Oil reservoirs in foreland basins charged by thrustbelt source rocks: insights from numerical stress modelling and geometric balancing in the West Carpathians

Abstract

The West Carpathian thrustbelt advanced northeastwards over the European Platform. Its thrust sheets comprise sediments of the Early Cretaceous rifts that evolved on a passive margin of the European Platform, the Late Cretaceous-Paleocene basins formed by rift inversion, and the Eocene-Oligocene flexural basin. Geochemical analyses established a clear link between pooled oils in the foreland and the Oligocene Menilite Formation inside the thrustbelt. In order to understand the driving forces for this oil migration scenario, finite-element models of fault-propagation and fold-bend folds are used to study the mean stress distribution in the thrust sheets and the foreland. Mean stress has a profound control on the pore fluid pressure through the relationship affected by sediment porosity, and sediment skeleton and fluid compressibilities. Modelling results suggest that only faultpropagation folds are capable of generating foreland-directed mean stress gradients as they are characterized by a large foreland area of decreased mean stress, by coupled increased/decreased mean stress areas on advancing/receding sides of the ramp tip, and an overall mean stress decrease inside the thrust sheet in the direction towards the foreland. This interpretation is in accordance with the dominant fold-and-thrust style in the Western Carpathians inferred from balanced cross-section restoration. It shows that frontal faultpropagation folding was active during the late Oligocene-Early Miocene, providing an effective tectonic driving force for hydrocarbon migration from source rocks inside the thrustbelt towards reservoirs in the foreland.

Burial and Thermal History Modeling of the Paleozoic–Mesozoic Basement in the Northern Margin of the Western Outer Carpathians (Case Study from Pilzno-40 Well, Southern Poland)

“…Thus, it appears that the Lower Menilite Shales [55][56][57] are the main source of these oils, because Menilite Shales kerogen is mature in the Dukla Unit and in deep synclines of the Silesian Units [38,54,62]. Therefore, it appears that, as shown by Nemčok and Henk [63] based on numerical modeling, hydrocarbons migrated from the Menilite Beds to the foreland. Moreover, hydrothermal quartz from mineralized joints of the Outer Carpathians contains numerous fluid inclusions, which indicate the migration of hot (above 200 • C) methane-rich fluids from over-pressured sediments with fluid pressures of 50 to 300 MPa [80].…”

“…These hydrocarbons were generated during thermogenic processes and probably migrated through the fault zones from source rocks. The Lower Carboniferous and Middle-Upper Devonian strata, and/or the Ordovician to Silurian rocks [38,47,62] or the Menilite Shales [55,56,63], are regarded to be the sources of these hydrocarbons.…”

New Opportunities for Oil and Gas Exploration in Poland—A Review

“…However, according to burial and thermal history modeling of the Paleozoic-Mesozoic basement in the central part of the Polish Outer Carpathians (between Cracow and Dębica) [93], the Devonian and Carboniferous rocks cannot be an effective and important source rock for many Mesozoic petroleum fields, as their hydrocarbon potential was exhausted in the late Triassic to early Jurassic interval-before trap formation. The origin of hydrocarbons accumulated in the Mesozoic reservoirs is therefore in question: they may be generated from, e.g., Carpathian Oligocene Menilite shales and migrate to the Carpathian basement ( [93], see also [94]).…”

Proceedings of the Montana Mining and Mineral Symposium 2021

“…Syntectonic pluton emplacement raises the question of how to make room for magma in environments where crustal shortening, not extension, occurs on a regional scale. Finite element modeling (Nemcok and Henk, 2006) of an analogous fold and thrust belt explored for oil in the Western Carpathian Mountains of Romania showed the existence of an overall mean stress decrease inside the thrust sheet anticlines. Our interpretation asserts that in southwest Montana similar thrust sheet anticlines were also the loci of magma ascent, mineralization, and alteration processes in syncompressional environments at the top of frontal thrust ramps where "releasing steps" at ramp tops served as initial points of emplacement, subsequent pluton growth, and exceptional levels of chemical diff erentiation within underlying laccoliths.…”