Hydrothermal Dolomitisation Fronts: Implications on Reservoir Characterisation and Modelling (Jurassic, Lebanon)

Abstract

Abstract Hydrothermal dolomite (HTD) reservoirs are well-known from exploration and development of hydrocarbon plays in the Middle East. Petroleum geoscientists query for means to model the related reservoir properties. This contribution describes the field geometry, petrography and geochemistry of hydrothermal dolomitisation fronts (Late Jurassic, Lebanon) and aims to investigate the relationship between diagenetic evolution and reservoir quality. The main application is to provide a methodology for HTD reservoirs modelling based on the internal facies distributions of dolomite bodies, and diagenetic evolution (fluid-rock interactions). Dolomitising fluids circulated along faults and invaded the nearby facies, preferentially according to the original porosity/permeability properties of limestone host-rocks. Enhanced porosity due to the initial hydrothermal dolomitisation of micritic limestones is observed at the margins of the front. There, sucrosic planar-e dolomites possess relatively high porosity and permeability (~8% and ~10mD). The centre of the dolostone bodies exhibits over-dolomitisation, which resulted in an increased crystal stoichiometry (50–52 mole% CaCO3) and broad, light d18O values (-9.0 to -4.0PDB). Such recrystallisation process may have changed the porous sucrosic dolomite textures into (over-dolomitised) planar-s, interlocking crystalline textures with lower matrix porosity (<6%) and permeability (<0.5mD). Additional diagenetic phases postdate the hydrothermal dolomitisation, altering (positively or negatively) the dolostones with respect to reservoir quality. By identifying the complex diagenetic processes and associated fluid-rock interactions, a new approach for predicting and modelling HTD diagenetic evolution and reservoir quality is proposed. The significance of this contribution includes:identifying distinct dolomite facies in HTD reservoirs;stressing on the role of diagenetic evolution in enhancing or destroying reservoir quality; andproviding a new approach for modelling HTD reservoirs through fluid-rock interaction coupled with sedimentary evolution history. Introduction The Jurassic carbonate platform in Lebanon, as well as in the neighbouring countries, includes thick pervasive dolostones (exceeding 1000m in thickness) which are believed to have resulted through hydrothermal dolomitisation of pre-existing Early Jurassic, seepage reflux dolostones and Middle-to-Late Jurassic limestones [?1]. Dolomitisation took place during a time span stretching from the Late Jurassic to the Early Cretaceous (ca. 10 million years approximately), coinciding with major uplifts and subaerial conditions that occurred together with faulting and volcanic activities. Recent fieldwork led to the discovery of a 'tongue'-shaped dolostone body (350m in length) within the latest Jurassic limestone strata in central Mount Lebanon (Fig. 1). Sampling was performed systematically along four almost parallel profiles crossing the dolomite 'tongue' lengthwise. In total, 70 core-drilled samples were collected from the dolostones, as well as from the nearby partly dolomitised limestones, sandstones and volcanic deposits. This contribution discusses the field geometry, petrography, and geochemistry of this outcrop together with its diagenetic and reservoir properties, and suggests a new approach for modelling HTD reservoirs through the use of an inhouse developed software which takes into account the fluid-rock interaction and mineralogical changes. Statement of Theory and Definitions The detailed field mapping and sampling of the dolomite tongue-shaped front was necessary in order to characterize the various dolomite phases (textures) and to provide material for petrographic and geochemical analyses. Such laboratory analyses led to the characterization of the various diagenetic phases and the setting up of a precise paragenetic evolution. All of this, together with petrophysical analyses (porosity/ permeability) provided a comprehensive data base which was essential for modelling.