GEOPHYSICAL METHODS OF HYDROCARBON EXPLORATION M T H

GEOPHYSICAL METHODS OF HYDROCARBON EXPLORATION M. T. H

What is Geophysical Exploration? • The application of the principles of physics to the study of the subsurface, in search of hydrocarbon • Geophysical investigations of the interior of the earth involves taking measurements at or near earth’s surface that are influenced by the internal distribution of physical properties. • The objective of any exploration venture is to find new volumes of hydrocarbons at a low cost and in a short period of time.

Introduction • The usual sequence of activities once an area has been selected for exploration starts with the definition of a basin. • Petroleum exploration and production are concerned with the geological interpretation of geophysical data, especially in offshore areas. • There are three main geophysical methods used in petroleum exploration: Magnetic, gravity and seismic. • The first two of these methods are used only in the predrilling phase. Seismic surveying is used in both exploration and development phases.

Gravity Surveying

Interpretation of gravity maps presents many problems, the simplest of which are caused by different subsurface bodies producing the same anomaly on the surface. Example: distinguishing between a small sphere of large density and a large sphere of low density at similar depths is impossible. In some circumstances gravity maps may indicate drillable prospect by locating salt domes and reefs (because of their low density.

Magnetics

Seismic Geophysical Survey • The seismic methods are the most widely used of all geophysical methods used in petroleum exploration. • Seismic methods measure seismic velocity of rock layers to detect both lateral and depth variations and the objective is to determine the lithology and geometry of the layers. • A seismic wave can be thought of as shock wave (elastic wave) or vibration traveling through the ground. • The rate of travel, or velocity, of the wave is related to the density of the rock. • There are two types of elastic waves produced: 1) P-waves, which are primary or “compressional” waves, and 2) S-waves, or shear waves

Sound as a Tool Hydrophone Streamers Airgun Sound Wave

Activity Time = 0. 7 seconds Velocity = 1. 4 km/second V=D/T where: V = velocity Distance D = distance T = time

Procedure • The procedure used is to lay out a survey line with geophones set at equal spacings along the line. • A shock wave is produced at one of the stations by dropping a heavy weight or detonating an explosion at a “shot point”. • The shot point is the point on the surface directly above the zone of interest. • Ground motions caused by the explosion or impact are transmitted in the form of P-waves and S-waves. • A “seismic timer” is used to measure the travel time of the wave from the instant it is generated until the time the wave reflection is detected back at the surface. • Times are measured for each of the successive stations along the line. Either semi-graphical or computer methods are used to determine the velocities.