Evaluation of Foothills Natural Gas Plays Using Two Dimensional Balanced Cross Sections

Exploration for large gas fields in the Foothills has been an important part of many oil and gas companies’ exploration strategies in Canada during the last 5 years. Given the increasing demand for natural gas and the realistic expectation that we will continue to find large natural gas reserves in the Foothills, this is a trend that will continue for many years to come. One of the techniques used to evaluate plays in the Foothills is that of drawing balanced cross sections. This has a proven record of reducing risk and increasing the financial success of exploration wells in this area of complex geology.

The Foothills is a geologically defined compressional structural domain, and is located along the west edge of the Western Canadian Sedimentary Basin. The area involved extends over 50,000 square miles and has 2,500 oil and gas wells spread somewhat unevenly throughout it. Production is dominated by natural gas and this area has established {remaining?} reserves of 40 Tcf of gas in place, of which 21 Tcf are considered marketable. Since the 1950’s the Foothills has produced 13 Tcf and is currently producing about three-quarters of a Tcf of gas per year. Based on the latest data available from the Canadian Gas Potential Committee there appears to be a further 32 Tcf of gas in place left to be discovered in established plays. The three largest will be over one Tcf in size.

Drawing cross sections in this structural domain requires the integration of many diverse data sets into one cohesive model. This model is frequently visualised by a series of dip sections tied by one or more strike sections. To prove that these sections are balanced it is normally considered sufficient if they have passed two tests. The first test determines whether the area of rock in a deformed cross section is the same as in an undeformed cross section. If it is, then this is termed a viable cross section. The second test is used to evaluate the cross section, to see if it honours the deformation model of the structural domain in which it belongs. If it does, then it is admissible. If a cross section is both viable and admissible then it may be called a balanced cross section.

To test the viability of a cross section several key conditions have to be met. Firstly, all of the deformation has to take place in the plane of the cross section. Next we have to assume that a significant volume of rock has not been added to, or removed from, the domain between the pre-deformation and post-deformation stages. Lastly, we must draw the cross section parallel to the principle direction of transportation. If these conditions are met then we can proceed with balancing a series of 2D cross sections. If they are not met then it may be necessary to carry out a balancing exercise in 3D.

For a section or a series of sections to be considered admissible it is necessary to define the deformation model for that particular structural domain. This involves the careful integration of surface geological mapping, dipmeter data, seismic mapping and large-scale regional mapping of outcrop data into a series of sections. In general, the Foothills may be divided up into two types of deformation models, the fault bend fold and the detachment fold model (Figure 1). The fundamental difference between these two models is related to the amount of shortening transmitted across the structure. In the fault bend fold model you will have nearly equal amounts of displacement on both sides of the structure. This is very different in the detachment fold model where the amount of displacement on one side of the structure is very much less than that on the other side.

With recent increases in computer power it is now possible to routinely use a third test. This test of the kinematic model is designed to evaluate the viability and admissibility of the intermediate steps generated between the pre- and post- deformation sections. By using the palinspastic restoration as a starting point and inverting through the forward model an evaluation of the kinematic history of the structure is possible.

In the oil and gas industry today it is important to know when to use the tools we have available to us. Across section needed for a land sale need not be balanced in the same way as one that is needed to develop a drill location in a naturally fractured reservoir. For a quick evaluation of a cross section a useful technique would be to check that the flats and ramps developed along a fault match those in the hanging wall and footwall. This would not prove that the cross section balances but it should indicate common problem areas.

If an evaluation of a seismic section was needed and there was no time to convert it from time to depth then it would be possible to measure the line length of the formations to see if they are constant between the pre- and post- deformation sections. This would be termed a line length balanced cross section. This does contravene some of the conditions we outlined above for drawing viable cross sections. However, it would provide a quick look technique that is useful under certain circumstances.

When a more detailed understanding is needed, then it is possible to determine if a cross section area balances. This is a valid balancing technique and would provide an answer to the viability and the admissibility of a cross section. However, if it is carried out on just the pre- and post- deformation models then it may not balance in the intermediate stages of the kinematic model. By using a full palinspastic restoration and forward model it is possible to overcome this. Then by drawing a cross section that has been proven to be viable and admissible in a kinematic sense it is possible to have a balanced cross section that is suitable for use in the Foothills.

To meet the time constraints imposed by the realities of today’s business environment, we need to be able to balance cross sections in the most cost-effective way. The data needed to prove that a series of cross sections balances should include a deformed state cross section, an undeformed cross section and a deformation model. With more time it would be desirable to include a kinematic model. This provides an added dimension to the deformation model and allows an in depth evaluation of the formation of the structure. This is useful for plays where the timing of hydrocarbon charge or the generation of fractures is a key risk in the evaluation of the prospect.

With the help of faster computers to manage the databases and to drive new modeling algorithms, much of the hard work has been taken out of this process. But at the same time we need to understand the concept of balanced cross sections better than ever before. By doing so we will be able to use them as an effective tool for evaluating the large natural gas plays in the Foothills of the Western Canadian Sedimentary Basin.