312
7 THE SUBSURFACE ENVIRONMENT
Fig. 7.3. Illustration through a basin showing causes of geothermal anomalies. Thin arrows show directions of
water flow. Thick upward pointing arrows indicate positive anomalies ("hot spots"), thick downward pointing
arrows indicate negative anomalies. (A) Negative anomaly due to deep meteoric water penetration. (B) Positive anomaly due to upward discharge of hot water along a permeable fault. (C) Positive anomaly due to discharge of superheated water from overpressured clay formation. (D) Positive anomaly within low conductivity overpressured clay diapir. (E) Positive anomaly above highly conductive salt dome. (F) Positive anomaly
above igneous intrusion; note peripheral convection currents.
by the overlying denser sediments. In this way overpressured clay may generate diapirs that are analogous to salt domes. Water moves to the crestal position of mud diapirs where it is retained in the closed pressure system. Thus heat energy is retained and
domes are positive geothermal anomalies. Where hot water is able to escape from overpressured systems (up faults for example), local positive geothermal anomalies will occur (Fig. 7.3).
An understanding of geothermal gradients and anomalies is useful because the rate
of diagenetic reactions is thermally controlled. As will be shown when discussing porosity gradients in sands (see Section 8.5.3.2), porosity tends to be lost faster in areas of
high geothermal gradient than in areas with low gradients.
7.2 SUBSURFACE PRESSURES
It is axiomatic that pressure increases with depth beneath the earth's crust. According
to Terzaghi's law, the effective overburden pressure is equal to the total vertical pressure exerted by the rock and pore fluid, minus the fluid pressure (Terzaghi, 1936; see
also Skempton, 1970). This relationship is also expressed in the Hubbert and Rubey
(1959) equation, which states that the total, or overburden pressure, is equal to the sum
of the lithostatic and fluid pressures. The lithostatic pressure is the pressure of just the
7 THE SUBSURFACE ENVIRONMENT
Fig. 7.3. Illustration through a basin showing causes of geothermal anomalies. Thin arrows show directions of
water flow. Thick upward pointing arrows indicate positive anomalies ("hot spots"), thick downward pointing
arrows indicate negative anomalies. (A) Negative anomaly due to deep meteoric water penetration. (B) Positive anomaly due to upward discharge of hot water along a permeable fault. (C) Positive anomaly due to discharge of superheated water from overpressured clay formation. (D) Positive anomaly within low conductivity overpressured clay diapir. (E) Positive anomaly above highly conductive salt dome. (F) Positive anomaly
above igneous intrusion; note peripheral convection currents.
by the overlying denser sediments. In this way overpressured clay may generate diapirs that are analogous to salt domes. Water moves to the crestal position of mud diapirs where it is retained in the closed pressure system. Thus heat energy is retained and
domes are positive geothermal anomalies. Where hot water is able to escape from overpressured systems (up faults for example), local positive geothermal anomalies will occur (Fig. 7.3).
An understanding of geothermal gradients and anomalies is useful because the rate
of diagenetic reactions is thermally controlled. As will be shown when discussing porosity gradients in sands (see Section 8.5.3.2), porosity tends to be lost faster in areas of
high geothermal gradient than in areas with low gradients.
7.2 SUBSURFACE PRESSURES
It is axiomatic that pressure increases with depth beneath the earth's crust. According
to Terzaghi's law, the effective overburden pressure is equal to the total vertical pressure exerted by the rock and pore fluid, minus the fluid pressure (Terzaghi, 1936; see
also Skempton, 1970). This relationship is also expressed in the Hubbert and Rubey
(1959) equation, which states that the total, or overburden pressure, is equal to the sum
of the lithostatic and fluid pressures. The lithostatic pressure is the pressure of just the