Qualified Structural Interpretation, Geological Realism and 3D Thinking
Peter Boult¹, Brett Freeman², and Graham Yielding²
¹GINKGO ENP GNG, St. Morris, Australia
²Badley Geoscience Ltd., Hundleby, Lincolnshire, UK
Our notion of reality in interpretation and structural geology usually follows a series of careful observations and ideas that eventually crystallize into a best case model. In most other branches of science the strength or reality of such models (or hypotheses) is increased by the number of robust tests that either refine or fail to disprove the original idea. Geological models in the E&P sector differ inasmuch as the starting point for testing is usually an interpretation of seismic or other remote measurements rather than the direct observation of an effect. This means that whatever tests we are able to apply have, themselves, significant margins for error and are further compounded by the intellectual issue of constructing a 3D view of the perceived reality, initially in the interpreter's head. This model building is an early stage of the E&P process and it is often merely a springboard for further geological or engineering purposes. However, errors and uncertainty at this early point propagate throughout the subsequent workflow and arguably, amount to the single biggest factor affecting perceived value and in particular drilling risk. Again, unlike other branches of science where a negative test can be viewed, to some degree, philosophically, a negative result from a drilling plan is a minor economic disaster.
A majority of structural geologists will have undergone undergraduate and postgraduate training in 3D thinking by way of producing geological maps and cross sections from the 2+D data of surface exposures and local structural measurements. The scientific method of prediction (3D structural rules) tested by observation is key to the process of mapping structures in the field. Indeed, since 3D is the universal currency of structural geology, they probably selected their professional path on the basis of their innate ability to think in 3D.
Most interpreters are not structural geologists. However, possession of 3D spatial skills and the building of 3D geologic models of faults and horizons from seismic interpretation is fundamental to the creation of a reality for E&P managers, based on which they make multi-million dollar decisions. Modern interpretation and modelling software is very sophisticated. It is now easily possible to generate beautifully persuasive realities that owe more to software design and programmatic flare than to geological realism. Furthermore the availability of the 3D interpretation/modelling canvas has had little impact on the requirement to understand the structure in 3D as opposed to the apparent structure of a single, auto-tracked horizon. Unfortunately, a pretty map or 3D model does not necessarily equate with reality. A good structural model should go beyond the aesthetic, it should include more than fault and horizon surfaces. It must also honour the rules of structural geology. These have been formulated by researchers in the field and in laboratories over the last 150 years. Hypotheses have been discussed, disproven, reformulated and approved by consensus through the peer reviewed scientific literature and interpreted structural models should be tested by these same rules.
The industry, to a large extent still harbours the cultural divide of geology and geophysics and to a large degree this division is reflected in the approach that an interpreter will take to generate his/her model. An out and out geophysicist is more likely to be interested in squeezing the last drop of physics from the seismic cube than testing that the picks make sense in the context of a structural model. There is also the issue of time and corporate inertia. Interpretation is a lengthy process and even in the face of expert opinion and advice a poor model may stand because (a) the interpreter will be reluctant to agree that the model is wrong and (b) it is too late to make significant changes because the resulting cellular model is already in use by sedimentologists and engineers. Thus the quest for good 3D interpretation is also hindered by poor management that, more often than not, still requires 2D, paper-based reports to make their decisions. The most pragmatic way to combat this is to allot more time to the interpretation so that when the 3D structure is challenging, simple, structural geological rules can be applied iteratively as the interpretation evolves.
Simple structural geological rules can greatly assist the interpreter to create a robust 3D model. Managers and JV partners can use the same simple rules to judge whether the model with which they have been presented is viable or equally importantly, not viable. Working in 3D is also an obvious step forward. If the interpreter favours traditional "flat screen" map and section based views the final product will tend to be inferior. On the other hand if their work is undertaken in a 3D environment and simple, structurally derived, attributes for fault and horizon surfaces make sense when plotted in appropriate graphs then the geological model can be considered to be more robust.
In order for the industry to fulfil its goals of 25 years ago, i.e. to embrace a truly multidisciplinary approach to E&P, then it needs to think carefully about the 3D process. It needs to encourage software vendors to develop innovative and integrated tools to test 3D interpretations during the interpretation process and it needs to provide exposure to these kinds of ideas through training across the board. Most importantly, since it is managers who control the E&P workflow, they, above all need to be educated about the advantages of using a structurally-qualified 3D model.
AAPG Search and Discovery Article #120140© 2014 AAPG Hedberg Conference 3D Structural Geologic Interpretation: Earth, Mind and Machine, June 23-27, 2013, Reno, Nevada