Gérard Massonnat

Abnormal reverse faulting above a depleting reservoir

Subsurface deformation is observed during pumping of some hydrocarbon fields. Deformation 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 balloon 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 structures observed around magma chambers, allow us to reinterpret Paul Segall9s numerical model of poroelastic stresses caused by changes in the distribution of pore fluids and draw a new pattern of active faults.

Dependence of joint spacing on rock properties in carbonate strata

Joint density is studied in relation to petrographic and petrophysical parameters in two sedimentary carbonate formations characterized by different diagenetic histories: the Kimmeridgian limestones of the Chay Peninsula (western France) with a mean joint density of 6.37 fractures per meter (fr/m) (1.94 fr/ft), and the Bathonian limestones of the Bouye outcrop (western France) with a mean joint density of 1.9 fr/m (0.58 fr/ft). The Chay carbonates are characterized by a lower CaCO3 content, a higher average porosity, and a lower sound velocity than values recorded in the Bouye limestones. The compressive strength and Youngs modulus of the Bouye carbonates are, respectively, 10 and 3 times higher than in the Chay carbonates. A statistical analysis was used to identify relationships between joint density and carbonate rock properties. When facies variations are marked, the joint density at outcrop scale is related to the mean bed thickness, the facies descriptors, and the Youngs modulus. When textural variations are more limited, the joint density is controlled by the porosity. At the scale of a sedimentary basin, 63.8% of the variation in joint density may be accounted for by Youngs modulus and the sparite/micrite ratio. The decrease in the sparite/micrite ratio reflects an increased number of grain boundaries in the carbonate rock, which limits grain deformation and enhances joint density. The variations in Youngs modulus depend essentially on the porosity and mineralogy of the studied rocks. Any increase in CaCO3 content or decrease in porosity is associated with an increase in the elastic properties of the rock and a reduction of joint density.

Barremian platform carbonates from the eastern Vercors Massif, France: Organization of depositional geometries

The Vercors Massif of southern France constitutes a world-class and unique opportunity to visualize depositional geometries and diagenetic transformations of a Barremian–early Aptian (early Cretaceous) carbonate system, the Urgonian platform, at a seismic scale and thus to constrain the three-dimensional continuity and connectivity of reservoir-prone facies deposited during periods of high and low sea level. The integration of detailed outcrop analysis, including diagenetic transformations, with the sequence stratigraphic interpretation of cliff provides opportunities to study the precise relationship between sea level changes and facies distribution, including reservoir-prone examples such as slope and fan deposits, to understand how local tectonics affect accommodation space and effects on sedimentation rates. This integrated stratigraphic framework developed at surface conditions provides an analog for major petroleum systems of the Middle East where platform carbonates are involved, such as the Aptian Shu’aiba or the Cenomanian–Turonian Mishrif Formation.