Page 67 - Petroleum Geology

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may be) against present depth. Time and temperature are not included ex-
plicitly: depth embraces time, temperature, intergranular pressures and pore-
water pressures. Temperature and pressures may be taken as linearly related
to depth from a practical point of view, but it is by no means certain that
time is. We cannot interpret such curves as describing the history of compac-
tion - that mudstone now 2 km deep followed the curve during its burial to
2 km. We cannot assume that a mudstone unit would have one history-of-
burial curve; it is much more likely that the very top and bottom would have
a history much like these curves, but that the centre's would be very different
because pore-water expulsion from the centre would have been retarded.
Compaction curves may be determined by direct measurement, and by in-
direct measurement through geophysical properties. Hedberg (1936, p. 254,
table 1) determined the bulk density, grain density and porosity of mud-
stones from a well in Venezuela by direct measurement, and from these he
estimated the overburden pressures and recorded the data against depth. The
deepest sample was from 6175 ft (1882 m), the shallowest from 291 ft (89
m). He regarded porosity as a better indicator of compaction than bulk den-
sity, and pointed out that the relationship is not between depth and porosity
(although this has practical value) but between pressure and porosity. He
found that the depth in feet was approximately numerically equal to the
overburden pressure in pounds per square inch (psi), that is, an overburden
pressure gradient of 22.6 kPa/m. He recognized three distinct compaction
trends:
- From 0-800 psi (to 5.5 MPa) he found the exponential relationship:
P = 67.214 G-0.'047

where P is the porosity in percent, and G is the overburden pressure in psi,
which was estimated from the measurements.
- From 800-6000 psi (5.5-41 MPa) he found the linear relationship:
P = 34.86 - 0.00421 G
- From 6000-10000 psi (41-69 MPa) he found the linear relationship:
P = 13.93 - 0.0006935 G

While we accept Hedberg's insistence that pressure, not depth, is the cause
of compaction, we also accept (as Hedberg did) that there is practical value
in porositydepth relationship. Since the depths and pressures listed by Hed-
berg (1936, p. 254, table 1) are very closely related by G = 0.70 z1.04 (where
z is the depth in feet) we may substitute this into his formulae above, obtain-
ing :
p = 69.82 z-o.1086
P = 34.86 - 2.93 X z1-04
P = 13.93 - 4.82 X z1-04