Hydrocarbon:Source rock and

Origin

Both inorganic and organic theories have been suggested to explain theorigin of oil and natural gas. Although some gases (especially CH4) andindividual components in petroleum may originate by inorganic processes,the weight of evidence strongly supports an origin in organicmatter produced at and near the earth’s surface.

Inorganic hypotheses

Cosmic origins?—Consolidation of H and C during consolidation of theEarth? Carbonaceous chondrites and space dust contain hydrocarbons—evidence of primary organic source?

Petroleum should be more widespread in space and time if there was acosmic source.

Reactions of metal carbides within the Earth?—

FeC2 + 2H2O = C2H2 [acetylene] + Fe(OH)2Al4C3 + 12H2O = 3CH4 + 4Al(OH)3

Fischer-Tropsch reaction:CO2 + H2 = CO + H2O, then CO + 3H2 = CH4 + H2O

There is no evidence that metal carbides exist in the mantle.

• Hydrocarbons in igneous rocks as evidence?—Hydrocarbons, including bitumens, can befound in igneous rocks:

• 1. In vesicles and inclusions in alkaline igneous rocks (e.g. Arendal, Norway). Origin iscontroversial.

• 2. In thermal aureoles around basic intrusions in sediments. Can be explained bydistillation of kerogen in surrounding sediments due to heat of the intrusion—petroleummay be incorporated in the igneous rocks as they cool.

• 3. In weathered and fractured igneous rocks. Normally explained by hydrocarbon migrationinto the rocks from a sedimentary (organic) source.

• Mantle degassing?—Polymerization of inorganic gases such as CH4 that are produced inthe mantle. It is difficult to produce the range of complex hydrocarbons by polymerization;also problems of permeability and porosity in lower crust.

• Main problems with inorganic theories of petroleum genesis:

• Poor correlation between petroleum and volcanism

• Paucity of Precambrian oil

• Isotopic evidence favours organic origin

• Petroleum is “optically active” – linked to organic origin

• Presence of homologous series

• Geological association with sedimentary basins

• GENERAL MODEL FOR ORIGIN AND MATURATION OF PETROLEUM

GENERAL MODEL FOR ORIGIN AND MATURATION OF PETROLEUM

• Most organic matter [C] fixed by photosynthesis in upper 100–150m is recycled in the water column by passing through the foodchain.

• Phytoplankton (diatoms, algae: primary producers of OM) areoxidised or eaten by zooplankton. Both types of plankton are thenconsumed by other higher organisms. They defecate, producingpellets that contain the indigestible part of the organic matter.

• The pellets sink relatively quickly to the bottom, whereas planktonare commonly degraded in the water column.

• The organic matter that arrives on the ocean (or lake) floor can thenbe consumed by benthic organisms.

• Only a few percent of the organic matter produced is buried insediments, especially in the deepest parts of the oceans.

• High organic productivity in the oceans depends mainly onadequate sunlight and availability of nutrients.

• In surface waters, sunlight generally is not a limiting factorexcept seasonally (winter) at high latitudes.

• Nutrients (mainly N and P) have a very heterogeneousdistribution in marine waters. The highest concentrations arecommonly found in coastal regions, where they are land-derived (e.g., soil erosion with leaching to rivers), and in zonesof upwelling. Upwellings are present mainly on the westernmargins of the continents

Propane

Isooctane

• ~100 mg of crushed rock placed into the Rock-Evalinstrument.

• first heated to 300°C for 3 minutes in inert atmosphere(helium).

• "free hydrocarbons" are thermally desorbed from thesample.

• The abundance measured using a flame ionisation detector(FID) and recorded as the S1 peak in units of mg HC.g rock-1.

• The sample is then pyrolysed by increasing the temperatureof the furnace from 300°C to 550 or 600°C at a rate of25°C.min-1

• The amount of hydrocarbons (pyrolysate) generated duringthis period is again measured using a FID and reported as theS2 peak, in units of mg HC.g rock-1 (equivalent to kgHC.tonne rock-1).

• The temperature at which the maximum rate of pyrolysategeneration occurs is referred to as the Tmax, measured in °C.

238U Spontaneous Fission

Arrhenius equation

Combined isoreflectance andstructure map, Woodford Shale,Vitrinite reflectance values ingeneral increase with depth ofburial,

but strong cross-cuttingrelationships of the isoreflectanceand structure contours

suggest that there may havebeen local thermal disturbancessuperimposed on the burial-relatedmaturation.

Mineralogical indices. Diagenesis of clay-richsediments as a function of depth andtemperature

Plot of temperature versus diagenetic illitecontent measured by X-ray diffraction.Increasing illite content severely reducespermeability. The main thermal interval forillite formation is 120–150 °C.

Plot of temperature versus quartz cementcontent and porosity

Timing of quartz cementation derived fromthe combination of temperatures ofcementation from fluid inclusion analysis(green bars) with paleotemperature fromburial history (red line), North Sea.

The temperature range of quartzcementation is based on fluid inclusionanalysis of authigenic quartz cements. Quartzcementation took place 35–10 Myr ago.

Maximum depths of burial can be estimated fromVR values from empirical relations

(Barker & Pawlewicz 1986):

ln (Ro) = 0.0096.T −1.4

Saturated hydrocarbons: each carbonatom is completely saturated withrespect to hydrogen

Aromatics: unsaturated hydrocarbonswith cyclic structures

NSO compounds/ heterocompounds:contain atoms other than carbon andhydrogen (N, S; O). Resins and theasphaltenes.

Gross composition of 636 crude oils in terms of the three main groupsof compounds found in petroleum. Normal (non-degraded) crudestypically contain 60–80% saturates, and less than 20% NSO compounds.

Typical distributions of n-alkanes, cycloalkanes and aromatics

~<35-500oC

Bombay High

Bermer

Assam Bombay highCambay shales