Weathering

1. The destructive effect of the atmosphere on the lithosphere is called weathering. The disintegration of rocks is effected through the action of agencies such as wind, rain, temperature changes, plants and bacteria.

2. Atmosphere is the most important factor in weathering, the atmospheric gases and temperature changes being the most significant agents. Additional help in the weathering processes comes from water and chemicals dissolved in it, and from light and organisms. The various agents may work together or may be combined in carrying on weathering.

3. There are two general groups of weathering processes – one mechanical, and the other chemical.

4. In mechanical weathering the rock breaks up or crumbles with little or no change in the composition of the material. It is essentially a physical process of disintegration.

5. In chemical weathering the composition of rock or mineral matter is altered during its breaking up. It is essentially a process of decay or decomposition.

6. The presence of micro-organisms in the upper layers of the earth crust increases the intensity of chemical reactions. Organisms alter rocks both mechanically and chemically.

7. It is often very difficult to draw a line between different types of weathering as in nature they usually act simultaneously.

Where to look for petroleum

1. The earth’s surface is always undergoing movements. They are shown by the displacements in the earth’s crust by the uplift of the land and its sinking in other places. All movements of parts of the solid earth are usually referred to as diastrophism.

2. Near the surface, at atmospheric temperatures and pressures, rocks tend to break when subjected to great stresses such as earthquakes. However, deeper into the crust at higher temperatures and pressures the rocks become flexible. Instead of breaking, they tend to warp or fold.

3. If stress is applied gradually, rock layer bends without breaking. So, a bend of a rock layer or a wavelike structure is a fold. The upfolded structures are called anticlines. The downfolded structures are synclines.

4. When rocks near the surface break, or fracture they are classified as joints and faults. A joint is a fracture that has opened without displacement of adjacent walls. Otherwise, the fracture is called a fault.

5. The study of geologic structures has been of prime importance in petroleum geology, because folded and faulted rock strata commonly form structural traps for the accumulation and concentration of fluids such as oil and natural gas.

6. But also, oil can be found in stratigraphic traps caused by unconformities. An unconformity is a surface of erosion or nondeposition that separates younger strata from older rocks. Geologists distinguish among three types of unconformities: the nonconformity, the disconformity, and the angular unconformity.