Modelling of 3D Geological Structures

By:

Zawar Muhammad Khan

Outline: What is 3D Modelling

3D Seismic Interpretation

Steps of 3D Modelling

Data Required for 3D Modelling

Basic Rules for 3D Structural Modelling

3D Structural modeling process

Fault Network Modelling

Horizon modeling

Case Study

Conclusions

References

What is 3D Modelling: 3D modeling is the process of developing a

mathematical representation of any three-dimensional surface of object via specialized software.

3D modelling in geology is based on subsurface data from boreholes, and /or seismic and geological maps. There are different strategies of model techniques, depending on the complexity of the regional geology.

The basic and first step required for 3D modelling is 3D seismic Interpretation.

PICKING THE FIRST INLINE

PICKING THE FIRST CROSSLINE

INLINES & CROSSLINES PICKED FOR ALL

WELLS

TIME SLICE PICKS ADDED

3D subsurface modeling is used for:

(1) improving data interpretation through visualization and confrontation of data with each other and with the model being created.

(2) generating a support for numerical simulations of complex phenomena in which structures play an important role.

Steps of 3D Modelling: A: Data georeferencing.

B: Picking of relevant structural objects.

C: Creation of fault network.

D: Horizon modeling.

Data georeferencing.

Picking of relevant structural objects. (Curves with spherical nodes denote faults; curves with cubic nodes denote stratigraphic contacts)

Creation of fault network

Horizon modeling

Data Required: The typical input data for a 3D structural modeling

project can be quite diverse and may include 1. Field observations(for instance stratigraphic

contacts and orientations, fault planes). 2. Interpretive maps and cross-sections. 3. Remote sensing pictures. 4. Reflection seismic. 5. Borehole data.

Basic Rules for Structural Modelling:

The surface orientation rule states that a geological surface is always orient-able.

Surface non-intersection rule states that any two surfaces should not cross each other, except if one has been cut by the other.

Surfaces (Horizons and Faults) should fit the data within an acceptable range depending on data precision and resolution.

Well data should generally be honored much more accurately.

Relationships between the geological interfaces should be correct.

Structural modeling is generally achieved in two steps:

1. Fault surfaces are first built to partition the domain of study into fault blocks.

2. Stratigraphic horizons are created.

Structural modeling process:

Fault Network Modelling:

Faults are very important in structural modeling, for they partition space into regions where stratigraphic surfaces are continuous.

Therefore, it is important to generate faults and to determine how faults terminate onto each other before considering other geological surfaces.

Defining the connectivity between these fault surfaces is probably the most important and the most consequential step in structural modeling.

This can usually be done by considering the geometry of both fault data and surrounding horizon data to assess the fault slip.

Indeed, the fault slip should always be null at the fault boundary, therefore, when a horizon on either side of a fault is significantly offset near the fault boundary.

This suggests that the fault should be extrapolated or projected terminates onto another fault.

This information can then be used to fill the gap between the branching fault and the main fault.

Horizon modeling

Horizon construction may be achieved fault block by fault block, from horizon data.

The logical borders must then be defined interactively to ensure that horizon borders are properly located onto fault surfaces.

This block-wise approach is adapted for simple models with few faults.

Each step of the process is manually controlled, and can be specifically adjusted to the goal at hand.

Case Study

THE DEVELOPMENT OF A 3D STRUCTURAL-GEOLOGICAL MODEL AS PART OF THE

GEOTHERMAL EXPLORATION STRATEGY – A CASE STUDY FROM THE BRADY’S GEOTHERMAL SYSTEM,

NEVADA, USA

Egbert Jolie, James Faulds, Inga Moeck

The focus for the Brady’s geothermal system located in the Basin and Range province is on the identification of structural controls on fluid flow and permeability anisotropy.

The data used as input parameters of the 3D model is

1) detailed geological maps,

2) borehole data,

3) processed 2D seismic and gravity data,

4) a digital elevation model

Based on these data, four representative cross sections have been developed as a major input for a preliminary 3D geological model.

Development of Cross Section

Study area with fault traces (red), well locations, and developed cross sections (B, C, D, E).

Process of data preparation for the development of a 3D structural model. The entire model is based on four geological cross sections and a geological map.

Fault modeling

Final fault model with 61 fault planes. The dominant strike direction of the fault system is NNE (~N30E).

Horizon modeling

Horizon model of the Brady’s geothermal system. The top horizon represents the early to late Miocene.

Results

3D geological model with the proposed stimulation well 15-12.

The 3D structural-geological model of the Brady’s geothermal system comprises of:

61 fault planes.

Eight geological units.

71 large fault blocks.

The dip angles of the faults vary from 45° to 80°.

The major strike direction is NNE.

The known geothermal reservoir is located at a depth of 600-1,500 m below surface.

Production wells target the NNE-striking Brady’s fault, which has a dip range from 70°-80°.

Compartmentalization due to faults has effects on the hydraulic conductivity, as it can isolate or connect production wells from each other.

Conclusions: 3d geology models are the future oriented base for subsequent

hydrogeological and environmental modelling, as well as for geotechnical and GIS based spatial planning.

3D models can be used as a consistent data base as well as structural model for subsequent modelling of groundwater flow and transport.

Understanding the spatial organization of subsurface structures is essential for quantitative modeling of geological processes.

The quality and reliability of available data should be considered to define the data integration strategy.

Although the direct generation of compatible geological structures often remains a problem, visual quality control is a must to detect inconsistencies.

References: Chilès JP, Aug C, Guillen A, Lees T (2004). Modelling the

geometry of geological units and its uncertainty in 3D from structural data: the potential-field method. In: Dimitrakopoulos R, Ramazan S (Eds.) Orebody modelling and strategic mine planning, Perth, WA: 313–320.

Culshaw MG. (2005) From concept towards reality: developing the attributed 3D geological model of the shallow subsurface. Q J Eng Geol Hydrogeol 38(3):231–384