AUTHORS: Scifoni A., Di Cuia R., Riva A. (G.E.Plan Consulting, Italy)
YEAR: 2008
GIC Annual Conference (Milan, IT)
A dedicated study was conducted over four dolomitised early Jurassic carbonate sequences cropping out in the southern Alps (northern Italy). The studied sequences (Mt Zugna and Loppio Fms, Calcari Grigi Group) represent the geologic records among those deposited on the Trento Platform. The Group directly overlies the upper Triassic Dolomia Principale Fm and is followed by the pelagic Rosso Ammonitico in the central–eastern part of the Trento Platform, while on the western margin by the San Vigilio Group.
We study four areas of the Asiago Plateau in which the carbonate sequence are pervasively dolomitised, with an undulated front generally limited to the Mt Zugna Fm deposits. The salients and recesses of dolomitising front are materialised by dolomite vertical bodies clearly recognizable in the field, characterised by quite massive-brecciated facies.
Dolomite textures vary from unimodal to polymodal and comprise also high porosity deposits, arranged in lenticular deposits, 1 to 10m in length, 1m in height and mostly confined by the bedding surfaces, with typical macropores, frequently filled by tardive dolomite and calcite cements.
The facies associations range from fine to medium-grained unimodal to strongly polymodal dolomites, associated with saddle dolomites and calcite cements, with the complete transitions in beneath. Dolomite breccias are usually characterized by rounded to un-rounded clasts (1 to 300cm), in a unimodal matrix, with well developed, thick (up to 1 cm) tooth-saw or by equant spar drusy mosaics.
The facies associations are variably composed of four crystalline phases (Dc1 to Dc3 and CC4). Dc1 represents the dolomite matrix, with small (100m) planar-s dolomites, with cloudy and dark crystals, while Dc2 is composed by planar-s to planar-e bright dolomites (100 to 500m), clear in transmitted light, while in CL they appear as zoned luminescent larger crystals, coupled with dark blue dull dolomites. The Dc3 is characterised by planar-c dolomites, with medium- large-sized zoned crystals (200 to 500m), with strongly luminescent dolomite rims and crystals overgrowing. The Dc3 represent the latest documented dolomite phase in the paragenesis, usually filling secondary porosity previously and sometimes is represented by a saddle dolomite. The Cc4 calcitic phase is the last diagenetic phase, which fills the residual spaces and/or the secondary pores, hence reducing the overall porosity toghether with the Dc3. Crystal dimension are very large (> than 1mm), directly depending on the empty pore volumes. This phase is normally dull with some zoned portions.
The diagenetic evolution is characterised by a first dolomitization stage, marked by well zoned dolomite crystals with a dull core. Following this stage, the porosity development is thought to increase, marked by the corrosion of older crystals, on which the blue dull dolomites developed. The appearance of saddle dolomite marks the onset of the porosity reduction stage, ending with the infill of the vugs with calcite cement.
Based on core samples and plugs, the porosity associated to these dolomitic bodies can be up to 25-30% with an extremely good connectivity. Matrix porosity and permeability, directly measured on plug analysis, vary respectively between 0.5-25% and 0.05-40 mDarcy. These petrophysical data appear strongly related to the diagenetic facies associations.
By considering this, three main reservoir facies are proposed: i) variable porosity (0.5-11%) but narrow permeability values (1.5-5 mD); ii) high porosity and permeability values; iii) low permeability (<1md) and porosity (<5%).
Matrix represents more than 10% of the total porosity, with values ranging from 1% to 15-20% with an average around 4-5%, related to the smallest pores represented by values lower than 10%.
Porosity variations are thougth to describe a trend which follows the diagenetic evolution, characterised by an initial porosity reduction followed by a porosity increase and finally by a porosity reduction related to deposition of late calcite cements. The connectivity is given by small-scale touching pores in the “matrix porosity” and by large scale touching pores in the “large vugs porosity”.