Principles of Structural Geology
Our Place in the Structural Geology World
The purpose of structural balance is to create a geological model which is: (i) internally consistent, (ii) relies only on known assumptions, (iii) makes some predictions, (iv) highlights interpretational problems and (v) highlights alternatives"
This remains as true today on as it was when Alan first pioneered the approach to tackle uncertainty in the North Sea and is the basis of our approach to geological modelling to deliver real commercial benefits. While other companies make modelling software for visual communication alone, our kinematic modelling tools help clients to challenge their assumptions to produce more accurate models. An interpretation remains the geoscientist's best estimate of the truth. The better the interpretation, the better the results. Our tools enable users to:
- Discriminate between alternative interpretations
- Generate key information on geohistory
- Predict palaeo- basin architecture
- Analyze hydrocarbon migration, sediment dispersal and salt movement, strain and fracture prediction
- Reduce structural risk
- Save time and money
Structural Modelling - Predicting What Can't Be Seen
The aim of the structural approach is not just to describe, but to predict the unseen parts of the system and to be able to use the structural development to predict changes through geological time. Structure modelling is the core skill at the heart of all Midland Valley activities. Our approach is based on expertise built up over 25 years as leaders in this discipline. As well as developing the technology, we ourselves use the tools in consulting projects and we work closely in collaboration with academic groups around the globe to ensure that we are always at the forefront in our field. The challenge we have faced has been to develop tools and workflows that work routinely with industry data and which are accessible to the generalist as well as the specialist. In this we have been guided not only by our own experience through commercial project work around the world, but also from the close relationships we have built with our clients' expert and business unit teams and academics worldwide. The tool sets we provide are the result of this marriage of practical need and the best technical and scientific approach.
The basic input for all of our analysis is the observed field, borehole, well, and geophysical and remote sensed interpretation as well as heritage interpretations that are routinely available to industry geoscientists. The techniques we have developed for structure modelling allow the deformation and development of the structure to be analysed and visualised from sediment deposition, intrusion and tectonism through to final present day configuration.
Approach - analytical modelling tools
Geologists at Midland Valley have been providing structural geology consulting, training and advice for over twenty years. As a direct result of our field work and experience, we have developed a market-leading range of software products to help the Oil and Gas, Mining and CO2 and Nuclear Waste industries operate with greater safety and efficiency. These applications and workflows not only enable geoscientists to build valid structural models but to discriminate between alternative assumptions they have made about the geology. With our toolset users can continually refine models over the course of a project to reduce risk and enhance decision making. The benefits for companies are significant:
- reduced risk
- lower costs
- increased speed
- improved accuracy
- proved recovery - better reservoir models
Our Restoration Algorithms
Move includes both geometrical and geo-mechanical options for restoration. Geometrical algorithms provide fast and flexible proxies for deformation that follow the traditional approach of using incline shear for extensional systems and fault bend folding for thrust belts that have an established track record. The geo-mechanical approach requires some knowledge of rock properties in order to get best results and handles non-plane strain situations as well as a complex mix of hard and soft linked faults. Midland Valley’s geo-mechanical approach has been specifically designed to give fast reliable restoration of complex structure and integrates with our fracture modelling tools. Good practice in restoration is to use a variety of restoration algorithms to establish sensitivity and to develop an understanding of deformation history. Sensitivity studies can usually be carried out in 2D before the most appropriate parameters are carried forwards to a full 3D restoration or a 3d geo-mechanical solution.
Inclined Shear (Restoration Menu)
The inclined shear algorithm is most applicable to extensional tectonic regimes, where anticlinal rollover structures have developed on non-planer normal faults. Furthermore, this algorithm can be applied to the restoration or forward modelling of inverted basins and growth faults, where the thickness of beds may vary. The inclined shear algorithm geometrically models the relationship between fault geometry and hangingwall deformational features. Inclined shear, models homogeneous deformation throughout the hangingwall rather than discrete slip between beds (i.e. flexural slip). The inclined shear algorithm in Move maintains the area between beds in 2d and volume in 3d. As the algorithm conserves area or volume it is possible to mix shear angle either between steps or to chose different shear angles for different horizons taking care that markers do not cross-over. Some workers chose to do this to simulate different mechanical properties eg. between sandstones and shales. The deformation of the hanging-wall is modelled by moving each point in the hangingwall by the same horizontal distance (the heave), following a path parallel to the fault. Each point on the hanging-wall surface can be considered to be on a pin, which does not change its length as it is moved over the fault.
