A recent petroleum-geologic evaluation of the Central Carpathian Depression, Southeastern Poland

Abstract

A subregional investigation of the Central Carpathian Depression within the Eastern Polish Carpathians provides some constraints on the character and distribution of reservoir quality sandstones, an analysis of the styles of structural deformation that can be expected and a description of oil to source correlations and some of the factors responsible for oil quality variations. The Paleocene-Upper Cretaceous Istebna Beds consist of conglomerates, sandstones and pebbly mudstones deposited by channel-fill and lobe deposits and by mudflows. The Istebna Beds are 50–1800 m thick with porosities from 10 to 20% in the conglomerates and sandstones and approximately 5% in the mudstones. The Eocene Ciezkowice Sandstones are composed of conglomerates, pebbly sandstones and sandy mudstones deposited by deep-water processes similar to those of the Istebna Beds. The Ciezkowice Sandstones are 50–500 m thick with 10–25% porosity. Both the Istebna and Ciezkowice show direct relationship between reduction in porosity and increased present-day burial depth and reduced permeability. Analysis of existing subsurface well-control and surface geologic maps facilitates the development of a new structural model for traps within the Silesian Nappe. Typical traps are complexly folded, asymmetric anticlines developed in the hanging-walls of large-displacement thrust faults. In addition, balanced cross-section construction allows the estimation of structural shortening ranging from 37 to 47% produced by an Early Miocene compressional event. Geochemical analyses indicate that the waxy oils encountered in the Carpathian overthrust and foredeep are derived from source beds containing a mixture of land-derived higher plant organic material and marine organic matter. Biomarkers within these oils suggest a Late Cretaceous or Tertiary age for the source of these oils. The higher sulfur levels and higher C 29/C 30 hopane ratios of the oils derived from foredeep fields indicates the additional contribution of a Jurassic carbonate source rock. The Lower Istebna shales appear to be of sufficient organic richness to have contributed oil to some of the overthrust fields. The biomarker maturity parameters of the oils within the Central Carpathian Depression are very high, suggesting that trapped oils were derived from a late stage of oil generation. Shallow burial depths, combined with a complex migration history and a number of degradation processes explain the variation in petroleum character within the oil fields of the Central Carpathian Depression.