Petrographical studies (Granier, 1993, et seq.) of the "Dalle Nacrée" Formation (Upper Bathonian - Lower Callovian oolitic limestones) in the subsuface of the Paris basin have resulted in notable advances in our knowledge of the architecture of oolitic deposits and as a consequence the geometry of the associated reservoirs. Four key criteria have been found useful in building sedimentologic and reservoir models:

1. ooid fabrics: after the publication of an article by A. Strasser (1986), my former colleague, Robert Boichard (TOTAL), focused his attention on ooid microstructures. He found that at the base of the "Dalle Nacrée" the ooids are radial, grade upward to concentric in the middle and are micritic at the top. This demonstrates a gradual change in the bio-physico-chemical conditions under which they were formed.
2. early lithification: at that time I was interested in early lithification that enabled me to identify sedimentary discontinuities, that is the bored and encrusted surfaces capping "sequences" (actually pseudo-parasequences sensu Granier, 1993) of which the top was subjected to early diagenetic lithification, and to recognize reactivated surfaces caused by submarine erosion. These in most cases are followed by beds containing perforated or encrusted pebbles derived from the erosion of the pre-existing early lithified seafloor.
3. hierarchy of the types of discontinuities: then I found that the evolution of the ooid fabrics was not as gradual as construed previously but that transitions between the three types were abrupt. Each of the sudden changes is at a discontinuity representing a stratigraphic gap which gives rise to a saltatory evolution to the generation and deposition of ooids. There are two categories of discontinuities – bored/encrusted surfaces and erosional surfaces:
   • those of the first order of magnitude are regional in extent and bound genetic depositional units, that is the large units defined by the discrete ooid fabrics that characterize them;
   • the others, of a second order, are local and the ooids immediately above and below them are similar.
   This concept of a hierarchy among discontinuities has as a practical consequence the introduction of a new tool for correlation (time-line) based on the "broken" gradient / saltatory evolution seen in the ooid fabrics, that is the distinction of three stratigraphic levels: a lower unit of radial ooids, a median unit with concentric ooids and an upper unit with micritic ooids. Studies in Burgundy of analogous terrains (outcrops and quarries) confirm the validity of these hypotheses. These sedimentary discontinuities are generally well-marked on well logs (GR, RHOB, NPHI). Their first role as barriers - to mineralizing fluids and later to the migration of oil and gas - has been clearly established.
4. porosity distribution: In these reservoirs there are essentially two types of porosity:
   • an effective porosity, essentially primary intergranular, that might be named more precisely "residual primary". It is the porosity that remains because cementation of the pore space was incomplete. It is not a "secondary porosity " caused by dissolution: cathodoluminescence studies support this distinction;
   • an intragranular microporosity, its amount related to the type of ooid present (from bottom to top: radial concentric, micritic) which brings us back to our starting point. This microporosity in which connate water is trapped increases from bottom to top of the "Dalle Nacrée" depending on the type of ooid: micritic ooids are more microporous than concentric ones and they in turn are more microporous than the radial ones. This determines a sliding scale of porosity cutoff ranging from about 4% at the base to ~8% at the top of the formation.

The key logs ("logs fondamentaux") of most exploration wells in the Paris basin are now in the public domain. From these logs it is quite easy to pick the several discontinuities and in any one set of wells to identify the first order discontinuities using a simple rule: pseudoparasequences thin sequentially upward from bottom to top of the parasequence to which they belong. As these first order discontinuities bound the discrete ooid units we do not need to confirm their category by thin section to identify the three main stratigraphic units that forms the "Dalle Nacrée" Formation. Within one unit significant variations in thickness are common in adjacent wells of any one field. Some transects are similar to that of the modern Lily Bank oolitic shoals presented by A.C. Hine (1977). Thicker zones represent the crest of large amalgamated sandwaves that migrated in space and in time on a wide ramp. These changes of thickness are accompanied by lateral change in facies. Good correlation exists beween the occurrence of certain facies (clean oolite) and the best reservoir properties, the reverse is also true: echinoid-rich facies are commonly tight, strongly bound by syntaxial rim cements. So the qualities of a flow unit may be predicted from the sedimentological model. The hydrocarbon trapping is commonly a combination of stratigraphy (trap at a disconformity) and structural geology (anticlinal trap), but it may also involve sedimentological parameters, such as facies distribution (for instance the relative amount of echinoid remains and their genetically related early diagenesis) as for example in the "Villeperdue nose".