The Fault Parallel Flow (FPF)
The FPF algorithm is best suited for modelling hangingwall movement on faults from fold and thrust belts where the majority of the deformation occurs discretely between bed interfaces (i.e. flexural slip). The Fault Parallel Flow (FPF) algorithm, developed in collaboration with the University of Keele (Kane et. al, 1997 and Egan et. al, 1997), is designed to kinematically model geological structures in the hangingwall where deformation is accommodated by fault-parallel shear. The Fault Parallel Flow toolbox uses an algorithm assuming particle flow parallel to the fault surface and parallel to the plane of cross section (plane strain assumption). Recent work has found that the Fault Parallel Flow algorithm may also be suitable for modelling in extensional regions. Compared to geometric construction models for move on fault restorations like Flexural Slip, the Fault Parallel Flow algorithm is not restricted to simple ramp-flat-ramps with a dip less than 30°; thus, the Fault Parallel Flow algorithm may be applied to faults with a complex geometry.
Mass Spring Restoration
The geo-mechanical restoration approach uses a linear elastic assumption with boundary conditions and constraints built in as defaults. These defaults can be user modified to allow scenarios to be built and compared using our scenario analysis tools to test assumptions and sensitivity. The algorithm uses a mass-spring approach to provide fast compute speed and flexibility to investigate a range of geological models without the need to build fully water tight finite element models.
Midland Valley places key emphasis on new technologies that enable us to visualise our ideas, animate concepts and interact with the data and model in a flexible manner - a process that matches as closely as possible the way we think as creative geo-scientists. In their day, traditional paper maps, sections and elaborate isometric diagrams were at the cutting edge of the geosciences, offering fascinating new insights into geology. Today, these paper based models have become outdated, and now seem to owe more to artistic skill and panache in the tradition of Escher than the real technical, scientific and commercial needs of the modern geologist. The ability to animate the idea that has led to our present interpretation provides a powerful tool not available to the paper and pencil era. It not only allows us to present our colleagues with an interesting kinematic model, but to actively engage them in the geological process and allow them to see our ideas from their own viewpoint. Kinematic models help challenge interpretation and modify decisions. In recent projects, our kinematic models have facilitated new plays and the identification of additional reserves for the hydrocarbons industry, developed improved exploration and production models in the mining sector as well as being used to help develop waste sequestration strategies for CO2 and radioactive materials. Improved understanding of the present day geology and its evolution reduces uncertainty and risk in sub-surface interpretation and prediction.
Model Building - A Question of Interpretation
Geological models on maps, sections and in 3D need to be correctly interpreted if they are to be properly utilised. Moving from the traditional map and sections to the 3D computer image still sets the interpreter the challenge of joining the data control into a coherent whole. In the real world there are no gaps or missing volumes! The most critical issue we have to address in building the 3d framework model is not just how to join up and honour the data, but to do so in a way which makes geological sense. Move provides a unique environment to validate and check the geological sense of our model building choices through the use of restoration and forward modelling tools. Our tools are designed to provide geoscientists with the freedom to try ideas before committing to the discipline of the completed model. Model building is the crux of data analysis and decision making. As well as building models for structural analysis, we can provide constrained models for basin modelling, image analysis, and reservoir modelling, either by carrying out a specific project or providing you with the software tools to carry it out yourself. Where complex structures are involved we can deliver the critical detail that is not captured by other products.
All the modelling tools are augmented by the ability to constrain the modelling with a geological rule base and to be used in conjunction with the forward modelling tools to check assumptions and interpretation. Move provides a tool-set that is designed specifically for geologists by geologists and allows the user to work in both 2d and 3d views that are fully linked to ensure that changes to the model in 2d are correctly extrapolated to 3d and vice-versa.
Palinspastically Restored Geometries
Our restoration tools are commonly used to predict palaeo-architecture of basins and mountain belts. The construction of valid palaeo-accommodation space provides control for models of sedimentation and prediction of sediment fairways. During such a restoration the effects of compaction, isostasy, folding and faulting are removed to arrive at a robust prediction of past basin geometry. These palaeo-geometries are used as input into sediment modelling and hydrocarbon and mineral systems analysis.
