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Date: 2015-10-07; view: 708.

Unit 2. Geophysical Exploration: Seismic, Stratigraphic and Electrical Methods

C. Discussion

Topics:

1. Geophysical data that can provide petroleum engineers with information concerning the planning of drilling activities;

2. Gravity and magnetic methods.

A third method of exploration is the seismic method. The central physical property upon which seismic prospecting is established is the variation in speed of the transmission of elastic earth waves or sound waves through different geological structures measured by time. There are two principle seismic methods: refraction and reflection. Refraction prospecting consists of elastic earth waves, initiated by some concussive force, traveling down to a dense or high velocity bed, then being carried along that bed until they are rerefracted up to seismic detector locations on the surface some distance from the shot point. What is recorded is the time required for the sound wave to reach each detector location from the shot point. The speed of transmission of the waves through different geological structures is proportional to the density or compactness of the formation. Unconsolidated formations such as sands and shales transmit waves with a low velocity, weak sandstones and limestones with higher speeds, and massive crystalline rocks such as limestones, rock salt, schists, and various igneous rocks with very high speeds. The refraction method aided petroleum explorers in locating salt domes that transmitted elastic earth waves at high rates of speed.

The reflection method of seismic exploration is based on the echo of sound waves off layers of varying density rock, which are reflected at a high angle back to the surface. The Geophysical Research Corporation began experimenting with the seismic reflection method in 1926 and by 1929 had seismic crews employing the method commercially throughout West Texas and the Gulf Coast. In 1931 Petty Geophysical Engineering Company of San Antonio invented and implemented the reverse profile method of reflection shooting that became the standard method of shooting throughout the industry. Now most seismologists, instead of using dynamite to make shock waves, use a machine called a thumper to produce elatic shock waves.

A final method of exploration is the study of stratigraphy. Stratigraphic exploration consists of establishing correlations between wells, matching fossils, strata, rock hardness or softness, and electrical and radioactivity data to determine the origin, composition, distribution, and succession of rock strata. Sample logs, driller's logs, time logs, electrical logs, radioactivity logs, and acoustic logs help geoloists predict where oil bearing strata occur.

Sample logs, compiled from well cuttings and cores, are used to identify key beds and lithologic sequences. A core is a narrow column of rock that is taken from the top to the bottom of a well and shows rock in sequential order as it appears in the ground. Core samples also provide information on porosity, permeability, and saturation of rock in the well. Cuttings are not a continuous record like core samples, but provide a means for identifying sections within larger thick layers through fossil and mineral deposits.

The driller's log provides basic information to the stratigrapher concerning depth, type of rock, density, fluids, and other miscellaneous data. The driller's log keeps track of the time required to drill through various strata and the recognition of key beds he drills through. This data is correlated with other information to enhane te chance of finding oil.

Early electrical methods of exploration in the 1920s tested electrical resistivity and electro-magnetic potential but proved to be more successful at locating metallic ores than oil and gas. Oil and gas have conductivity properties that differ from water, which conducts electricity more readily. Occurrences of oil and gas can be located by this difference in resistance. The most useful application of electric testing has been in the development and impact of well logging. Schlumberger electric well logging is now standard in the industry. These logs record the conductivity of interstitial water in rock, the movement of drilling mud into porous strata, and the movement of formation water into the well bore.

Radioactivity logs, which record both gamma-ray and neutron values, have been in use productively since 1941. Because radioactivity can be measured with precision it can be used to identify different layers within beds. Radioactivity logs give an indication of the type of rocks and fluids contained in those rocks. Acoustic or sonic logs are used to measure the porosity of a formation. This tool measures the speed at which an acoustic or sonic impulse is carried through a specified length of rock. The speed of sound through the rock gives an indication of the porosity and can be helpful in locating reservoirs. Maps, including contour, isopach, cross sections, and three dimensional computer images, also aid the petroleum explorer in locating oil and gas. Contour maps give details of subsurface structural features enabling geologists to visualize three dimensional structures. Contour maps include information about porosity, permeability, and structural arrangements such as faults, pinch-outs, salt domes, and old shorelines. Isopach maps show variations in thickness of a given subsurface formation and are used in calculating the size of reservoirs and secondary recovery operations. A cross section map is a diagram of an imaginary vertical cut along a straight line that reveals subterranean features of a given area much like looking at a road cut. Three dimensional computer maps construct images of subterranean strata as deep as thirty miles. The best way to gain a full understanding of subsurface geology and the potential for natural gas deposits to exist in a given area is to drill an exploratory well.

Geologists also examine the drill cuttings and fluids to gain a better understanding of the geologic features of the area.