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)