Influence of the diagenetic oxidation and allochthonous organic matter input on geochemical and petrological results – Insights from the Upper/Middle Triassic strata, Polish Basin

Abstract

Set of 24 Lower Keuperian (Upper Ladinian) and Grabfeld Formation (Lower Carnian) grey/black claystones core samples from the central part of the Polish Basin was examined by geochemical (Rock-Eval and Gas Chromatography-Mass Spectrometry) and petrological analyses. The Rock-Eval and petrological analyses revealed that the analysed organic matter is mostly mature/overmature (1.0–2.5%Ro). A combination of biomarker maturity indicators and petrological analyses suggest that autochthonous organic matter did not reach 0.9%Ro, and the higher mature organic matter particles have an allochthonous origin. Most samples contain significant input of allochthonous organic matter, which influenced the results of some geochemical indicators describing the origin of organic matter and palaeoenvironmental conditions.Investigated strata contain mostly mixed planktonic-bacterial and terrigenous organic matter. A higher contribution of terrigenous organic matter was observed within the Grabfeld Fm., where geochemical traces of fluvial-deltaic palaeoenvironment was recorded. Deposition of both strata occurred mostly under anoxic bottom waters in brackish or normal-saline marine conditions. However, an elevated salinity level was observed within Grabfeld Fm. sample from Byczyna 1 borehole.All samples contain phenylnaphthalenes (PhN), phenylphenanthrenes (PhP), and aromatic ketones, widely used as diagenetic oxidation indicators. The juxtaposition of two ratios based on phenanthrene (P), methylphenanthrenes (MP), and PhP was proposed to define the degree of organic matter diagenetic oxidation. Combined results of ƩMP/(ƩMP + ƩPhP) and P/(P + ƩPhP) revealed that Grabfeld Fm. contains more oxidised organic matter than Lower Keuperian strata. Moreover, the distribution of the PhN isomers – 1-PhN and 2-PhN – is controlled mainly by maturity level only in the least oxidised samples, where P/(P + ƩPhP) values range from 0.8 to 1.0. In other samples, the PhN distribution is controlled by the diagenetic oxidation degree. For this reason, the distribution of 1-PhN versus 2-PhN should not be used as a maturity indicator within more oxidised samples.