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Oil and gas potential of the Neogene Carbonate formations in the sedimentary basins of NW Crete

Makridou Zafeiria

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Summary

A reservoir is a rock having the ability of containing fluids such as gas, oil or water. In order to be productive it should be large enough in dimensions and having enough pore space; the connection between the pores and the permeability of the reservoir controls its capacity of fluids to flow. Sandstones and carbonates are the most common reservoir rocks. High values of porosity commonly characterize coralline algae limestones introducing them as potentially good reservoirs and thus globally many carbonate formations which characterized by abundant coralline algae host prominent hydrocarbon fields. Though, carbonate reefs have been proved significant as hydrocarbon exploration targets, the study of the divergent environment of facies can be extremely helpful in identifying patterns for source, reservoir or (rarely) seal settings. This Thesis is relevant to hydrocarbons exploration potential, through the study of the evolution of carbonate facies and their ability to act as reservoir. Neogene carbonate strata of West Crete comprise several bio- and lithofacies that represent various depositional environments. The present work is based on the study of an outcrop exposed along the eastern coast of Chania city. It describes a significant in size (both in length and thickness) Upper Miocene mostly carbonate succession of coralline algae and Rhodolith-beds from the island of Crete. Moreover, it highlights the presence of coralline algae as a major reef-like contributor while it introduces this new occurrence in the coralline algae/Rhodoliths palaeo-distribution global and Mediterranean map. Analytically, the studied ‘Koumpeli’ outcrop with a length of 1900 meters and a thickness of 45 meters, can be subdivided into four distinct lithofacies (A to D). From the base to the top, the succession is composed by two sets of sandstones (a yellowish and a reddish one, parts A and B respectively), passing upwards into rhythmical marine, coralline-red-algal carbonate facies (C) that underlie neritic bioclastic limestones (D). It is the first time that such an extensive occurrence of coralline algae is described from western Crete, therefore the present work is mainly concentrated in this facies (C) in particular. The prevailing biological feature of coralline algae of the outcrop corresponds to an array of palaeo-environmental conditions; the structure and appearance of their development are sequence stratigraphically interpreted to be related as to a Transgressive–Highstand System Track of relative sea-level change (TST-HST). These well-built structures are representative for the Mediterranean region, as a result of events that date back just before the dawn of the Messinian Salinity Crisis (MSC), when the Mediterranean became dry; rhodolith beds were extensive in Miocene platform carbonate and siliciclastic sediments both in the Mediterranean and Paratethys. Investigations based on paleoreconstructions and actualistic comparisons with the depth distribution of living rhodoliths comprising similar coralline algal assemblages, imply a wide range of palaeodepths of development ranging from 10 to 100 m.Nine sedimentological sections were logged in high resolution along the studied outcrop, five of them directly related to the red-algal facies (part C). Recorded data included the thickness of each bed and the depositional environments that deduced by combining lithologies, sedimentary structures and the (macro-) palaeontological content of the deposits, as well as the nature of interbedding surfaces (erosional, subaerial exposure, angular etc). Vertical facies successions and geometries at the outcrop scale were interpreted in terms of progradation, aggradation or backstepping of depositional environments reflecting the interplay between accommodation and sedimentary flux in time. In an approach of estimating possible reserves of the hydrocarbon content in this individual occurrence, the true volume of the reservoir was evaluated using three different scenarios (different reservoir dimensions and porosity values) that were assessed accordingly. The results offer a good insight into the nature of Late Miocene deposits of Chania region and their ability to act as reservoir rocks. In this context, this research also provides a basis for further evaluation of the hydrocarbon potential of Neogene rocks in Crete Island. Also, in order to test another scenario, that of the potentiality to act as a Source Rock, laboratory geochemical (ROCK-EVAL) analysis of the organic matter was performed in the matrix of the rhodoliths. The samples are characterized by poor content in organic matter; subsequently they show a rather poor for oil or gas generating potential. Although carbonates have abundant algal organic matter mixed into them there were probably other factors that acted, for example the high energy of hydrodynamic regime and led to the oxidization of the organic content.

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