borehole Deviation, and Borehole Azimuth. The diagram on the right plots three of these curves in the Orientation curve track. The fourth piece of information, Borehole Azimuth, can be calculated from the other three. This functionality is built into dipmeter and borehole imaging software packages.
Modern software can interrogate digitized archival dipmeter data to establish the spatial geometry of bedding. Borehole elongation, which occurs due to stress anisotropy, can help determine the trajectory of the minimum and maximum horizontal stresses acting on the borehole. In the absence of additional data, information about present-day stress can be crucial for proper well placement and stimulation design.
The diagram on the left displays a lower-hemisphere, equal-area (Schmidt) stereonet plot. This plot shows contoured poles-to-planes, with superimposed rose petals indicating the compass direction of bedding dips. Below the stereonet, there is a histogram scaled from 0° to 90° in 10° increments, illustrating the distribution of bedding dip angles.
On the right, a tadpole plot presents the depth and geometry of bedding picks derived from dipmeter data, plotted against the gamma response (scaled from 0 to 200 API). The tadpole plot indicates specific dip angles with vertical lines scaled from 0° to 90° in 10° increments, and the tadpoles point in the direction of the dip.
Analysis of the dipmeter data reveals that the lower section yielded significantly more bedding contact picks. While there is a wide range of dip directions and angles, the overall average bedding dip trend shows a direction to the north-northwest (NNW) at an angle of 349°.
The dip azimuth walkaway (or vector) plot is used to identify general trends in the characteristics of bedding dip. This diagram is created by aligning bedding tadpoles head to tail, starting from the bottom of the dipmeter data. Significant changes in bedding dip direction generally indicate post-depositional structural deformation, such as folding or faulting, which disrupts the typical horizontal positioning of bedding contacts. Additionally, changes in bedding dip direction can occur due to large-scale sedimentary processes, including sequence boundaries or periods of non-deposition or erosive removal (unconformities).
In this case, the minor variations in the walkaway plot likely reflect the lower angle bedding dips. Overall, the trend of the walkaway diagram is towards the north-northwest (NNW).
It has been known for some time that significant differences in the magnitudes of the minimum and maximum horizontal compressive stresses, referred to as stress anisotropy, can be observed in directional changes in borehole diameter. Specifically, boreholes drilled in rock exhibiting stress anisotropy commonly develop an elliptical cross-section due to the breakage or spalling of the rock material near the minimum horizontal compressive stress. The degree of ovality is influenced by factors such as mud weight, pore pressure, and the magnitude of the stress differential.
The departure of a borehole from a circular cross-section is measured using calipers, and this information is included in most dipmeter and borehole image data sets. By mapping the variation in caliper measurements, we can establish the direction of maximum elongation, which indicates the orientation of the minimum horizontal compressive stress. The maximum horizontal compressive stress is orthogonal to this direction.
In the examples presented, the elongation direction is represented by the compass orientation of the black caliper breakout tadpoles in the accompanying diagram. When plotted on a stereonet, these data form a tight cluster that reveals the minimum horizontal compressive stress direction oriented NW-SE (indicated by the red arrow). The maximum horizontal compressive stress (shown by the blue arrow) is 90° away from this direction.
Between the depths of 670 feet to 705 feet, the calipers in the far-right track indicate a separation of 1½ inches. The second track from the right displays mirror image calipers scaled in tenths of inches, which assist in visualizing the caliper differential of the breakouts. The Caliper Breakouts track, located on the far right, indicates the compass direction of the hole’s elongation. This orientation is the minimum horizontal compressive stress (Shmin) direction.