Petroleum originates from a small fraction of the organic matter deposited in sedimentary environments. Out of the average “source rock”, 99% is mineral matter and 1% organic matter. Of this organic matter, 90% is kerogen and 10% bitumens. This organic matter is usually a combination of marine- and terrestrially-derived organic (plant) and zooplankton (animal), which constitutes more than 95% of the life in the oceans.
Studies of present depositional environments show that the optimum requirements for accumulation and preservation of organic matter include:
A large supply of organic matter.
An intermediate rate of sedimentation of fine-grained material.
An oxygen-poor environment to reduce oxidation and aerobic microbial degradation of dead organic matter.
Continental margins have all the requirements for organic accumulation. Deltaic deposits have a rapid sedimentation rate, but it is also an aerobic environment causing degradation of the organic matter. The “benthonic zone” of the continental margin has an anaerobic environment, but little organic matter or sedimentation. Therefore, within the continental margins, only certain areas (lagoons, estuaries, deep basins within the margin) have both organic contributions, sedimentation and a reasonable anaerobic environment.
Kerogen is a general term describing any insoluble organic matter in sedimentary rocks. It is insoluble in water, non-oxidizing acids and organic solvents. It is best described as a heterogeneous, highly polymerized organic material, with a large amount of inter-bound aromatic rings. Kerogen has a variety of different side-chains emanating from the general aromatic nucleus. Kerogen is dispersed within sediments and is intimately associated (absorbed) into the mineral/rock.
All kerogen types experience chemical alteration during maturation. This results in the formation of petroleum, and generally begins with the loss of oxygen, followed by hydrogen, to arrive at a form of hydrocarbon.
STAGES OF PETROLEUM MATURATION:
Once an organism dies, the process of petroleum generation begins. The majority of organic matter is totally oxygenated and degrades to carbon dioxide, water and small amounts of mineral matter. The organic matter that is deposited in a “reducing” environment suffers only minor oxidation. The process of petroleum generation is divided into three stages; diagenesis, catagenesis and metagenesis. These divisions are artificial, since the process is a continuous one, without clear-cut boundaries.
The first stage in the transformation of freshly deposited organic matter into petroleum is called diagenesis. This process begins at the sedimentary interface and extends to varying depths, but usually no deeper than a few hundredm meters. In cases where the geothermal gradient is extremely low, diagenesis may extend to a depth of 2,000 meters.
During this time compaction of the sediments occurs, the temperatures and pressures are low, causing transformation to begin under mild conditions. During diagenesis, organic matter loses a great deal of oxygen in the form of H2O and CO2. The only significant hydrocarbon formed during diagenesis is microbial methane. Diagenesis causes a decreasing O/C, with only a slight decrease in H/C.
As burial continues, the kerogen formed during diagenesis is exposed to increasing temperatures and pressures. Catagenesis is the stage of thermal degradation of kerogen that forms oil and gas. This stage typically occurs between the depth of several hundred to several thousand meters.
The metagenesis stage is reached at great depths, or in areas of high geothermal gradients at shallower depths. Metagenesis usually begins at depths of approximately 4,000 meters. At this stage, kerogen has very little hydrogen remaining and is forming methane as its only hydrocarbon product. Towards the end of metagenesis, virtually no hydrocarbons are being generated from the kerogen.
THE OIL WINDOW:
The formation of oil from kerogen depends on the amount and type of source material present in the sediments and the thermal history of the kerogen. It has been conclusively determined that temperature is the most important factor affecting the generation of oil and gas from organic matter. During the generation of petroleum from kerogen, both temperature and time play key roles.
Kerogen, exposed to relatively high temperatures for a short period of time, will mature to about the same extent as kerogen which is exposed to relatively low temperatures for a longer period of time. Thus, the time and temperature history of a kerogen determines occurrence and depth at which kerogen generates oil or gas or both.
The depth range over which oil generation occurs is known as “the oil window“. This oil window is usually different for most sedimentary basins. It may cover several thousand meters or may be confined to less than a thousand meters. The depth range for the oil window is mainly a function of the past geothermal gradient and is similar to the present geothermal gradient if the subsidence history is continuous. If this assumption is correct, then the oil window can be estimated from the current downhole temperatures. However, if the tectonic history of the basin has been active, then such assumptions can lead to gross errors. The determination of the oil window is best performed by geochemical means using T(max), bitumen extraction, gas chromatography and optical methods such as vitrinite reflectance.