Localization of potential migration pathways inside a fractured metamorphic hydrocarbon reservoir using well log evaluation (Mezősas field, Pannonian Basin)

Abstract

The basement of the Pannonian Basin is composed of metamorphic rock complexes that serve as either a pathway for fluid migration or hydrocarbon storage in the fractured rock body. Mezősas field is located in the northern part of the Békés Basin, the deepest sub-basin of the Pannonian Basin. There are three major rock units that exist in the area. An orthogneiss-dominated realm occurs at the lowermost part of the basement, sillimanite biotite gneiss in the middle part, and amphibolite gneiss at the top of the basement. Lithology identification and interpretation of metamorphic rocks from well logs are difficult especially when it involves the basement formation. In addition, core samples from the basement are limited since the process to retrieve the cores is rather difficult and costly. Hence, the aim of this study was to distinguish different metamorphic rock types using well logs and statistical methods using the available data, various cross plots, and a multivariate statistical analysis, namely, discriminant function analysis. The results from this approach showed that the different metamorphic rocks could be grouped statistically, and the boundary of each realm could be determined in each studied well. Thus, one-dimensional lithology columns could be developed, and the spatial arrangement of the lithologies could be defined. From the cross section and the newly developed geological map that integrates all the results revealed the existence of low-angle thrust faults and normal faults inside the metamorphic basement of the study area. From the results too, it was proven statistically that in this Mezősas field, there exists four different lithologies instead of three. Amphibolite and amphibolite gneiss characteristics could be differentiated statistically and using cross plots. In conclusion, the discriminant function analysis is a suitable tool to evaluate complex well logs information and the internal structure of the Mezősas area could be reconstructed.