Between 1957 when the lacq gas field was first put into production and 1967, a 3 cm subsidence put into production and 1967, a 3 cm subsidence occurred. This period corresponded to a 30 MPa depletion. From 1967 until 1989, further depletion of 25 MPa induced an additional subsidence of 2.5 cm which this time coincided with more than 1000 microearthquakes and earthquakes 44 with magnitudes greater than 3 and 4 with magnitudes greater than 4.

This paper gives a brief review of earlier case histories. It then goes on to describe a local seismic monitoring network which was installed on Lacq field. The method used for processing and interpreting the seismological processing and interpreting the seismological data takes into account the lithologies and the mechanical properties of the rocks and the source parameters. From the information obtained, the global mechanism of the gas field deformation appears to be the lowering of a rigid block located at the top of the structure by a successive of reverse movements along preexisting fractures and faults. preexisting fractures and faults. The paper goes on to demonstrate the practical consequences with regard to safety and gas production, and the contribution to our production, and the contribution to our knowledge of reservoir behavior over the course of time. close monitoring of subsidence and seismic events in a new and useful aid to understanding the reservoir response to fluid withdrawal.


Several human and industrial activities can trigger or produce seismic events: surface or underground nuclear explosions important quarry blastings, underground accidental explosions etc…. The question becomes more complex as soon as fluid movements are concerned.

Three main types exist: pure fluid injection injection of fluid (or pore pressure increase) due to surface daming and fluid extraction.

Triggering seismic events by fluid injection has been proven to be a straight forward phenomenon, as shown by the observations and phenomenon, as shown by the observations and tests performed in Colorado.

In the case of dam and reservoir filing, this can sometimes be straightforward when seismic events occur very close to the dam or the reservoir at the initial filings (Hoover dam 1939, U.S., Hsing fengkiang 1962, China, Monteynard 1963, France, Kremasta 1966, Greece, Nurek 1972, U.S.S.R.) More questionable are the cases where events occur after several fillings for example: in Marathon 1938, Greece, Kariba 1962, Zambia, Koyna 1967, India, Oroville 1975, U.S., Assouan 1981, Egypt, etc… In these cases, newtonian effects of the reservoir weight have been invoqued as the possible cause of triggering the seismic events. The pore pressure increase in depth and the correspondent effective normal stress decrease, allowing fractures and faults to slip, has also been invoqued as a possible cause of triggering.

Pure fluid extraction (water, oil or gas) is the Pure fluid extraction (water, oil or gas) is the most questionable of these three cases. If up to now most of the oil and gas fields are operated without inducing any noticeable seismic events, some rat cases have been found to cause microseismic or seismic activity. For example, during the 1920's a series of earthquakes were observed to coincide with the production of the Goose creek oil field in Texas. In Italy, the production of gas from the Caviaga field caused production of gas from the Caviaga field caused an earthquake of magnitude 5.5 in 1951. IN 1956, Caloi assumed and was able to confirm in 1970, that the gas production was the main cause of the earthquake.

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