Occurrence characterization and geological significance of polar organic matter in carbonate rocks

Abstract

Organic matter in various occurrence states exists in carbonate rocks, which provides essential information for understanding diagenesis and sedimentary environment. However, the source, occurrence state, and hydrocarbon generation mechanism of carbonate organic matter are unclear. The method of automatic rapid extraction is used. Through the principle of similarity and compatibility, strong polar methanol is selected as a suitable solvent for extracting organic matter in carbonate rocks. Infrared spectroscopy is used to identify fatty acids, aliphatic ketones, fatty esters, and aliphatic hydrocarbons with oxygen-bearing groups. The aliphatic compounds, combined with the analysis of pyrolysis gas chromatography-mass spectrometry, characterize the occurrence and thermal evolution of organic matter. The results show that these organic matters are partly derived from the pyrolysis of biological lipids and partly connected with kerogen or geological macromolecules in the form of polar covalent bonds, mainly ester/ether bonds. The detection of high-abundance saturated fatty acids and n-alkan-2-ones confirms that carbonate organic matter can be physically or chemically adsorbed on the surface of carbonate minerals or encapsulated within crystalline minerals. It can interact with metal cations to form organic ligands. The evolution of pyrolysis products of carbonate organic matter at varying temperatures is related to the occurrence state of organic matter and thermal desorption/cleavage of chemical bonds, which is an effective approach for studying carbonate organic matter.