Geological solutions concluded by petroleum geochemical data in Western Greece

Τα προηγούμενα χρόνια έχει πραγματοποιηθεί μια λεπτομερής γεωχημική μελέτη πετρελαίου στη Δυτική Ελλάδα. Εντοπίστηκαν αρκετοί ορίζοντες πιθανών μητρικών πετρωμάτων πετρελαίου, προσδιορίστηκε το παράθυρο πετρελαίου στις πιο σημαντικές υπολεκάνες, ενώ από τη μελέτη συσχετισμού πετρελαίων προσδιορίστηκαν οι διαφορετικές ομάδες πετρελαίου της περιοχής, που έχουν παραχθεί από τα διαφορετικά μητρικά πετρώματα υδρογονανθράκων. Τα αποτελέσματα αυτά είναι πολύ σημαντικά και απαραίτητα για την έρευνα πετρελαίου στην περιοχή. Όμως, εκτός από αυτά τα δεδομένα, κάποιες επιπλέον γεωχημικές παρατηρήσεις μπορεί να αποβούν εξίσου σημαντικές στην επίλυση ορισμένων γεωλογικών προβλημάτων της Δυτικής Ελλάδας.- Ένα πολύ σημαντικό πρόβλημα είναι η απόθεση και διατήρηση του οργανικού υλικού στις λεκάνες της Δυτικής Ελλάδας.- Η δολομιτίωση σε συνάρτηση με τη γένεση πετρελαίου είναι ένα πρόβλημα.- Ένα άλλο θέμα είναι ο υπολογισμός του πάχους που διαβρώθηκε από τους υπερκείμενους σχηματισμούς.- Ο προσδιορισμός της παλαιογεωθερμικής βαθμίδας είναι επίσης πολύ σημαντικός.Οι δύο τελευταίοι παράμετροι είναι απόλυτα απαραίτητοι για τον προσδιορισμό της ωριμότητας του οργανικού υλικού.Η μελέτη όλων των παραπάνω παραμέτρων συμπληρώνει τη γεωχημική μελέτη της Δυτικής Ελλάδας, και σε συνδυασμό με άλλες γεωλογικές μελέτες μπορεί να δώσει λύσεις σε προβλήματα που σχετίζονται με την έρευνα υδρογονανθράκων στην περιοχή.A detailed petroleum geochemical study has been performed in the previous years in the Western Greece. Several source rock horizons have been identified, the oil window has been calculated for the most significant sub-basins and the oil correlation study has distinguished the different oil groups of the area, generated from different hydrocarbon sources. These results are very significant and useful for the oil exploration. But, further to these, some more geochemical observations can also be very important on solving some geological problems of the area.- A major problem is the deposition and preservation of the organic matter in the Western Greece.- The dolomitization in relation with the oil generation is also an issue.- Another issue is the calculation of the eroded overburden formations thickness.- The Paleogeothermal gradient determination is also very important.The last two parameters are absolutely necessary for organic matter maturity calculations.The study of all the above parameters completes the geochemical study of the Western Greece, and in relation with other geological studies can provide solutions in the petroleum exploration of the area

The source rock horizons of the Ionian Basin (NW Greece)

