Stratigraphic Cross-Sections
Stratigraphic (strat-i-GRAPH-ic) cross-sections differ from structural cross-sections in that
underground structure or elevation is ignored in the stratigraphic type.
"Strat" cross-sections allow the PG to see some things more clearly.
Geologists make a whole lot of stratigraphic cross-sections as they try to
figure out what the rocks are doing.
The stratigraphic cross-section shown here illustrates a vertical view through six wells. Electric logs are laid out in a line with paper and ruler or (much more likely these days) a computer.
The PG has identified an easily-picked formation or marker-bed (at the top of the cross-section), and has drawn a straight line called a "datum" (day-dum) through it. Then, each electric log is "hung" on the datum line. Even though the formations in each well are at different depths, due to the underground structure, "hanging" the line on the marker-bed datum allows the PG to see the relationship of the formations more clearly, and proceed to the important work of connecting them properly. This kind of cross-section does not show a true representation of the vertical depth change between wells. For that, the geologist must make a structural cross section.
The PG then looks at the various formations that she can identify on the electric logs. In this case, several sandstones are visible. The PG tries to connect the formations so the cross-section will look similar to real geology she has observed on field trips, or studied in school. This process of showing on paper what we think the real world looks like is called "modeling", because we are making a model of what we think we would see if we could dig it up!
The most interesting thing about this particular cross-section is the way sand 3 disappears between wells 3 and 4. This is called a pinch-out, and is a very common thing in petroleum geology. This is one of the things that makes geology interesting, puzzling, and challenging!
If the oil or gas is contained only within sandstone 3, the two wells on the left (10 and 4) would be dry holes, because no sand 3 was present
Below is a modern stratigraphic cross-section generated using a computer and image files of the well logs. In this case, the petroleum geologist has chosen a "hot" shale (high gamma-ray reading) for his datum . All of the well logs are "hung" on the datum. The green and yellow areas represent "pay zones". These "pay zones" are intervals that the petroleum geologist believes may produce oil or gas. The red bars show where some of the zones have been perforated for production.
The next stratigraphic cross-section was generated with a computer using digital well logs. Notice how the rocks are displayed between the well logs as the geologist believes they lie underground. The brighter yellow colors represent the better sand quality in the well. This kind of display is especially useful to help explain things to non-geologists.
