The Oman Drilling Project was conducted as a part of the International Continental Scientific Drilling Program (ICDP) from 2017 to 2018, and several boreholes including four across the crust-mantle transition zone were drilled in the program and acquired a full suite of slim and conventional wireline logs, high-resolution electrical borehole images, and geochemical spectroscopy logs.
In this study, we applied advanced borehole image display methods to crust-mantle transition log data to maximize the data value and increase the visibility of major lithology changes as well as fracture classification, which is useful for geomechanics understanding. The first innovative approach created a pseudo radius image from standard resistivity or ultrasonic types of borehole images by converting the measurement values to the borehole diameters. This radius image can be displayed in 3D to maintain the measurement value contrast. Another approach provided a rich borehole image by integrating the existing standard borehole image and multiple petrophysical logs to compose a true colored image from the 3D RGB color space.
First, two pseudo radius 3D images, one using a static normalized borehole image and another using dynamic normalized borehole image, were generated. Second, two integrated color images, one using whole interval (=single-zone) data and another using zoned interval data, were generated by merging of the dynamic borehole image (assigned as green component), Fe (assigned as red component), and Ca (assigned as blue component) from geochemical spectroscopy log in this complex crust-mantle transition lithology.
The generated pseudo radius 3D images and integrated color images were compared with the automatic geological facies classes and other available core and log data. The pseudo radius 3D images show pronounced fractures and alterations. The natural fractures and drilling-induced fractures can be distinguished more efficiently from 3D pseudo radius image displays. The integrated color images show change of Fe and Ca elements, which matched with other logs and cores at the three major lithologies which are 1) olivine rich dunite zone; 2) plagioclase feldspar, olivine, and clinopyroxene mixed gabbro zone; and 3) olivine and orthopyroxene dominated harzburgite zone. In the single-zone color image, the dunite interval is composed of several different colors such as green, reddish green, and bluish green colors; the gabbro interval is dominated by the blue color; and the harzburgite interval is dominated by the green color. Additionally, the four-zone color image shows detailed color changes that matched with the automatic classes result.
The automatic classes result well represented lithology and texture change in the previous study, however, these classes need to compare carefully with borehole image and other logs to understand geological meaning. The advanced borehole image display methods can reflect petrophysical change within the borehole image, hence, quick formation evaluation can be done visually and directory on new generated images.
