ABSTRACT:

The Zagros Basin, which extends in a NW-SE orientation across southwestern Iran, is a large tectonically disturbed sedimentary basin that is oil-rich and which has an elevated natural seismicity level. Much of the region appears to be close to criticality, and anthropogenic perturbations can lead to triggering of seismic activity. This article assesses the baseline seismic rate and induced seismicity linked to oil and natural gas development activities in the reservoir structures in the Zagros Basin in the south and southwest of Iran. According to the location analyses of the earthquake data and the b-values map, some reservoirs have a particularly high b-value and these sites of concentrations of induced earthquakes were selected for further analyses. Among these selected reservoirs, Field B, one of the biggest producing oil fields located in the southwest of Iran, was studied. The injection and production history of the field is compared with the annual seismicity recorded for the field and a good correlation was found. The data are discussed in the context of criticality in the natural stress field. A few simple calculations of injection and depletion effects suggest that the seismicity field represents a valuable data source for improved geomechanics management of oil and gas production activities in critically stressed cases.

1. INTRODUCTION

  • The existing fault planes have high stress concentration, which the induced stresses are sufficient to produce failure

  • Fluid injection increases the pore pressure, which decreases the effective stress and induces microseismic cracks within the reservoir (ML<3),

  • Pore pressure decreases as result of fluid exploitation in the reservoir, changing the stress field in the surrounding area, causing induced seismic instabilities within a few kilometers from the reservoir (ML = 5).

  • Massive hydrocarbon exploitation or recovery in giant oil and gas fields removes such a high load from the upper crust and induces large earthquakes (ML = 6) far from the reservoir.

It has been become crystal clear that some man-made (anthropogenic) activities can affect the crustal stresses and trigger earthquakes. These seismic instabilities are usually occurred as a result of different activities such as water impounding behind high and large dams; huge surface mining; deep underground mining; injection of high pressure and high temperature fluids into the ground for different purposes (e.g. waste disposal, secondary oil recovery or power generation from geothermal areas); oil & gas exploitation; and large underground explosions [1] Induced earthquakes occur when one of the following situations is dominated in the considered area [1]: Induced earthquakes in hydrocarbon reservoirs usually occur as a result of fluid injection into the reservoirs during enhanced oil recovery or fluid production for hydrocarbon exploitation. Three basic mechanisms have been introduced for induced seismicity in hydrocarbon reservoirs as below [2]:There are now many case histories related to induced earthquakes in oil and gas reservoirs.

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