INDONESIAN PETROLEUM ASSOCIATION (IPA) REGULAR COURSE, SHERATON LAMPUNG, 26-30 AUGUST 2013

3. Overburden Rocks & Source Maturation

by: Awang Harun Satyana

OVERBURDEN ROCKS

mature source/kitchen

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.

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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.

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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.

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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

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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.

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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.

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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.

tectonic subsidence histories for basins from different tectonic settings will affect the thickness of overburden rocks

Angevine et al. (1990)

Magoon and Dow (1994)

Burial History Plot and Generation of Petroleum

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).

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Optical maturity (VR-vitrinite reflectance and SCI-spore coloration index) : a wide temperature range from about 30 to > 250C; sediment sample

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Molecular maturity : from about 70 to 180 C; oil and sediment sample

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AFTA (apatite fission track analysis) : up to maximum of about 115 C; inorganic components in sediments

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TTI (time temperature index) : maturity level obtained by rocks exposed into a range of temperatures during a length of time.

Effects of Maturity on Organic Matters • 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 

The major changes to organic matter that occur with increasing maturity include three stages of evolution : diagenesis, catagenesis, metagenesis.

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Diagenesis : convert organic debris derived from living organisms into kerogen, temperature < 100 C, mediated mostly by bacteria

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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

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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)

Profile of VR vs. Depth

Dembicki Jr. (2009)

Hydrocarbon generation depends on the kerogen Type In the source rock as well as its time and temperature history.

Dembicki Jr. (2009)