Page 278 - Origin and Prediction of Abnormal Formation Pressures

250 H.H. RIEKE, G.V. CHILINGAR AND J.O. ROBERTSON JR.

TABLE 10-4
Major ionic concentrations in g/1 of seawater and Cretaceous oilfield formation waters [Quantou (Kq),
Denglouku (K~), and Shahezi (Ksh) formations] in the Shiwu Fault Depression

Pressure system Na + + K + Ca 2+ Mg 2+ HCO 3 CI- SO 2- g/l** n
Normal pressure 0.9407 0.0540 0.0073 0.3965 1.1202 0.2928 2.85 29
(Kzq2 - Kzq4)
Normal pressure 0.9545 0.0660 0.0097 0.5002 1.0564 0.3696 2.98 36
(Kid3 -- K2ql)
Subnormal pressure 1.3417 0.4480 0.0219 0.6466 2.1837 0.4603 5.13 19
(Klsh -- Kid2)
Seawater * 10.88 0.400 1.350 0.142 19.00 2.7000 34.50
n = the number of water samples analyzed. * Data from Snoeyink and Jenkins (1980). Modified after He et
al. (2000, table 1, p. 151). ** Total dissolved solids.

is no pressure transition zone between the normal and underpressure sections. The
low-pressure zone's upper boundary (seal) does not follow any particular stratigraphic
horizon, and the seal was possibly created during diagenesis (He et al., 2000). Table 10-4
shows that the total dissolved solids of the oilfield waters from both zones are much
lower than seawater. However, all the major ionic concentrations in the subnormal
pressure zone are higher than those in the two normal pressure zones. Sulin's water
type classification shows that CaC12 occurs only in the underpressure systems, whereas
NazSO4 exists only in the normal-pressure systems.
Why is the subnormal pressure zone's chemistry data relationship similar to the
expected chemistry of pore waters in the overpressured zones? The basin's burial and
thermal history was analyzed using a two-dimensional mathematical model to simulate
the evolution of the abnormal pressure history. Results from the computer simulation
indicate that the subnormal pressure zone evolved from a high abnormal pressure zone.
He et al. (2000) interpretation of the results indicated that the present-day underpres-
sured zone was overpressured in the Early Cretaceous when the basin experienced high
depositional rates. Since the end of the Cretaceous, tectonic uplift and erosional cooling
eliminated the overpressures, and a reduction in the geothermal gradient occurred cre-
ating a decrease in the formation temperatures. It was revealed in this investigation that
the chemistry of pore waters in the subnormal-pressure zones can reflect the chemistry
of previous high-pressure zones, and that the evolution of the structural geology can be
important in modifying or preserving pore-fluid chemistry.

South Caspian Basin
The retardation of the compaction processes in the South Caspian Basin is dis-
tinguished by the following environmental factors that have created and maintained
high-pore pressures in the basin's thick sedimentary deposits (Buryakovsky, 1993a,b,
1993c). The pressure environment is characterized by: (1) high sedimentation rate up to
1.3 km/m.y.; (2) thick sequence of Quaternary-Pliocene age sediments that contains up
to 10 km of sand-silt-shale out of a total of 25 km; (3) low heat flow and low formation
temperatures (105-110~ at a depth of 6 km); (4) wide development of mud volcanism;