DOLOMITES WITHIN THE MESOZOIC CARBONATES OF SORRENTO PENINSULA (SOUTHERN APENNINES - ITALY): GENETIC MODELS AND RESERVOIR IMPLICATIONS

Abstract

Dolomitization is undoubtedly the most intensively studied diagenetic process in carbonate rocks. It can induce porosity and permeability changes resulting from a complex interplay of intrinsic and extrinsic factors, which together with the variety of dolomitizing fluid circulation schemes, makes the prediction of the dolomite geometric distribution and of petrophysical features very challenging. In Southern Italy the dolomitization processes and the induced porosity/permeability changes have never been investigated in detail. Only the Upper Triassic interval of the Apenninic carbonate platform succession outcropping on the Monti Lattari Belt (Southern Apennines) has been studied by Iannace (1991) and Iannace & Frisia (1994), which demonstrated a fundamental difference in dolomitization style between Norian and Rhaetian-Lower Jurassic successions. The present research aims to a detailed genetical and petrophysical characterization of the dolomitized bodies outcropping along the Monti Lattari belt from the Lower Jurassic to the Lower Cretaceous. The goal was to complete the study of the dolomitization processes along the carbonate succession of the Monti Lattari belt, started by Iannace (1991), and to furnish also a complete characterization of the petrophysical properties of the observed different types of dolomites. The research approach was to combine detailed sampling and analysis of well exposed outcrops along road cuts and quarries with a large scale reconstruction along a transect showing platform domains alternating with intraplatform basins. The workflow has been the following: • Field work including geometric and stratigraphic observations and sampling. • Petrographic analysis of dolomite types and their porosity (Optical microscopy, SEM and catodoluminescence analyses). • Geochemical analyses on the separate dolomite phases (O, C, and Sr, trace elements, Ca% and fluid inclusions). • Petrophysical analyses including: Helium-porosimetry, Petrographyc and Digital Image Analysis, Mercury Injection porosimetry, Nitrogen Permeability and Sonic Velocity. The field study and the petrographic analyses have shown that the Lower-Middle Jurassic stratigraphic interval mainly consists of a widespread massive dolomite which irregularly replace the carbonate bodies in the Liassic interval and only partially replaces the Dogger facies. This dolomite (called Dolomite2), which affects also the Rhaetian portion of the succession, is made of coarse crystals with both planar-s and planar-e mosaic with low porosities and permeability. Its occurrence and geometry, together with the light oxygen isotopes signature and the Mg/Ca ratio close to the stochiometry, allow to ascribe this dolomite to a late diagenetic event related to a large scale circulation of marine fluids through the Jurassic carbonate platform driven by thermal convection. In this stratigraphic interval also another type of dolomite has been locally recognized (Dolomite1). It consists of fine grained crystals with a very low porosity which only partially replace the carbonate bodies. It shows sedimentary structures which indicate, together with the stable isotopes results and the XRD data, a very early diagenetic process, likely related to reflux and tidal pumping mechanisms of fluid circulation. At last, a third type of dolomite (saddle type dolomite, called Dolomite3) followed by precipitation of poikilotopic calcite has been discriminated. These last two diagenetic phases are concentrated along faults and fracture systems and their oxygen isotopes in addition to the fluid inclusions results, allow to relate them to a precipitation from warm fluids (about 130°C) raised along extensional faults during a very late stage of diagenesis. The Lower-Middle Cretaceous interval has been studied, in cooperation with Shell, because considered as possible analogue of Val D’Agri reservoirs. In fact, the high similarities between the most productive intervals of the Apulian Platform (Cretaceous in age) and the coeval rocks outcropping in the Apenninic Platform (Monti Lattari belt), allow the characterization of buried bodies via outcropping facies. This interval consists of partially dolomitized bodies, usually stratiform, alternated with low porosity micritic carbonates. Petrographically, it has been possible to distinguish two main different types of dolomites: Dolomite A made of fine crystals (10 to 50µm) with a low porosity mosaic and Dolomite B made of coarse crystals (70 to 130µm) with both a tight mosaic and a more porous one (planar-s and planar-e respectively). They are Ca enriched and have positive oxygen isotopes data which indicate an early diagenesis from a normal marine water. The invoked processes for their formation are a capillary rising of fluids in an evaporitic setting, for Dolomite A and a reflux of slightly saline water for Dolomite B. Finally, also in this stratigraphic interval, a third less abundant type of dolomite has been distinguished: Dolomite C (saddle type) followed by precipitation of poikilotopic calcite. Again, these last two diagenetic phases are concentrated along fractures and fault systems and can be related to warm fluids (130°C, as indicated by fluid inclusion microthermometry), raised along extensional faults. As a consequence, considering the similarity with the Jurassic, and also with some Raethian samples collected by Iannace (1991), these two last diagenetic phases can be ascribed to a unique late diagenetic event that is the rising of warm fluids along the extensional Neogenic faults, which represent the last tectonic phase affectiong the Apenninic fold and thrust belt. From a petrophysical point of view, the dolomites belonging to the two analyzed intervals show very low porosity values. The integration of petrophysics and petrography, show that the main factor affecting the porosity and permeability values is the crystal size and packing which is strictly related to the limestone precursor facies. In fact, both in the Jurassic and in the Cretaceous, the presence of two different textures (Planar-e and s), due to the facies variations of the host rock, strongly drives the porosity differences. As a result, considering an hypothetical hydrocarbon reservoir, both for the Jurassic and for the Cretaceous, the potential permeable horizons could have represented by the layers with the more porous Planar-e mosaic. Finally, in order to have a complete petrophysical characterization of the sampled dolomites, a detailed study on sonic velocity variations has been carried out. The investigation, aimed to analyze the influence of the intrinsic and extrinsic parameters on the sonic velocity variations in low porosity dolomites, involved the characterization of dolomites coming from the same succession but having different stratigraphic heights (Cretaceous and Jurassic from the Apenninic Platform, sharing the same burial and tectonic history) and also of dolomites having the same age but coming from two domains with different burial history (Cretaceous dolomites from the Apenninic and Apulia Platform). The main result of this part of the study has been that, in low porosity dolomites (<10%), the factors affecting the sonic velocity propagation drastically reduce. As a consequence, pore types and mostly crystals size result to be the main controlling factors on the sonic velocity variations. In conclusion, the data collected in this PhD thesis, together with the previous studies on the Sorrento Peninsula and on the Tethyan domain, demonstrate that there is a stratigraphic control on the observed different types of dolomites in terms of geometry, petrography and geochemistry. This control appears to be related to the different impact that surface-related, climatic controlled diagenesis and subsurface late diagenetic processes had in the different moments of the Mesozoic. The Norian and the Cretaceous appear as time favorable to the formation of early dolomites. On the other hand, the Rhaetian and the Lower-Middle Jurassic were times characterized by widespread fluid circulation episodes took place during the Jurassic and leaded to the formation of large discordant bodies of coarse grained dolomites. These processes have been active not only in the Lattari Mountain platform but likely affected also other platform domains of Apennines and Alps.