3. overburden rocks & source maturation
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INDONESIAN PETROLEUM ASSOCIATION (IPA)
REGULAR COURSE, SOLO – CENTRAL JAVA, 4-8 JUNE 2012
3.
Overburden Rocks &
Source Maturation
by: Awang Harun Satyana
mature source/kitchen
OVERBURDEN ROCKS
Overburden Rocks
Overburden rock, an essential element of the petroleum system, is that
series of mostly sedimentary rock that overlies the source rock. it is usually
the largest part of the basin fill.
Generation of hydrocarbons from thermal degradation of organic matter in
the source rock is determined by thickness of the overburden rock in
conjunction with the physical properties and processes that determine
temperature in sedimentary basins.
Thickness of the overburden rock is a by-product of the fundamental forces
and processes that control the structural development of the sedimentary
basin in which the overburden rock is found.
Source rock temperature is largely determined by thickness and thermal
conductivity of the overburden rock, heat flow, and ground surface
temperature.
Overburden Rocks
Overburden rocks = burial sediments/rocks
Because of burial, a source rock generates petroleum, a reservoir rock
experiences a loss of porosity through compaction, a seal rock becomes a
better barrier to petroleum migration, and if oil and gas are kept in a trap at
an optimum temperature, biodegradation is prevented.
The main zone of oil generation occurs between 100° and 150°C (Quigley et
al., 1987). For these temperatures to be reached, a source rock must be
buried by overburden rock through the process of sedimentation. The extent,
depth, and timing of hydrocarbon generation from the source rock thus
depend on the sedimentation rate and the geothermal gradient.
For a typical geothermal gradient of 25°C/km, most oil generation takes
place at depths of about 3--6 km. However, there is a tremendous range of
natural variability associated with both sedimentation rates and geothermal
gradients in sedimentary basins.
Angevine et al. (1990)
tectonic subsidence histories for basins from different tectonic settings
will affect the thickness of overburden rocks
Burial History Plot and Generation of Petroleum
Magoon and Dow (1994)
Factors Determining Temperature in Sedimentary Basin Fill
Deming (1994)
Maturation of Organic Matters
Following its incorporation into sediments, the composition of organic
matters change radically at both bulk and molecular level. These changes
are resulted from increased burial and are in response to the combined
effects of microbial activity, temperature, and time. Together, these
processes result in an increase in the maturity of kerogen (and
subsequently, petroleum).
Optical maturity (VR-vitrinite reflectance and SCI-spore coloration index) : a
wide temperature range from about 30 to > 250C; sediment sample
Molecular maturity : from about 70 to 180 C; oil and sediment sample
AFTA (apatite fission track analysis) : up to maximum of about 115 C;
inorganic components in sediments
TTI (time temperature index) : maturity level obtained by rocks exposed into
a range of temperatures during a length of time.
• Optical maturity parameter
– darkening of spores and pollen
– increase of the reflectivity of vitrinite particles
• Molecular maturity parameter
– loss of oxygen containing functional groups followed
by isomerisation at chiral centres
– increase of aromacity in cyclic compounds
Effects of Maturity on Organic Matters
Effects of Maturity on Organic Matters
The major changes to organic matter that occur with increasing
maturity include three stages of evolution : diagenesis, catagenesis,
metagenesis.
Diagenesis : convert organic debris derived from living organisms into
kerogen, temperature < 100 C, mediated mostly by bacteria
Catagenesis : Thermally degrade kerogen into petroleum,
• temperature 100-150 C breakdown of labile kerogen to oil
• temperature 150-230 C breakdown of both oil (to gas) and of
refractory kerogen to gas
Metagenesis : generation from kerogen is complete, internal change of
the residual kerogen to graphite, temperature > 230 C
Peters and Cassa (1994)
Mechanism of Petroleum Generation and Destruction
Tissot and Welte (1984)
Brooks et al. (1987)
Mahmud et al. (2006)
Selley (1985)
Merrill (1991)
Hunt (1996)
Spore Coloration Index (SCI)
Clayton and Fleet (1991)
Clayton and Fleet (1991)
Selley (1985)
Selley (1985)