Many interpreters rely on their workstation for their interpretations (auto-picking) and for their final maps. Yet the maps generated in a computer are almost always wrong, sometimes significantly so. The failure to review and validate these computer-generated maps results in inaccurate reserve estimation and in the drilling of many dry holes every year.
Before accepting a map as final, it is important to conduct a self-audit. That is a thorough review of the interpretation and the map to ensure that they are geologically sound and valid. More often than not, the self-audit will allow you to find mistakes or problems that you need to fix before considering the map final. This talk will walk through the various steps you can use to conduct a self-audit to ensure that your final map is geologically and geometrically valid.
The self-audit is a six step process:
Step 1: Does the map honor the data?
Review the final seismic interpretation to see if the computer honors that data. In the process of gridding and smoothing, the computer will generate a set of horizon picks that differ from your original picks. Once done, the computer will use its picks and ignore yours.
There are many contouring algorithms. Depending on which algorithm you use, you can get radically different looking maps from the same data set. Many of these algorithms will give you maps that no longer honor the data.
Step 2: Do the contours exhibit contour compatibility?
For almost all faulted structures, the contours will exhibit contour compatibility as the cross from one fault block to another. That is, the contours will exhibit a similar strike on both sides of the fault. Computers contour each fault block independently, resulting in maps that do not exhibit contour compatibility. You will have to hand edit the computer-generated contours to establish contour compatibility.
Step 3: Do the contours honor vertical separation?
Just as computer-generated maps do not exhibit contour compatibility, they routinely fail to exhibit the proper amount of offset across the fault. The contours should change value as they cross a fault by the amount of vertical separation, which is the same as the missing section (or repeat section) measured in faulted wells.
Step 4: Does the map match the seismic?
Since the computer uses its grid over your interpretation, and the various contouring algorithms can significantly impact the final appearance of your map, it is common to see final maps that no longer match the dip or fault offsets shown on the original seismic interpretation. It is important to look at a number of your seismic profiles and ensure that the map is in agreement with your interpretation.
Step 5: Are the fault traces properly positioned?
Most interpreters pick fault sticks and then use those to draw fault polygons onto their horizon maps. Although the fault polygons are close to the actual position of the fault relative to the horizon, they are not precise, and many wells have been drilled into the fault gap as a result. You need to learn the steps needed to integrate the horizon contours with the contours of the fault surface. This integration process is the only way to ensure that the fault traces are properly positioned.
Step 6: Does the map honor the geology?
You would be amazed, and disheartened by how many prospect and field maps are made that have no resemblance to the geology of the structure they are intended to portray. Again, this is often a function of the gridding and contouring algorithms, although it is occasionally caused by interpreters using the wrong models. As you review your maps and seismic to make sure they are in agreement, also make sure that they both portray the geology.
STRUCTURE OF THE E-SYMPOSIUM
This e-symposium consists of a one-hour recorded video presentation, along with material for one full day of independent study. The one-hour e-symposium can be accessed from any computer anywhere in the world using a high-speed internet connection. When you register, you will receive via email information about accessing the recorded video and the other independent study materials, to be accessed and studied at any time. You will be able to email responses to the readings, along with your study question answers for CEU credit (if you sign up for the extended package).