ENDEEPER - Rock Knowledge and Reservoir Evaluation Systems

In order to support the systematic quantitative microscopy analysis through the PETROLEDGE system, we have developed an electromechanical microscope stage, called STAGELEDGE, which is controlled by the software. The systematic quantitative petrographic analysis requires a uniform scanning of the thin sections, using a grid of steps defined according the rocks textures and fabrics. This approach not only guarantees that the thin sections can be representatively explored, but most important, that the spatial coordinates of each quantified point can be recorded in a virtual map.

Our approach is based on the creation of a digital map of the points quantified and described on the thin section, in which virtual features can be created. Although such map does not eliminate the need for the real thin section, it has some significant advantages over the traditional approach that uses the original section both for description and for later verifications. This “virtual thin section” is keyed to the recorded petrographic description referenced through a defined ontology. The “virtual thin section” constituted by the digital map and the description can be sent over the Internet to several petrographers, who can independently evaluate the interpretations previously derived from the description. This involves neither costs for shipping the real thin sections, or risks associated with a possible loss of the physical thin sections. Moreover, the virtual thin sections show potential for unlimited documentation via hyperlinks to images, video, audio, and text provided by expert petrographers, as well as to other resources available on the Internet and related to the contents of the sections. This sets a new level of rich documentation, turning the virtual thin sections into ideal training tools. This situation is illustrated in Fig. 1, which shows an image corresponding to a portion of a sandstone thin section containing links to different media formats: audio, video, other images, websites, and other observations. These resources allow complementing the information conventionally captured by the petrographers during the quantification. The images should have less than two megabytes, in order not to slow down the system performance.

Fig. 1. A sandstone thin-section image with links and observations: A: Video file C:video.mov; B: WEB link; C: Sound file C:quartzExplain.mp3; Observations: Grains of quartz (q) covered by coatings of iron oxide (arrows); zoned and partially dissolved crystals of dolomite (d); microcrystalline kaolinite (k) partially filling the intergranular pores (impregnated by blue epoxy resin); uncrossed polarizers.

The PETROLEDGE system controls the steps of the STAGELEDGE, allows the user to select constituents and features described in the domain ontology and associates them to the current position under analysis in the thin section. Fig. 2 illustrates a PETROLEDGE interface showing the constituents of a given rock sample and a partial menu providing specific ontology terms. The quantitative petrographic analysis identifies and saves the location of every constituent positioned in each of the coordinates in the virtual net, controlled by STAGELEDGE. The PETROLEDGE interface depicts different minerals using colors, as presented in Fig. 3. Thus, with just a quick glance, the geologist can have a good idea of the spatial distribution of the constituents and pores identified in the thin section.

Fig. 2. Rock sample composition interface with the description ontology for constituents.

In the process of creating the digital map, the thin sections are digitized with a regular flatbed scanner. Since flatbed scanners can capture images at different resolutions, it is necessary to specify the selected resolution in points per inch (ppi), thus relating pixels to real distances in the thin section. According to our experience, the use of 600 ppi provides satisfactory results. The resulting images are used as a base to place the additional documental information. This stage requires the careful correction of the scale, tilt and coordinates of origin of the scanned image, in order to provide a correct association with the origin and scale provided by STAGELEDGE. Once the image has been captured and referenced to the actual position in the equipment, the documentation can be referenced to the real spatial coordinates.

The digital map of the rock uses the digital thin section image as a base map on top of which the additional documental information will be placed. At the end of the process, an extensive documentation of the thin section is provided. For example, it is possible to locate where the 10th detrital quartz is located and then visualizing it. Moreover, the system guarantees that all descriptions would be performed based on a formal and complete petrographic vocabulary, defined in the domain ontology. This feature will provide extra capabilities by allowing the automatic geological interpretation and correlation with the captured information.

Fig. 3. Thin section with points of different constituents, identified by different colors.

The constituents and pores that can be found in a thin section are fully described in a domain ontology, as well as the attributes and domain of values of them. The ontology also describes in which way the instances of qualified constituents can indicate the rock-formation environment. This is expressed by knowledge graphs, a one-level tree where the root node represents the interpretation hypothesis and the leaf nodes represent visual chunks identified by the experts in the image of rock as pieces of evidence necessary to support the interpretation.

The uncertainty of interpretation is represented in the knowledge-graph by a threshold value that represents the minimum amount of evidence needed to indicate it. Also, the chunks have an influence factor and are combined to increase the influence and the certainty of the interpretation stated. By their side, the chunks represents in an AND-OR tree the several ways in which way an evidence can be recognized in the rock, such as, possible minerals, possible habits, locations, etc. For each significant feature identified in the thin section, the user can capture a photograph and associate it to the coordinates of the described point or annotate the captured image itself, describing the special characteristics that must be considered. Thus, the quantitative analytical process generates a map containing the documentation of the most important diagnostic features for reservoir evaluation. According to the user interest, the system can selectively show the location of the special features, as exemplified in the left window of Fig. 5, where the segments indicate the trajectory of the analysis and the white dots shows the position of the selected constituent. As the user moves the mouse over a point for which a picture has been taken, it is automatically shown in right side window (Fig. 5). This documentation will provide further validation to the reasoning process or may reveal possible errors in feature identification.

Fig. 5. Interface showing the virtual thin section with a superimposed map of the analytical pathways and points (left). As the mouse cursor passes over a feature point (white dot) for which a picture has been taken, the photograph is shown on the right. Note that the photograph itself contains a series of annotations in the form of hyperlinks.