The possibility of oil-field subsidence is an important environmental consideration in both urban and rural areas. Current policies of various agencies require a subsidence-susceptibility appraisal prior to drilling operations. A hostile environment for oil-field operations can mean the Los Angeles and Santa Barbara areas as well as north of the Arctic Circle. Estimations of subsidence susceptibility are divided into two basic methods--the comparative-empirical and the analytic. The comparative-empirical method is considered best when undeveloped areas are examined. In this method, the geologic history, structure, and competency, of the area are compared with known subsiding areas. In the analytic method, core compressibility data, estimated stress changes, and sometimes, structural configuration, are applied mathematically to the estimation of reservoir compaction and subsidence. Many of the necessary data are not available until field development is well under way. Some combination of these methods should be used wherever possible.

Monitoring techniques include precision leveling, horizontal movement measurement, in-zone compaction measurements by collar logging, mechanical extensometers for shallow zones, tide gages, and special seismograph installations. Operational monitoring of casing derangements might also be important to locate compacting intervals.

Application of these and other techniques pertaining to the geologic aspects of environment will be increasingly important in the future.

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