Active Vocabulary

Date: 2015-10-07; view: 549.

Unit 1. Geophysical Exploration: Gravity and Magnetic Methods

Chapter 3. Exploration Methods

C. Discussion

Topics:

1. Unconventional gas (general characteristics).

2. Development of unconventional oil and gas.

LEARNING OBJECTIVES:

Having worked through this chapter the student will be able to:

• define the objectives of an exploration;

• list physical properties of rocks and minerals measured at a distance;

• describe the types of information necessary for planning of drilling activities;

•describe main types of computer assisted exploration and different types of

logging.

Through the early 1990s, finding oil and gas was largely a matter of luck. Early explorers looked for oil seeps to the surface, certain types of rock outcrops, and other surface signs that oil might exist below ground. This was a "hit or miss" process. But science and technology quickly developed to improve the industry's ability to "see" what lies below ground.

The exploration for oil and gas now typically begins with geologists examining the structure of the earth, and determining areas where it is geologically likely that petroleum deposits might exist. The goal is to find a convergence of the geologic elements necessary to form an oil and gas field. These elements include a source rock to generate hydrocarbons, a porous reservoir rock to hold them and a structural trap to prevent fluids and gas from leaking away. Traps tend to exist in predictable places – for example, along faults and folds caused by movement of the Earth's crust or near subsurface salt domes. Finding these subterranean features requires a careful blend of science and art. For example, structural geology involves gathering and interpreting information from above ground to deduce what lies underground. Geologists obtain this information by examining exposed rocks or, when difficult terrain limits access, by examining images from satellites and radar.

The physical properties and effects of subsurface rocks and minerals that can be measured at a distance include density, electrical conductivity, thermal conductivity, magnetism, radioactivity, elasticity, and other properties. Exploration geophysics is often divided into subsidiary fields according to the property being measured, such as magnetic, gravity, seismic, electrical, thermal, or radioactive properties.

Geophysical data can provide petroleum engineers with vital information concerning the planning of drilling activities, and the planning for field development. This information includes:

· the distribution of reservoir properties (its internal architecture), and the properties of the surrounding rocks;

· the geometry of the rock bodies that comprise the petroleum system, including the over-burden, the under-burden, the side-burden, and the reservoir itself (the boundary of the hydrocarbon-bearing region can be called the reservoir envelope), along with the characteristics of the petroleum kitchen and its associated migration routes;

· the size and distribution of the aquifer;

· the presence of internal compartments or disruptions of the reservoir (such as faulting);

· possibility, direct indications of hydrocarbons;

· sometimes, the orientation of fractures;

· in some cases, indications of high pore pressures.

However, these benefits are not always realized in practice. The unfulfilled potential of geophysics is largely related to problems with data quality, but sometimes the reduced benefits are related to inappropriate techniques and limited interpretation skills. Understanding the basic controls on data quality (resolution) will help the engineer appreciate the limits of the data – although these limits are being consistently pushed back as technology develops in all areas of the geophysical method (acquisition, processing, interpretation and visualization).

Oil companies generally held the use of geology in low regard prior before the 1920s, when geophysical methods of exploration that enhanced the oil prospector's knowledge of subterranean strata began demonstrating an advantage for finding oil. Tools used by oil and gas explorers were fairly basic and depended on fundamental variables in the earth's physical condition: gravity change, magnetic field change, time change, and electrical resistance.

Because the density of rocks varies, the gravitational force they exert necessarily varies. If very light rocks are found close to the surface, the gravitational force they exert will be less than those of very heavy rocks. With this in mind, geophysicists attempted to locate salt domes, which would be associated with minimum gravity, by using the torsion balance instrument.

The pendulum method, another variation of the gravity method, relied on the period of a pendulum's oscillation adjusted by variations in gravity due to changes in altitude and latitude. The pendulum method was superseded by the gravity meter. Advances in gravity instrument technology afforded geophysicists better equipment with which to make more accurate determinations. The most common gravitational instrument in use today is the gravity meter or gravimeter, which measures variations in the earth's gravitational field by the gravitational pull on a mass balanced against some form of elastic force.

A second method of exploration is the Magnetic method. Most oil occurs in sedimentary rocks that are nonmagnetic. Igneous and metamorphic rocks rarely contain oil and are highly magnetized. By conducting a magnetic survey over a given area, a prospector can determine where oil-bearing sedimentary rock is more likely to be found. Two types of magnetic instruments are used to measure the slight difference in magnetism in rocks, the field balance and the airborne magnetometer. The field balance is used on the earth's surface to measure magnetism in specific locations. The airborne magnetometer is used to measure the magnitude of the earth's total magnetic field over a large area.