- This is a second lecture on a three part series on seismic stratigraphy. In the first lecture, we discussed what seismic stratigraphy is, and its applications. Today we're going to discuss a very powerful tool in interpreting seismic called seismic sequence analysis. The first step in interpreting stratigraphy from seismic is seismic sequence analysis. This interpretation methodology identifies stratigraphic packages in the seismic composed of relatively conformable strata. Now keep in mind, the subject area uses a plethora of terminology, but it's important to remember that it's simply identifying conformable packages in your seismic data. The diagram here shows you an idealized example of what these conformable packages look like. The first step is to identify reflection terminations, which are denoted by the arrows. These reflections terminate against unconformities and discontinuities, which are traced here by the bold lines and the dotted lines. Conformable packages are bound by these unconformities and discontinuities. One of the key surfaces we'll want to identify is the sequence boundary. The sequence boundary is a major erosional unconformity. So why is it important to carry out this type of analysis? To begin with, it allows the interpreter to apply the concepts of sequence stratigraphy and predictive depositional environments to the subsurface. It will aid in interpreting and mapping your reservoir, source, and seal. It will help in tying the seismic to well information, and then extending that information into inter-well regions and over large distances. It will also aid in identifying plays and prospects, and in well planning and field development. This workflow was introduced to you in the first lecture. It contains the general steps in the seismic interpretation process. Before beginning, it's important to prepare your data for analysis, such as identifying your key lines with well ties and plotting out base maps. The general information on the regional setting should also be analyzed and established in order to orient you in your interpretation. For example, are you on a shelf margin setting, or in a basin setting? The ideal place to interpret sequence stratigraphy is in the shelf margin setting. In this slide, I've outlined the four steps involved in interpreting sequence stratigraphy from seismic. In the first step, you're going to identify reflection terminations or lapouts. The next step is to connect these terminations and identify and correlate unconformities and discontinuities. In the third step you will identify these candidates for major surfaces, such as sequence boundaries. And then finally you will correlate the surfaces throughout the data. So the first step in identifying these conformable packages in your seismic is to identify lapouts, which are also reflection terminations. I'm gonna sketch here what those reflection terminations look like. Now the most important lapout is the angular truncation, which is an erosional termination of dipping reflections up against a reflection of lesser depth. Now sequence boundaries will generally dip at this type of termination. Now, a toplap is where progressively younger reflectors terminate against an overlying reflection. An onlap is where a reflection terminates upticks against a reflection of greater depth. And finally a downlap is where a reflection terminates against a reflection of a lower depth. Now let's look at some examples of lapouts in seismic. When marking lapouts on your seismic, it's a great idea to use a red pencil. The color will stand out, and you may be able to erase it. In these examples, we have onlap on the first picture, onlap on the center picture, and downlap on the last picture. Once these reflection terminations are identified, you can trace them out and identify the unconformities and discontinuities in your seismic. Here they're marked by a dotted red line. Let's look at some more samples that are a little more complex. The first picture has toplap, onlap, and downlap. The center picture has toplap and downlap, and the picture on the right has onlap and toplap. It may take some time to start pulling out where the discontinuities are and the unconformities, but eventually a picture will emerge if you're consistent with getting those reflection terminations correct. Here's another example of multiple reflection terminations and multiple discontinuities. And it's particularly challenging here to figure out which reflections are continuous and which ones are not, which ones are discontinuous. Once you've identified your reflection terminations, and your unconformities and discontinuities, you can begin to identify surfaces that would make good candidates for major seismic stratigraphic surfaces. In particular, the most important surface we can identify is the sequence boundary. The sequence boundary can be identified in seismic by onlap and angular truncation. The best place to identify the sequence boundary is in shelfal portions and shelfal margins of your data. Oftentimes the sequence boundary will become continuous in the basin area. Here is an example of seismic from offshore South Africa. The upper part of the figure is the uninterpreted line, and the lower part is the interpreted line. In the upper part of the section, you can see the sequence boundaries have been identified and correlated through the data, and are characterized here by onlap. The lower part of the data contains prograted clinoforms. These clinoforms are prograting to the south-southwest, and are characterized by downlap and toplap. Here's some pointers for success or best practices when interpreting seismic. To begin with, print out some key lines on a reduced scale. By beginning interpretation on paper, you'll be able to see the big picture or trends that you may overlook when working on a workstation. Begin your interpretation using dip lines. Dip lines show the most detail of your stratographic architecture, compared to other lines of different orientations. Begin by focusing on those lapouts. Try not to get bogged down on the different terminology in sequence stratigraphy and just focus on lapouts. Remain unbiased. Try not to target certain surfaces that you wanna interpret to begin with. Remain unbiased, and then the surfaces and unconformities will start to make sense and come together. Always know what your regional setting is. Are you in a shelf margin setting, a basin setting? Certain stratigraphic surfaces will be expected to look differently in different settings. Remember that you'll focus on identifying those major angular truncations, and that those angular truncations and sequence boundaries will be conformable in basin areas. When correlating your major surfaces, remember to stay low without crossing reflections when you're correlating sequence boundaries. And when you're on the shelf, stay high when correlating in those areas. Seismic sequence stratigraphy is a powerful tool in interpreting seismic data. Identifying your stratigraphic packages will help in the interpretation of lithology, depositional environment. It helps you correlate in other types of data, such as ages, well log data, reservoir information. And it helps to find your stratigraphic architecture and its subsequent implications on compartmentalization and connectivity. The sequence stratigraphic framework is useful in interpreting and mapping reservoir, source, and seal, and correlating over large distances and in between wells. And it will help identify plays and prospects as well as aid in well planning and field development. The next lecture in the series on seismic stratigraphy is seismic faces analysis and will follow this lecture in this three part series.