Abstract: Stratigraphic Controls on Facies: A Carbonate Outcrop Study as Reservoir Analog, Vercors Plateau (SE France)

Katie Joe McDonough

Lower Cretaceous ramp carbonates of Vercors National Park provide ideal outcrops from which to unravel the distribution of petrophysical properties within a potential carbonate reservoir. Superb exposures of carbonate shelf clinoforms from topset to toeset allow facies analysis along clinoforms from different positions in the geometric stacking pattern.

Changes in the component facies, stratal geometry, and dominant depositional process of progradational/aggradational units are found to coincide with position in the stacking pattern (e.g., within "landward-stepping" vs. "seaward-stepping" units). Because stacking pattern position is a manifestation of the accommodation regime under which units were deposited, petrophysical characteristics are in turn critically tied to accommodation regime. This study documents the strong stratigraphic control on carbonate sedimentology.

Position in the stacking pattern governs the following sedimentologic and geometric attributes.

In landward-stepping units:

1. Sediments are characterized by skeletal (dominantly foraminifer), peloid packstones and grainstones. Grain sizes are smaller, sediments are generally better sorted, and detrital quartz content and degree of micritization are higher in sediments from these units. Coated grains have more coats and more coat micritization. Porosity is lowest in these units.

2. Stratal geometries are characterized by low-angle clinoforms (<6°) and corresponding aerially wider facies tracts than seaward-stepping units.

3. Physical processes in the upper shoreface environment are dominated by tides and storms.

4. Lateral offset of facies tracts is small in comparison with seaward-stepping units.

In seaward-stepping units:

1. Sediments are characterized by poorly sorted, coarse-grained to granule bioclastic grainstones with lower silica content than their landward-stepping counterparts. Grain sizes are much larger, sediments are generally more poorly sorted, and detrital quartz content and degree of micritization are lower in sediments these units. Porosity is highest in these units.

2. Stratal geometries are characterized by high-angle clinoforms (up to 22°) which correspond to narrower facies tracts.

3. Physical processes in the upper shoreface environment are dominated by storms.

4. Lateral offset of facies tracts is large in comparison with landward-stepping units.

The results of these ties between stratigraphy and carbonate sedimentology have far-reaching implications for development geology.

Firstly, the accommodation regime under which sediments were deposited (given by position within the stacking pattern) may exert a stronger control on final reservoir quality than water-depth alone. Furthermore, some potential reservoir facies may only develop under particular accommodation regimes (e.g., tidal sandwave facies develop only during times of increasing accommodation). Reservoir evaluation must therefore include consideration of stratigraphic position.

Secondly, the degree of reservoir compartmentalization varies with position in the stacking pattern, so that development strategies must vary with the stratigraphy. In general, decreasing accommodation regimes result in more compartmentalized reservoirs in the facies tracts investigated.

Lastly, the difference in stratal geometries means that potential reservoir facies tracts stack with variable amounts of offset depending on where in the stacking pattern they occur. Potential reservoir facies tracts are aerially narrower in seaward-stepping units (steep clinoform angles) and wider in landward-stepping units (low clinoform angles). The stacking pattern determines whether multiple reservoirs are vertically stacked, and the degree of fluid communication across unit boundaries. Again, development strategy must be tied to the stratigraphy.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994