Here’s the short answer…
The fossils themselves are not direct indicators of pressure the way splintered ‘cavings’ are. This has to do with:
- Stratigraphy
- Recognizing the natural barriers between distinct permeable zones and
- Predicting and detecting where those natural barriers were when a borehole rips them, so they can be replaced with cement or put behind pipe.
While biostratigraphic data is largely unheralded, it is not unneeded. It can be used as insurance to guard against the unexpected; a behind-the-scenes secret weapon. Its usefulness has been proven time and again in reducing well costs.
The longer, more technical answer for engineers, or the ‘engineering minded’, is…
Most of the natural barriers to pressure are associated with finely grained, highly laminated sediments deposited during geological times when the sea level was high, or a maximum flooding surface (MFS) was in place.
When sea levels were high, the microscopic critters (such as foraminifera or calcareous nannofossils) flourished, died and fell to the seafloor. Eventually, those critters helped create finely grained, highly-laminated, muddy sediments. In fact, the carbonate content of a marine rock is a function of organism productivity (more critters) and reduced sedimentation rate (less coarsely grained sediment influx).
Most casing/liner and cementing programs are based on recognizing distinct permeable zones and natural pressure barriers. This is the first evidence operators have to submit in order to calculate worst-case discharge. Subsequent engineering is based on this calculation.
In areas with highly expanded stratigraphy, like the Gulf of Mexico, the traditional formations do not exist as they do onshore in the United States. The monotonous sequence of sand, mud and marl in deep water is difficult to correlate properly. This is especially true when salt-evacuated mini-basins have different geological histories. Fossils become the easiest high-level calibration to other correlation techniques.
Once a geoscientist recognizes where the MFSs are, the GR, Seismic and XRF data can be aligned. Thus, they begin making more sense. Depending on the geological history, many of these natural barriers are region-wide.
While biostratigraphic data is largely unheralded, it is not unneeded. It can be used as insurance to guard against the unexpected; a behind-the-scenes secret weapon. Its usefulness has been proven time and again in reducing well costs.
At Paleo Data Inc., we want to help you use clues from the past to create efficient future drilling plans. Contact us today to find out how we can fit into your workflow.
