Exploring of Oil and Gas

Earth scientists in the petroleum industry – including geologists, geophysicists, geochemists and paleontologists - study what has happened to rocks that may be buried thousands of meters below surface, how those rocks were formed and affected by events stretching back millions of years, and how to identify traps where oil and gas accumulated within rock formations. An explorer may have a well-developed theory or intuition why an area should contain oil and gas. A first-hand look at outcrop geology and surface features sometimes helps to confirm the basic requirements - that there may be sedimentary rocks, potential reservoirs and hydrocarbon-bearing source rocks in a sedimentary basin. Within a basin, the explorer’s first step is to examine all the information already known about the area. This might include academic papers, surface geology observations, any wells drilled, data from relevant agencies or departments and previous exploration results from nearby or similar areas. Geophysicists can identify the structure, configuration, thickness and depth of new sedimentary basins by measuring slight variations in the Earth’s gravitational and magnetic fields and by measuring the time taken for seismic energy waves to pass through and be reflected from sedimentary layers.

In a typical trap, gas accumulates on the top of the reservoir as a “gas cap” over the oil, which in turn overlies the water-saturated zone in the reservoir. This occurs because natural gas is lighter than oil which is lighter than water. However, all three fluids are often intermingled in parts of the reservoir. Porosity is the ability of rock to hold oil and gas like water in a sponge. A trap requires three elements:

• A porous reservoir rock to accumulate the oil and gas- typically sandstones, limestones and dolomites

• An overlaying impermeable rock to prevent oil and gas from escaping

• A source for the oil and gas, typically black waxy shales.

There are 6 common oil and gas traps:

1) thrust fault

2) normal fault

3) stratigraphic pinch-out

4) reef

5) anticlines

6) salt dome

If it is impossible to obtain the geophysical data from regulatory bodies, the seismic survey is required. In a seismic survey it is necessary to lay out a line or several lines of sensitive receivers, called geophones or jugs, on the ground. Then explosions or mechanical vibrations are created on the surface. The geophones record the energy reflected back as seismic waves from rock layers at various depths. Geophysicists and geologists examine the seismic data for the presence of suitable traps and for similarities with other petroleum-producing areas. If the results seem promising, they use the seismic data to pinpoint where to drill a well.

2. Answer the following questions:

1. What do geologists, geophysicists, geochemists and paleontologists study?

2. What are the basic requirements for presence of oil and gas?

3. What is necessary to know about a basin?

4. How can geophysicists identify the structure, configuration, thickness and

depth of new sedimentary basins?

5. Which form does natural gas accumulate in?

6. What is lighter: oil, water or gas?