Facilitates the real time geo-steering of wells and boreholes in complex structures
For the specialist the techniques are compatible with and complimentary to analogue and physical property approaches, but have the key commercial advantage of being fast to use and modify in the light of changing data inflow. This facilitates the real time geo-steering of wells in complex structures, reducing technical risk and cost. Strain and detailed structural analysis allow geologists to identify missing interpretational components and include them in the model. Alternative algorithms can be applied to build up an understanding of the uncertainty and alternatives and thereby identify the key elements of the model which are sensitive to the commercial outcome.
Validation and Risk
A completed model in either 2D or 3D is often considered finished and valid if horizon / fault ties are complete and when wells and volume information are attached to the model in the correct locations. However, these "completed" models may not be geologically permissible either in terms of the representation of the present structure, or, commonly, in terms of the geological evolution which is implied by the interpretation. We provide a complete suite of tools to check the geological, as well as geometrical, validity of the model based on known and quantifiable geological constraints. At its simplest this can involve unfolding and unfaulting the model (restoration) to check that all of the rock volume implied in the interpretation is correctly represented and that the processes by which it has arrived at its current configuration can be validated for all geological time steps. In more complex cases the sediment, erosional, compactional, intrusive and isostatic history can also be accounted for in the validation process.
A model which does not restore and is not balanced cannot be a valid representation of reality. A balanced and restored model is both valid and geologically permissible and always represents a lower risk solution.
Reducing risk produces real commercial benefits
Most drilling surprises and production problems arise from an incomplete or inappropriate understanding of the geology. In most cases this is related to the structure, either during reservoir deposition or to subsequent structural development. While improving data quality is always important, the key to understanding and managing geological uncertainty lies in modelling the geological process, identifying alternative interpretational strategies and comparing the outcomes of multiple scenarios. The restoration process is central to this approach and can provide a precise quantification of the uncertainty in modelling. Midland Valley has assisted clients to reduce risk in exploration drilling through validation of the geological model and increasingly is helping clients improve drilling success in production in a variety of geological environments. This includes crestal drilling and down flank sub-thrust step outs in mature fields where lower than 50% success in drilling has been eliminated through better understanding of the updip geology. Midland Valley continues to research and develop new fundamental approaches to risk and uncertainty modelling and is working on innovative new techniques for whole model uncertainty characterisation. This approach includes data, interpretation and algorithm uncertainties as model attributes and as well as novel visualisation techniques for communication and management of risk. Midland Valley's unique strength is that we are always focused on providing the best solutions to the interpretation of geological structure to resolve commercial and societal problems. This requires us to focus not only on the best science but on the pragamatic issues of problem solving to time and budget.
Fracture Analysis - a key input for Reservoir, Mineral and Sequestration Engineering
All rocks are fractured, and increasingly the fracture system has a critical effect on production and recoverable reserves. In such fields successful reservoir management requires accurate fracture models to assist in the planning of wells and discrete fracture network models as key input to the reservoir model. Similarly the mining engineer will require predictive models for fractures in planning and exploration potential frequently hinges on the prediction of mineral fairways controlled by the structure. In many more cases, end stage or fault compartment production will be influenced by sub-seismic faults and fractures, which were not anticipated in advance. Midland Valley's products provide a unique environment to develop fracture concepts and models within a holistic geological framework. Structural development and strain history are critical in the generation of fracture systems in the real world. Stress history, coupled with diagenetic and fluid history, control which fractures are open systems. These histories can be used to generate fracture models and realisations based on a range of assumptions within the context of the live structure model. This ability has led a number of key clients, notably BP, to identify Midland Valley's systems as a key new analytical tool in this challenging arena.
Offering optimal fracture reservoir management
A common approach during fracture modelling is to integrate strain history with well data to generate fracture networks. These allow attributes to control fracture distributions and densities as well as using stochastic methods. Well fracture data can be used either to control the modelled fracture system, or to test a range of geo-realistic fracture models as part of a probabilistic approach. This technique is now being used to successfully guide fracture reservoir management in a wide range of reservoirs, from thrust belts to basement fracture systems to carbonate systems. Critically the approach has proved valuable in providing fracture models and scenarios for exploration and early field appraisal with a wide range of data quantity and quality. A number of projects have also been completed that provide detailed fracture and fault models for production planning in large fields where gas clouds have decreased seismic resolution.