Page 104 - Geology of Carbonate Reservoirs

ROCK, TIME, AND TIME–ROCK UNITS 85

A A
Sea Level
T5
T4
T3
T2
T1

A' A'
Figure 4.4 An illustration of time - transgressive facies and Walther ’ s law. Note the way
depositional facies migrate landward during transgressive sea - level phases and move seaward
during regressions. At an arbitrary starting time, T1, the limestone facies migrates landward
and deposition continues through T2 time but without change in facies identity. This is called
′
time - transgressive deposition. The vertical column A – A shows what would be found if a
′
borehole were cut at the point marked by the line A – A . From the top of the column down-
ward, the sandstone overlies the limestone. The same facies are present as time equivalents
in a seaward direction along the surface, an expression of Walther ’ s law (Walther, 1894 ) .

identical in rock properties, may have formed in identical depositional regimes on
a platform, but they are not equivalent in age. The importance of these facts becomes
evident when one attempts to reconstruct the shape of an ancient platform from a
three - dimensional succession of layered rocks.
“ Absolute ” geological time is based mainly on ratios of stable daughter isotopes
and their unstable parent radioactive isotopes. For example, an isotope of uranium,
238 206 238
U, decays to Pb through a series of steps. The rate at which U decays is known
precisely; therefore if the Pb/U ratio of a rock sample is known, its age can be cal-
culated. Other geochemical techniques are available to determine the age of rocks.
The point is that these methods can provide age measurements in years of actual
time before the present whether fossils are present or not. The precision with which
age dates can be determined varies depending on the method used, the amount of
substance present for analysis, the purity of the substance, and the age of the rock.
Absolute ages are determined with margins of error as great as 5% or more for
8
dates of 10 years before present. Relative time based on index fossils is even less
precise and dates based on any particular fossil may vary by many millions of years.
Geochemical methods are not often used to determine the ages of reservoir rocks
because mineral species that contain datable isotopes are not always present and it
is expensive and time consuming to make the measurements. In general practice,
correlation for reservoir mapping and sequence stratigraphic applications can be
accomplished with marker or index fossils. Of course, the marker fossils must be
present. If they are absent, the precision of stratigraphic correlation along a specifi c
time surface decreases dramatically with distance between points of reference (wells
or outcrops), the likelihood of miscorrelation increases, and accurate sequence
stratigraphic applications are difficult if not impossible. A basic rule in sequence

stratigraphy is that sequences are bounded by unconformities and their correlative
conformities. The amount of time represented by the sequence - bounding unconfor-
mities must be established by identifying the ages of the beds above and below the