Achieving as accurate as possible models of complex geologic structures should be the goal of every geoscientist. Yet, there are significant challenges to modeling geologic structures. Data sources can be voluminous as well as erroneous. It’s rare to have a complete surface model with well-defined boundaries and layers, to begin with. Data often needs to be verified and corrected to improve model accuracy. Computational complexity needs to be balanced against computational time. As additional data are acquired, it should be easy to update the geologic structure model.
The process of modeling subsurface geologic structures can be made significantly easier and result in better quality models with the aid of an effective workflow and the ability to visually verify the results of the data as the model is being built.
EarthVision Workflow Manager for Modeling Geologic Structures
EarthVision is a powerful 3D modeling, visualization, and analysis software. Among its many capabilities is the Workflow Manager module for creating models of complexly layered, faulted, and non-faulted structures. It uses a workflow to help novice users navigate the process. Experienced users also benefit from creating and saving workflows that can easily be executed, modifying parameters to refine the model, as well as updating the model as new data are acquired. For the most experienced users, another module—the Geologic Structure Builder—can be used to build or edit a model on a fault block by fault block basis. The principal steps in the workflow for modeling geologic structures are:
- Define the range (AOI) for modeling efficiency.
- Define vertical, normal, and reverse/thrust faults using 2D or 3D surfaces.
- Construct fault-surface intersections and the resulting fault blocks according to an automated and/or user-specified fault hierarchy.
- Intersect and truncate structural horizons using a geologic sequencing technique based on user-specified depositional, erosional, and uncomformable surface relationships.
- Partition the geologic sequence into fault blocks and stratigraphic layers.
- Calculate property distribution models—such as porosity, permeability or temperature—within the geologic framework.
Below, we’ve expanded on each of these steps, explaining the key purpose of the step and its benefits. Note that within several of these steps the workflow provides a wide range of functions and tools to tailor the process to more accurately model geologic structures. We’ve eliminated that level of detail for the sake of brevity.
1. Define the Range for Modeling the Geologic Structure
Depending on the volume of data and processing power, models can become unwieldy. The X, Y, and Z ranges for the model to be calculated determine the geometry used in building and slicing the faces file that represents the 3D geologic model. For optimum efficiency in building the model, the range should cover only the area of interest.
2. Define Fault Surfaces
Faults are represented as 2D grid surfaces (or as isosurfaces from 3D grids) and are classified as major faults and local faults. Major faults define the fault blocks. Individual fault shapes can be verified using the 3D viewer while correlating the fault shape with the input data. Displaying the input scattered data along with the fault helps in detecting erroneous data. As part of this process, fault grids generated from noisy data can be significantly improved with a grid smoothing function.
3. Define the Fault Framework (Automatically)
A fault framework establishes the geometric relationships between different fault surfaces and defines boundaries between individual blocks. A “fault tree,” composed of fault blocks is used to define the relationships—how faults intersect. The model range is divided into smaller sections by specifying that a fault cuts through the region. A parent fault block can be subdivided into several children with child fault blocks being further subdivided. The process continues until all faults and their relationships in the model have been defined. The workflow offers the option of manually defining the fault relationships or automating the process.
Creating a reliable fault framework is crucial prior to modeling horizons or properties. Best practices recommend creating a 3D model of the faults upon completing the first version of the fault framework to visually verify the relationships. Viewing this 3D display allows the user to determine if fault interpretations are reasonable and agree with known local geological conditions. If not, users can make modifications to correct the geologic model.
4. Define Zones and Their Geologic Sequence
Having established the fault framework, define the layers or zones occurring within each fault block. A zone represents everything between the surfaces above and below the zone. Zones are specified by the vertical sequence of layering in each fault block, the bounding surfaces of each layer, and the geologic operation that defines the relationship between the intersecting surfaces. Sequence information is added only for fault blocks without children, as these are the only fault blocks used in the final model.
5. Define Horizon Surfaces
The process of defining horizons can be complex—numerous horizons with each horizon crossing numerous faults. A model with 20 fault blocks and 5 horizons can result in up to 100 individual fault block/layer compartments. The Workflow Manager automates this process using the fault tree to sort scattered data by fault block, then grid all scattered data groups independently.
Once horizon surfaces have been gridded, viewing the surfaces in 3D shows how surfaces interact with each other within the fault framework. Often the location of faults defined by horizon data doesn’t correspond with the position of the fault surfaces themselves. This resulting inconsistency is clearly revealed by the analysis of a 3D model of the horizons and faults. With a 3D view, inconsistencies in horizon surfaces in relation to faults are highlighted and can be corrected to create a more accurate model.
6. Build the Structure or Property Model
When all geologic model information has been specified, including property data and isosurfaces parameters, the workflow creates the complete 3D faulted, zoned model with property values calculated in zones and fault blocks.
Property distributions are modeled within the geologic framework using three different techniques:
- Continuous Across Zones/Fault Blocks;
- Independent Within Zones/Fault Blocks; or
- Distributed Before Faulting Occurred
to account for how the distribution occurred in relation to the faulting and the layer geometry.
Individual fault blocks and zones are selected and any of three gridding algorithms—3D Kriging, 3D Minimum Tension, or 3D Trend Gridding—can be applied. Prior to gridding, the data can be transformed relative to the top/bottom of a zone allowing conformal kriging, conformal minimum tension, or conformal trend gridding. Gridding an entire zone in a geometrically reconstructed space allows connective gridding between different fault blocks of a zone. The workflow produces a framework where each input data point is labeled based on its fault-block and zone location.
EarthVision’s 3D viewing capabilities facilitate a detailed evaluation of the geologic model.
EarthVision’s 3D viewing capabilities facilitate a detailed evaluation of the geologic model. Individual zones can be displayed with their properties. Individual fault blocks can be made visible or hidden. Property isosurfaces can be displayed within zones. Fault surfaces can be displayed according to properties, zones, or faults.
Workflow Leads to Modeling Efficiency and Improved Accuracy
The workflow that drives EarthVision’s Workflow Manager process enables geoscientists to simplify the complexity of modeling geologic structures while simultaneously improving their accuracy through:
- the ability to visually review and verify the 3D structures defined by your data throughout every step of the workflow
- an extensive set of algorithms, functions, and techniques to improve data quality and consistency and the resulting geologic model
- workflow versions that support multiple iterations for comparison of models
- the ease of incorporating additional data as it is acquired and updating the model
- flexibility in determining computational complexity to minimize processing time
Upon completion of the geologic modeling workflow, any portion of the resulting model can be viewed in almost any combination of fault blocks or layers to provide reservoir teams detailed insight regarding the internal geometric configuration of the model.