Petroleum Occurrence
1. The occurrence of petroleum is not evenly distributed. Some deposits are at the surface of the earth
and others are buried at varying depth. Almost all of the world’s commercial supply of oil and gas is produced from underground deposits. The substance occurrences are exploited only as the result of drilling.
2. Petroleum Deposits. The petroleum deposits of the world may be classified as follows:
a. Occurrence Based on Location.
(1) Surface Occurrence.
b . Occurrence Based on Geographic Location.
c. Occurrence Based on Geographic Age.
3. Occurrence Based on Location. On this basis the main division is into surface and subsurface occurrences. A deposit of either type may be of small magnitude or may constituting a commercial deposit.
a. Surface Occurrence. Surface observations are still employed whenever possible. They are used extensively in the early prospecting of new territory. Surface observation include:
(1) Nothing direct indications of the presence of the underlying hydrocarbons.
(3) Utilization of maps and air photographs.
Petroleum occurs at the surface of the earth in a variety of ways. These are:
(1) Seepages and exudations of bitumen.
(2) Those associated with mud volcanoes , mud flows and springs.
(3) Dikes and vein filling of solid bitumen, bitumen – impregnated sediments drying up deposits etc.
(4) Cavity fillings.
4. Petroleum weather gas, oil or liquid asphalt may reach the surface along fractures, joints, fault planes
unconformities or any of the connected porous opening of the rocks.
5. Oil and Gas Seeps. Oil seeps are the most favourable type of any surface indication and they are also easier to find than other type. Oil may be recognized by its appearance, odour and inflammability.
Petroleum seepage are common in the sedimentary regions of the world and many pools and producing regions have been discovered by drilling seepage. Surface seepage, either oil or gas are frequently associated with water springs. The oil floats to the surface of the water and the gas bubbles out and
The gas may pass unnoticed if the gas seeps occur where there is no water. Gas seeps may be detected by:
a. Odour due to the presence of sulfur compounds.
b. Yellow colour in the rock surrounding the orifice.
c. A whistling noise when the gas emerges under pressure.
d. Eternal fire when the gas has become ignited by natural or artificial means.
e. Exploding bubbles when the seep is covered by water or mud.
Massive seepage of oil and gas under the oceans occurred at a number of places on the earth
.
Example : Surface deposits of Southern California, Venezuela, Trinidad and Baker of former USSR, ocean seepage of Santa Barbara of California.
6. Mud Volcanoes. High pressure gas water seepage through the mud volcano’s carry mud, methane and other hydrocarbon gas, sand, fragments of rock and occasionally oil. The chief significant of mud volcano’s to petroleum geologist is that they generally indicate the presence of gas. Mud volcanoes occur chiefly in areas of Cenozoic rocks that have been strongly deformed.
7. Occurrence of solid Hydrocarbon. The solid hydrocarbons occur chiefly in veins and asphalt occur in the pores or porous rocks such as sand stones. The spaces in which the solid hydrocarbons occur were either open up by pr of the solid hydrocarbons or were produced by tectonic forces.
In the solid vein and dike fillings the loss of the gaseous and liquid fraction probably occurred while the petroleum was filling the openings. The solid hydrocarbons found in veins altered to stiff and viscous.
Solid petroleum may be regarded as fossil or dead seepages from which gaseous and liquid fractions have been removed, leaving only the solid residues behind.
8. Oil Shale. Oil Shale consists of varying mixtures of organic matter with shale and clay. Most oil shale contains free petroleum which is recoverable by ordinary oil solvents. The organic matter in oil shale in the form of miner able is called karogen shale. Yields of most commercial grades kerogen are on the order of 25-50 gallons of oil per short ton of shale have been encountered . Kerogen varies in chemical composition by weight percentage within the following ranges: Composition of kerogen is as follows :
Carbon 69-8% Hydrogen 7-11 % Nitrogen 1.25-2.5 % Sulfur 1-8 % Oxygen 9-17 % 9
9. Subsurface Occurrence. Subsurface petroleum occurrences may be broadly divided into two categories accordingly to their size as:
a. Minor or Noncommercial petroleum deposit.
Often furnish clues that lead to the discovery of commercial deposit.
b. Commercial petroleum deposits are described by the terms pools, fields, provinces, sub provinces etc.
10. Pool. A pool may be a few acres or it may extend over to many square miles. Its content may be entirely gas or it may be entirely or mainly oil. If a pool has capacity of producing 50 million barrels or more oil it is
11. Field. When a group of pools are related to a single geo logic feature, either structural or stratigraphic, it is termed as a field. The individual pools comprised in a field may occur at various depths. Geologic features that are likely to from fields are salt plugs, anticlines and complex combinations of faulting, folding and stratigraphic variation.
12. Provinces. A petroleum province is a region in
which a number of oil and gas pools and fields occur in a similar or related geologic environment. Sub provinces may occur within provinces.
12. Occurrence Based on Geographic Location.
Petroleum deposit are unevenly distributed through out the world. The two out standing areas or regions are:
a. Middle East Region. Includes the provinces in Iran, Iraq, Kuwait, Saudi Arabia and some part of former USSR.
b. Gulf of Mexico - Caribbean Region. Includes the Gulf Coast provinces of the US and the provinces in Mexico Colombia, Venezuela and Trinidad.
Other important production regions in North America are Illinois, California, province of Comedy Tapioca and province of Mexico.
Europe – Poland, North Germany, Rumania, North sea. For
East – Java, Borne a, Sumatra, New Guinea, Myanmar, Japanese
island etc. Costal areas – Coast of southern of California, shores of Louisiana and Texas in the Gulf of Mexico, in the North sea and in the Persian Gulf.
Region Reserve (%) North America 12.6 South America 7.0 Europe 0.6 Middle East 62.5 Africa 5.0 Asia-pacific 3.5 USSR-China 9.0 Total :100.00 13
13. Geologic Age of Reservoir Rock. Geologic age of reservoir rock is a usefully means of classifying a group of pools and field or a province or sub-province . Two parallel classification are used:
a. Time necessary to from the rock (geo logic time unit)
b. The rock unit of the earth (time
