Page 315 - Origin and Prediction of Abnormal Formation Pressures

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

Yeh (1980) used D/H ratios of OH hydrogen in clay minerals to further investigate
the above three sequences of the buried Gulf Coast shales. The purpose of his
investigation was to show if there is (1) a relationship between the isotope ratios and
particle size, (2) effect of the smectite to illite conversion, (3) dehydration event, and
(4) if there is evidence of isotopic fractionation between residual and expelled pore
waters. Yeh indicated that the diagenetic clays (mixed-layer illite-smectite; diagenetic
illite) appear to be in oxygen and hydrogen isotope equilibrium with the pore waters
(calculated) at the time the most recent diagenesis occurred.
Detrital clays (smectite, illite and kaolinite) have undergone different degrees of
oxygen and hydrogen isotope exchange with the coexisting pore waters, which became
enriched with increasing depth of burial. Pleistocene shales that were buried at 1000 ft
having an in-situ temperature of about 68~ (20~ were not isotopically equilibrated
with their coexisting pore water, and had 3D value of -72%o from the fine shale fraction
(<0.1 txm). This value lies within the range of those of modem Mississippi River
sediments and detrital marine sediments of the Gulf of Mexico fight off the Mississippi
River delta (-50%o to -90%o). In general, Yeh (1980) found that the spread in ~D
values (-7 to -8%o) among the six different size fractions remained roughly constant in
the wells below the range of 7000 to 8700 ft. Inspecting Yeh's average hydrogen isotope
values for each of the wells results shows a spread ranging from -11%o to -21%~ in
the <0.1 Ixm fraction from a depth around 2000 ft to TD. He ascribed the progressive
change in the clay 3D values with burial depth to an enrichment (lower values) of the
pore-water 3D values associated with the clays. The percentage decrease in the smectite
in the mixed-layer illite/smectite per unit interval of depth (or temperature) is a direct
indicator of the amount of water released from the interlayer sites to the interstitial
water. This is attributed to the late-stage dehydration of mixed layer illite/smectite
to illite and is based on Yeh's statement that the percent change in the diagenetic
montmorillonite correlates with a corresponding percent change in ~D and not simply
with the burial depth or temperature.
Poulson et al. (1995) performed O-isotope analyses of water, C-isotope analysis
of dissolved inorganic carbon, and gas analyses (CO2 and CH4) on samples from
normal and overpressured horizons located in the Morganza and Moore-Sams gas
fields producing from the Tuscaloosa Formation, Louisiana, U.S.A. 3~8OsMow values
of the formation waters range from -2.2 to +8.7%~ showing that possible mixing
with a low-salinity, isotopically light water took place. Mixing analysis suggest that
the Tuscaloosa pore waters gave 8~80 ~ +8%o, which is normal for deep, basinal
waters, although the C1- concentrations indicate that there are two water populations
(approximately 35,000 rag/1 and 15,000 to 20,000 mg/1). They reported that there is
no systematic difference between the overpressured and normally pressured samples,
or between samples from the two fields. Evidence for the gases having a thermogenic
origin is based on the values for 3~3CpDB from dissolved inorganic carbon ranging
from -106 to -3.2%~, ~13CpDB values of CH4 ranging from -43.9%o to -40.8%o, and
~13CpDB values of CO2 ranging from -8.8%e to -6.3%o. In conclusion, the writers
believe that a considerable amount of research work still remains to be done in this area.