Answer the following questions. Please stick to the questions AND DON\'T PROVIDE

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Answer the following questions. Please stick to the questions AND DON'T PROVIDE INFORMATION THAT IS NOT RELATED TO THE QUESTIONS.

(1) Discuss all structures caused by the deformation of rocks. And how these structures influence the migration and storage of hydrocarbons in the subsurface?

(2)Discuss the formation of source rocks, th factors that control their maturation and hydrocarbon generation, and the main types of hydrocarbon migration?

(3) Discuss seismic exploration and how it used as a tool to help select sites for drilling exploration wells?

Explanation / Answer

1. Variety of structures form by the deformation of rocks, some are extensional, some constructional, or most combination of both. Petroleum traps are many a times can be seen in such structural traps. Here I am discussing only those structures important in hydrocarbon study.

a)Anticlines are one of the most common deformational feature and is also the most common oil trap. Major oil traps all around the world are located in anticlines. When beds are water saturated, oil being lighter than water, migrated upward along the flanks of anticline and collect beneath an impermeable trap.

b)Oil can also accumulate in synclines. Mainly when beds are water free, water moves downward due to gravity and collects along the troughs of syncline.

c) Monoclines and terraced also hold oil.

d) Oil is also found in homoclinal structures, where beds uniformly dip in one direction. Oil migrates upward along the porous and permeable, diping beds and gets collected when the upward continuation of beds are terminated by a seal rock along fault or a dike or unconformities.

e) Faulted structure also give rise to many important oil pools. Due to faulting, when an impervious bed comes against the updip continuation of porous reservoir rock, effective sealing happens, permitting oil accumulation.

(2) Answer:

Now it is universally believed that oil and gas are of organic origin. organic materials buried in marine mud, clays, have undergone changes to form hydrocarbons, they have then migrated to form oil pools, within the reservoir rock, capped by impermeable rocks.

Source materials: while crude oil is largely derived of animal proteins, natural gas have formed from plants. The lower lifeforms like algae, corals most abundant in muds, are major contributor. Ideal place of accumulation for oil and gas must be a reducing environment, with no prolonged exposure to oxidation and scavangers. Such conditions are fulfilled for silled basins and troughs, swampy areas, marshy areas, isolated lagoons. The conversion of organic matter into petroleum must involve removal of oxygen rich substances and preservation of lipids and hydrogen rich residuals of organisms. Activity of anaerobic organism plays a very significant role, additional factors being shearing stress during compaction, heat, addition of hydrogen from deep seated source, radioactive disintegration, presence of catalyst. Overall chemical change that take place involves complete oxidation of a part of OM to CO2 and complete reduction a part of OM to CH2. Formation of petroleum decreases when depth of burial exceeds 3000 m., and no petroleum forms at 5000 m. depth, under average geothermal gradient of 35 degree centigrade per km.

Hydrocarbon migration takes place in two ways – primary and secondary. Primary migration involves migration from source rock to reservoir and secondary migration involves migration within reservoir.

(3) Answer:

Seismic survey is the most important geophysical tehnique in hydrocarbon exploration. Exploration seismology is divided into two parts-reflection and refraction.

Reflection survey includes the processes-Data acquisition, Data processing and Data interpretation. Seismic reflection profile can be carried out both in land and offshore.

Seismic sources are-Explosives, Weight dropping, Dinoseis, Vibroseis, Land air gun. Seismic energy arriving on the surface of the earth is detected by geophone. Reflection coefficient is the parameter of quantification of the strength of a reflection generated at the boundary. This reflection coefficient is dependent on another parameter called acoustic impedance, which is formulated as the product of density and velocity of the particular rock layer. The reflection boundary is appeared as trough when the impedance of lower layer is greater than the upper one, whereas peak is appeared in opposite condition.

Another important parameter in seismic data is amplitude. Amplitude can be extracted by various algorithms from seismic data e.g. AAA, Coherency etc.

Anything apart from the signals is a noise which interfere with the observation and measurement of signals. Noise Attenuation is thus very important. There are different types of noise including---- ground roll, guided wave, scattering noise, cable noise, air wave, power line noise, random noises which can be attenuated by different type of filters, such as ground roll attenuation is done by F-K filter.

The study of Seismic stratigraphy can also be done from the seismic data. Seismic stratigraphy is the geologic approach to interpret stratigraphy from analysis of seismic reflection patterns.

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