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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
1Manuscript received October 7, 1997; revised manuscript received October 28, 1998; final acceptance May 30, 1999.
2Moscow State University, Geology Museum, 119899, Moscow, Russia; e-mail: gal@geodyn.pvt.msu.su
3Geosystems Institute, Varshavskoye Shosse 8, 113105, Moscow, Russia; fax (095) 954 54 00.
ABSTRACT
Present-day temperature and vitrinite reflectance (Ro) profiles are
considered key factors for calibration in numerical basin modeling; therefore, it is
important to understand how such profiles are generated. Basin modeling typically
considers factors such as rock consolidation, depth-variation of petrophysical
characteristics, changes in heat flow, and paleoclimate; however, our basin evaluations
have shown that other processes that influence temperature and Ro (vitrinite
reflectance) profiles commonly are inadequately studied. Modeling within the Urengoy field
of the West Siberian basin has allowed numerical estimates of some of these nonstandard
processes. First, a substantial increase in temperature (up to 5-10¡C) is expected from
changes in heat conductivity due to the formation of significant free gas, gas-condensate,
and condensate. Second, climate variations creating numerous periods of formation and
degradation of permafrost zones for the last 3.4 m.y. could decrease the modern rock
temperatures by 10-17¡C in the upper 1.5 km of the sedimentary section and up to 10¡C at
its base. Due to the short duration of these two processes, their effect
on organic matter
maturation is minimal; nevertheless, their roles in the formation of present-day
temperature profiles necessitate their inclusion in basin modeling. Third, modeling
results demonstrated that change in heat conductivity of rocks with dispersed organic
matter can increase temperature of sedimentary rocks by 3-5¡C and rock maturity by not
more than 0.02% (Ro). Finally, the sharp increase in vitrinite reflectance (% Ro)
with depth, typical for the deep sedimentary units of the Urengoy field and many other
continental rift basins, is possible to explain from hydrothermal activity. The influence
of these processes on subsurface temperatures and maturation of organic matter can vary
significantly depending on lithology, organic matter content, and the location of the
modeled section relative to the ancient rift system. The results of this study can help
one estimate the relative influence of these parameters.
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