The organic geochemical study of six geological sections and two wells in the Ionian Zone (NW Greece), allows us to distinguish five horizons of possible source rocks: The Vigla shales (Cenomanian-Turonian), the Upper Posidonia Beds (Callovian-Tithonian), the Lower Posidonia Beds (Toarcian-Aalenian), the marls at the base of the Ammonitico Rosso (Early Toarcian), and shale fragments incorporated within the Triassic breccias. These horizons have good hydrocarbon potential and an organic matter type I to II. In the deeper parts of the Botsara sub-basin, the oil window is located in the interval between 3700-5800 m depth. Consequently, the Triassic shales have already entered the gas window. The Lower and Upper Posidonia Beds, and the marls at the base of the Ammonitico Rosso are mature in terms of oil generation. The Vigla shales maturity corresponds to the early maturation stage. The timing of the principal oil-source horizons maturation is the Late Jurassic for the Triassic shale beds and the Serravalian for the Lower Posidonia Beds. The preservation of the organic matter in the Lower and Upper Posidonia Beds through Toarcian to Tithonian and in the marls at the base of the Ammonitico Rosso during Early Toarcian are directly related to the geometry of the syn-rift period of the Ionian Basin. The organic matter preservation in the Vigla shales is related to the sub-basins that were preserved by the continuation of halokinetic movements during the post-rift period. The geometry of the restricted sub-basins that were formed during the syn-rift and post-rift period of the Ionian Basin evolution favored water stagnation and the development of local euxinic conditions in the bottom waters; these conditions were accentuated during the anoxic events that are known to have affected the Tethys ocean during the Early Toarcian and Late Cenomanian. The organic rich shale fragments within the Triassic breccias were initially deposited as stratigraphic layers in sub-basins of the evaporitic basin. The establishment of evaporitic sedimentation in the entire basin favored the preservation of the organic matter. The processes that resulted in the formation of the evaporite dissolution collapse breccias caused also the fragmentation of the organic rich layers, which are present actually as organic rich shale fragments within the Triassic breccias. The surface occurrences of petroleum in the Ionian Zone of northwestern Greece are mainly attributed to the Toarcian Lower Posidonia Beds and to a lesser extent to the Late Callovian-Tithonian Upper Posidonia Beds and the Albian-Cenomanian Upper Siliceous Zone or Vigla Shales of the Vigla Limestones. Oil also originate from Triassic formations containing potential source rocks in Albania and Italy. In this article, analytical results are presented of six geological sections and two wells where two new oil-source rocks have been identified

Evolution and petroleum potential of Western Greece

This paper reviews previous data on the geological evolution of Western Greece, with special emphasis on the petroleum potential of the Pre-Apulian zone (including new data) and the Ionian zone, the two most external portions of the Hellenide fold-and-thrust belt. From the Triassic to the Late Cretaceous, Western Greece constituted part of the southern passive margin of Tethys, and siliceous facies are widely associated with organic-carbon rich deposits. Pelagic Late Jurassic units rich in marine organic matter constitute important hydrocarbon source rocks in the pelagic-neritic Pre-Apulian zone succession. Oil-oil correlation with an Apulian zone oil sample (from Aquila, Italy) indicates similar geochemical characteristics. Thus, the significant volumes of oil generated by the rich and mature source rock intervals identified in the Pre-Apulian zone are likewise expected to be of good quality. In the Ionian zone, four organic-carbon rich intervals with hydrocarbon potential have been recorded. The tectonic history of the Pre-Apulian zone, which is characterised by the presence of large anticlines, is favourable for the formation of structural traps. By contrast, locations suitable for the entrapment of hydrocarbons in the Ionian zone are restricted to small anticlines within largerscale synclinal structures. Hydrocarbon traps may potentially be present at the tectonic contacts between the Ionian zone and both the Pre-Apulian and Gavrovo zones. Major traps may also have been formed between the pre-evaporitic basement and the evaporite-dominated units at the base of both the Pre-Apulian and the Ionian zone successions. The degree of participation of the subevaporitic basement in the deformation of the Pre-Apulian and Ionian sedimentary cover will determine the location and size of these traps. Various scenarios regarding the deformation of the sub-evaporitic succession are examined in order to determine the hydrocarbon trapping possibilities of each model. The hypothesis of continental subduction (Early to Late Miocene) of the shared pre-evaporitic basement of the Pre-Apulian and Ionian zone eastwards of the Ionian zone is regarded favourably, as it appears to be compatible with the presence of a Phyllite - Quartzite - dominated (HP/LT) metamorphic unit beneath the Gavrovo-Tripolis zone carbonates in Peloponnesus and Crete. © 2007 Scientific Press Ltd