Mass Properties of Sedimentary Rocks and Gravimetric Effects of ...

GRAVIMETRIC EFFECTS OF PETROLEUM AND NATURAL-GAS RESERVOIRS A5
Foundation fpr financial assistance to permirt contributory
studies in the Po basin of Italy.
ROCK DENSITY IN SITU
A widespread practical technique has not yet been
devised for determining the density of rock in situ
underground without direct or indirect dependence
upon laboratory volume measurements of core samples.
Core-sample analysis provides the calibration
and control data needed for interpretation of gammagamma
and velocity logs in terms of density (Pickell
and Heacock, 1960). Until a praotical borehole gravimeter,
or some other sensitive device capable of producing
detailed and precise density logs, has become
a reality, we will remain dependent on core analysis.
Expressed in terms of parameters partly measurable
in a core sample of rock, the density in situ Cuts) is:
(I)
where
u,a=the density of the rock saturated with :fluid
in its natural condition underground,
u11=the dry-bulk density of the rock, including
its pores,
u1=the average density of the :fluid (or :fluids)
filling the pores under natural conditions,
=the total porosity expressed as a fraction,
and is equal to 1-u 11 ,
O"g
where
ug=the average density of the solid mineral
grains composing the rock. .
Other equivalent expressions for density in situ
obtained by substitution of various of the foregoing
terms in equation 1 are:
and
+
(2)
O"fO"b ( )
O"is=O"b u,-- 3
Consideration of the terms in equations 2 and 3 reveals
that knowledge of the densities of underground
rocks in situ from core analysis depends on evaluation
of the volume compressibilities-under low to moderate
pressures and ·a range of temperatures like those found
in nature-of the crystalline solid and the interstitial
fluid constituents of such rocks, and also depends on
the porosity or dry-bulk volume. The question of
whether core samples provide a satisfactory basis for
evruluating these factors has never been fully and satisfactorily
answered and is therefore examined here.
The first step in such an examination is consideration
of subsurface temperature and pressure gradients.
239-675-67-· -2
Ug
SUBSURFACE TEMPERATURE AND PRESSURE