Geoscience Reference
In-Depth Information
Fig. 9.1 Simple artificial example to illustrate construction of a RASC optimum sequence. Three
fossil taxa (a, b and c) have been observed at five equally spaced levels in three sections.
Circles
indicate observed lowest (
1
) and highest (
2
) occurrences of the taxa. Distance between levels could
represent a 10-m sampling interval between cuttings taken from three exploratory wells drilled in a
sedimentary basin. The
vertical heavy black lines
represent ranges of the three taxa in the
hypothetical situation that the sampling interval would be very narrow. Even if the sampling
interval is not narrow, it can be assumed that any observed range of a fossil taxon is too short
because it is unlikely that the lowest (
1
) and highest (
2
) occurrence would coincide exactly with
the truly first and last occurrence of the taxon elsewhere in the sedimentary basin (Source:
Agterberg et al.
2013
, Fig. 1)
and gamma ray peaks can be diachronous. In RASC “marker horizons” without
biostratigraphic uncertainty receive more weight in the calculations. Basic princi-
ples of RASC are introduced in the next section. A simple artificial example will be
used to illustrate ranking followed by determination of probable positions of
stratigraphic events in a RASC biozonation that can be used for CASC correlation
between sections. Scaling, equi-distant sampling effects and RASC analysis of
variance will be illustrated in Sect.
9.3
on the basis of large exploratory well
data sets.
9.1.2 Artificial Example of Ranking
Figure
9.1
shows three artificial stratigraphic sections in which three fossil taxa
(A, B and C) were observed to occur. Each taxon has first and last occurrence
(FO and LO) labelled 1 and 2, respectively. Consequently, there are six strati-
graphic events in total. The equidistant horizontal lines represent five regularly-
spaced sampling levels. The discrete sampling procedure changes the positions of
the FOs and LOs. As discussed in the previous section, any observed range for a
taxon generally is much shorter than its true range of occurrence. In our example,
the observed ranges become even shorter because of the discrete sampling. FOs and
LOs coincide in three places. In reality, the FO of a taxon always must occur
stratigraphically below its LO. It is because of the sampling scheme that these
two events can occur at the same level in our example (see circles in Fig.
9.1
).
Search WWH ::
Custom Search