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Flow Units in Carbonate Karstic Reservoirs: Insights from Outcrop Analogues (Urgonian Limestone and Upper Jurassic Dolostone, SE France)

A. Fournillon, F. Fournier, B. Arfib, F. Gisquet, P. Léonide, and J. Borgomano
Aix–Marseille Univ, CEREGE Centre Saint-Charles, Marseille, France.

Hydrocarbon accumulations in karstified carbonates are known worldwide (Loucks, 1999). They commonly exhibit high reservoir potential in hydrocarbon fields such as Kashagan field in Kazakhstan (Ronchi et al., 2010) and are regarded as relevant exploration targets (e.g. in the Oriente Basin Ecuador, after Lee & Castagna, 2007). Karst carbonate reservoirs are challenging for both exploration and production since heterogeneities occur at all scales, from millimeters to kilometers, and are generally below the resolution of seismic. The spatial distribution of karsts and paleokarsts is controlled by a vast number of factors. Among them, the nature of the initial carbonate sediments, and their subsequent eogenesis, and the impact of tectonic deformation on the base levels positions during the telogenesis are considered to have played a major role on karst development (Palmer, 1991).

The aim of this study is to characterize the influence of the petrophysical properties on the development of karsts and flow units in karstic reservoirs. This study is based on two karstified outcrops: Lower Cretaceous tight limestones and Upper Jurassic porous dolostones from Provence, South-East France. Both outcrops have undergone the same telogenic karstification phases since they belong to the same structural unit. The case studies are in present active aquifers. The characterization of the flow units are then based on the link between their static and dynamic properties. The static properties are inferred from thin-sections analyses, petrophysics measurements from outcrops and cores, and from field studies (characterization of shape, size and distribution of paleokarstic features). The dynamic properties are derived from continuous monitoring of three springs parameters: temperature, specific conductance and water head. These springs are the outlets of both studied aquifers. The springs records are analyzed as time series in order to quantify the influence of karst on fluid flow.

The static and the dynamic data suggest a major influence of petrophysical properties in telogenic karst development and then on the definition of flow units. In high porosity media, i.e. in sucrosic dolostone, the speleogenetical processes lead to the formation of a diffuse karst. This results in increasing porosity and permeability. Flow units in porous intervals are concentrated in the stratabound highly porous dolostones which acts as horizontal drains. In tight formations, the speleogenetical processes concentrate in the discontinuities, i.e. fractures and joints, and lead to the creation of a few big karst conduits. The flow units are then dominated by the karst pipe network.

 

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012