INTRODUCTION In many hydrocarbon fields, subsurface deformation is observed during oil or gas recov- ery. Subsidence is the most widely developed feature due to hydrocarbon extraction and mining (King and Smith, 1954). Generally, subsidence is centered over the field and associated with cen- tripetal horizontal displacements and faulting (Koch, 1933; Yerkes and Castle, 1970; Volant and Grasso, 1994). In oil fields, a fraction of the movements can be recovered by repressurization (Yerkes and Castle, 1970), proving that they are related to oil recovery. In some places, seismicity has been recorded during extraction (Segall, 1989; Grasso, 1992) and, in most cases, it van- ishes after it. Wetmiller (1986) and Feignier and Grasso (1990) recorded focal mechanisms formed during hydrocarbon exploitation that de- scribe reverse movements on steep faults, dips of 60°. This contrasts with general observations of reverse faults which, due to the current state of stress in the crust, have dips usually less than 30°. This paper uses the results of analogue ex- periments to clarify the geometry and arrange- ment of brittle deformation features associated with depleting oil reservoirs. Our model is a sand-silicone box in which reservoir depletion is obtained either by deflation of a balloon or by de- pletion of an isolated volume of undercompacted material. Experiments show that most of the faults that form during depletion are steeply dip- ping reverse faults. Such abnormal fault geome- try is discussed in the light of Segall’s numerical model (1989) of state of stress obtained around a contracting volume. MOVEMENTS AND FAULTING RELATED TO THE RECOVERY OF HYDROCARBONS In the Goose Creek oil field, subsidence com- menced in 1918, one year after the hydrocarbon recovery commenced, and reached 1 m in 1925 (Pratt and Johnson, 1926). In 1918, the down- faulted block was becoming submerged beneath sea level, attesting to depletion of the underlying reservoir. Two major faults developed, one on each side of the field, their strikes parallel to the long axis of the subsidence bowl. At Wilmington oil field, extraction com- menced in 1936 and by 1966, cumulative subsi- dence had reached 9 m (Yerkes and Castle, 1970). Leveling surveys have determined that the subsidence bowl was centered on the producing area and extended beyond it (Fig. 1). Horizontal displacements above mining areas (King and Smith, 1954) and gas or oil fields (Lee and Shen, 1969) are found to accompany subsi- dence. At the Wilmington oil field, centripetally di- rected horizontal displacements were 3.66 m from 1937 to 1966 (Fig. 1). Horizontal displacement reached a maximum halfway along the flanks of the subsidence bowl, and decreased progressively to zero at both the center and peripheries of the bowl (Fig. 1). Because displacement vectors are oriented radially toward the center of the subsi- dence, the center is subjected to compression while the periphery is subjected to radial extension (Kovach, 1974; Segall and Fitzgerald, 1998). In the Lacq gas field of France, the relation- ship between subsidence and gas pressure that has been observed (Grasso, 1992, 1993) seems to differ from many gas fields where reservoir pressure linearly decreases with time. Instead, for 20 years, the slope of the depletion has decreased weakly with time (Fig. 2). This may be attributed to an increase in rock intrinsic per- meability that is unlikely because permeability generally decreases during gas recovery or to a volume increase of the reservoir by widening of connected volume due to faulting (Rolando et al. 1997). Active faulting has been observed from dam- aged wells in Wilmington oil field (Kovach, 1974) and in Buena Vista Hills oil field, Califor- nia, where fault dip has been determined to be about 25° (Koch, 1933). There, displacement rates have been calculated to be about 4 cm/yr. These extreme movements have been attributed to fluid withdrawal (Yerkes and Castle, 1970; Segall, 1989). SEISMICITY AND SOURCE MECHANISMS The relationship between earthquakes and fluid extraction in oil fields is clearly established (Segall, 1989). In some fields, earthquakes oc- curred immediately after production com- menced, as in Rocky Mountain House, Alberta (Wetmiller, 1986), and Goose Creek (Pratt and Johnson, 1926). In other places, seismicity com- menced a few years after production. This is the case in the Lacq gas field, where the first signifi- cant earthquakes were recorded in 1969, 12 years after production began (Guyoton et al., 1992). In the Rocky Mountain House seismic zone, the seismic activity is in a flat thin zone below, and/or possibly within the reservoir (Wetmiller, Geology; February 1999; v. 27; no. 2; p. 111–114; 7 figures. 111 Abnormal reverse faulting above a depleting reservoir Francis Odonne Université Paul Sabatier, Pétrophysique et Tectonique, U.M.R. 5563 C.N.R.S., 38 rue des Trente-six-Ponts, Isabelle Ménard F-31400 Toulouse, France Gérard J. Massonnat Elf Exploration Production, Centre Scientifique et Technique Jean Feger, Avenue Larribau, Jean-Paul Rolando F-64018 Pau Cedex, France ABSTRACT Subsurface deformation is observed during pumping of some hydrocarbon fields. Defor- mation features include subsidence centered on the field and subsidence-related centripetal horizontal displacements and faulting. Focal mechanisms yield reverse movements on steeply dipping faults. In our sand-silicone analogue model, the reservoir is represented by a latex bal- loon or by undercompacted ground sand. Deflation of the reservoir results in formation of steeply dipping reverse faults bounding a downward-opened cone. The cone moves downward to follow the reservoir contraction. Faults along the cone are straight beneath a thick reservoir cover and tend to curve upwards with decreasing cover. Our results, similar to natural struc- tures observed around magma chambers, allow us to reinterpret Paul Segall’s numerical model of poroelastic stresses caused by changes in the distribution of pore fluids and draw a new pat- tern of active faults. Figure 1. Surface displace- ments at Wilmington oil field near Long Beach, Cali- fornia. Vertical downward displacements are shown between 1928 and 1966; subsidence bowl is cen- tered over producing area. Horizontal displacements are shown between 1937 and 1970; cen- tripetally di- rected displacements are maximum halfway along flanks of subsidence bowl and decrease progressively to zero at both its center and periphery (redrawn from Yerkes and Castle, 1970).