Characteristics comparison and occurrence mode of different types of soluble organic matter in lacustrine shale in the Dongying Sag, eastern China

Abstract

Soluble organic matter (SOM) in shale can be divided into free and adsorbed states. However, the characteristics and occurrence mode of different types of SOM remain unclear, which are of great significance to the unconventional oil and gas and hydrocarbon generation mechanism. This paper investigates the characteristics of different types of SOM, discusses the relationships between SOM and minerals, and clarifies the occurrence mode of SOM in shale. Twenty shale samples collected from the Dongying Sag were successively subjected to “organic solvent extraction - alkali hydrolysis - hydrochloric (HCl) acid hydrolysis - hydrochloric/hydrofluoric (HCl/HF) acid hydrolysis” treatments to obtain different types of SOM (extracted SOM, alkali-hydrolyzed SOM, HCl acid-hydrolyzed SOM and HF acid-hydrolyzed SOM), which were then separated into different group components by column chromatography. X-ray diffraction (XRD) and Diffuse reflectance infrared Fourier transform (DRIFT) were used to analyze the original samples and treated residues. Comparison of organic characteristics reveals: (1) Comparison of SOM content shows the order of extracted SOM (about 80%) > HF acid-hydrolyzed SOM (10%) > alkali-hydrolyzed SOM (7%) > HCl acid-hydrolyzed SOM (3%). (2) The extracted SOM is mainly composed of saturates (average content of 65.22%). Resins are the main components of the alkali-hydrolyzed SOM (38.28%) and HCl acid-hydrolyzed SOM (43.69%). For the HF acid-hydrolyzed SOM, the content of asphaltenes is the highest (76.52%). (3) Comparison of SOM polarity shows the order of HF acid-hydrolyzed SOM > alkali-hydrolyzed SOM > HCl acid-hydrolyzed SOM > extracted SOM. Based on the organic characteristics and the relationships between SOM and minerals, there are three occurrence modes of SOM in shale: (1) Free SOM in pores (extracted SOM) is free in the pores and physically adsorbed on pore surfaces. (2) Mineral-bound SOM (alkali-hydrolyzed SOM and HCl acid-hydrolyzed SOM) is chemically adsorbed onto the surfaces of clay minerals and carbonates, and is enclosed in the recrystallized carbonates. (3) SOM in inaccessible kerogen (HF acid-hydrolyzed SOM) is retained by inaccessible kerogen. These findings fully demonstrate the differences in the characteristics of SOM in different occurrence states, which are exemplified by the content, composition, polarity and adsorption. These differences provide a new insight for the study of unconventional oil and gas and hydrocarbon generation mechanism.