Friday, December 16, 2011
The factors of concern to study Petroleum System is the source rocks (source rocks),maturation (maturation), reservoir, migration, timing, trap (trap), rock sealing (sealingrock) and fracture gradient.
Source rocks are sedimentary deposits that contain organic materials that canmenghasilan oil and gas when the sediment is buried and heated.
Organic materials contained in the deposition of sediment to be known as kerogen (in Greek, means producing wax).
There are four types of kerogen:
Type I: organic materials Type I kerogen is the algae from pegendapan lacustrineenvironment and lagoon.Tipe I can mengkasilkan light oil (light oil) with kuallitas a good and capable of producing gas.
Type II: a mixture of plant materials and marine microorganisms. This type is the main ingredient of petroleum and gas.
Type III: Crop land in the sediment containing coal. This type is generally produce lessgas and oil.
Type IV: plant materials are oxidized. This type can not produce oil and gas.
The content of the source rock kerogen known as TOC (Total Organic Carbon), where the minimum standards for the 'economy' must be greater than 0.5%.
An important implication of the knowledge type of kerogen is a prospect we canpredict the type of hydrocarbons that may be generated (oil, gas, oil & gas or no gas).
Maturation is the process of change in biology, physics, and chemistry of the kerogeninto oil and gas.
Maturation process started since the deposition of sediment rich in organic materialdeposited. At this stage, the reaction occurs at low temperatures involving anaerobicbacteria that reduce oxygen, nitrogen and sulfur to produce concentrations ofhydrocarbons.
This process continues until the temperature reaches 50 degrees Celsius rocks.Furthermore, the effect of increasing the temperature to be very influential in line withthe reaction rate of organic materials kerogen.
Because the temperature continues to mengingkat in line with increasing depth, the heating effect naturally determined by how deeply buried source rocks (geothermalgradient).
Figure below shows the relative proportions of oil and gas for type II kerogen, which isburied in an area with a geothermal gradient of about 35 ° C km -1.
Seen that petroleum can be generated significantly above temperatures of 50 ° C orat a depth of about 1200m and then stopped at a temperature of 180 degrees or at a depth of 5200m. While the gas formed significantly in line with increasing temperature/ depth.
Gas produced due to factors referred to termogenic gas temperature, whereas thatproduced by bacterial activity (low temperature, shallow depth <600m) calledbiogenic gas. The figure below is an example of cross-sectional depth of the layers of rock sources,as well as temperature prediction curve by using the example above. From this cross section can be predicted whether the source is within the oil window, window gas,etc.. This method is known as the method of Lopatin (1971). Clearly, Lopatin methodbased only on temperature and ignore the effects of chemical and biologicalreactions.
Is the rock that can store and drain hydrocarbons. In other words, the rock should haveporosity and permeability.
This type of reservoir is generally sandstone and carbonate rocks with a porosity of15-30% (both primary and secondary porosity) and a minimum permeability around 1mD (milli Darcy) for gas and 10 mD for light oil (light oil).
Here are examples of the following reservoir porosity, permeability, etc.. (click toenlarge):
Migration is the process of oil and gas transportation from source to reservoir rocks.The process of migration begins with the migration of primary (primary migration), ie transport from source rock to reservoir directly. Then followed by secondary migration (secondary migration), ie, its migration within the reservoir rock itself (from reservoir to reservoir section in the shallow section).
The basic principle of the identification of hydrocarbon migration pathways is to create a map of the reservoir. The converse of river water in the earth's surface, hydrocarbons will pass through the ridge (the hills) of reservoir morphology. Regional hydrocarbon teraliri called drainage area (watershed analogy on the surface of the earth). If the trap is fully charged (fill to spill) to the spill point, then the hydrocarbons will be spilled (spill) to a more shallow. Here's an example:
Charging time of oil and gas in a trap is very important. Because we want the trap formed before the migration, if not, then the hydrocarbons had already passed before the trap is formed.
There are a variety of hydrocarbon traps: stratigraphic traps (D), the trap structure (AC) and the combination (E).
Seal is rock insulation system that is not permeable like claystone / mudstone, anhydrite and salt.
In the evaluation of prospects, fracture gradient curves needed them to predict the extent of overburden rocks capable of holding oil and gas. The thicker the overburden, the more the volume of hydrocarbon that is able to 'hold'.
Figure below shows the fracture gradient curves of gas, oil and formation water from a field. Based on this curve, if we have a trap with a thickness of overburden (c), then the thickness of the gas column is the maximum that can hold (ca), and the thickness of the oil column is (cb), the remaining hydrocarbons will be seeping out insulation.