Organic structural properties of kerogen as predictors of source rock type and hydrocarbon potential

Abstract

This study improves upon previously identified correlations between the chemical structure of kerogen and potential hydrocarbon (oil and gas) yields assayed by Rock-Eval pyrolysis. We propose a quantitative structure-catagenesis relationship that predicts the hydrocarbon generation potential of source rocks and of lacustrine, marine, and terrestrial origin (types I, II, and humic coals). We used one-dimensional solid-state 13 C Nuclear Magnetic Resonance (13 C NMR) spectroscopy with 1 H spectral editing to determine the abundance of carbon functional groups, including non-protonated and mobile groups. An NMR-based van Krevelen analysis readily separated the kerogen types. Single regression matrices of NMR-based structure parameters against Rock-Eval hydrocarbon yield revealed distinct dynamics of the kerogen types upon pyrolysis. Multiple regression showed that alkyl, oxygen-substituted alkyl, and carbonyl groups were strong contributors to hydrocarbon production, while oxygen-substituted aromatic carbons were strongly counterproductive. Catagenetic relationships established for kerogen provide insight into kerogen structure evolution upon pyrolysis, and can more closely constrain the mechanisms of hydrocarbon generation for use in sedimentary basin modelling.

Improved Kerogen Models for Determining Thermal Maturity and Hydrocarbon Potential of Shale

“…In the Marcellus Shale the evolution of most of the structural parameters of kerogen such as immobile alkyl without heteroatoms (with restricted rotation), mobile (freely rotating), and alkyl-substituted aromatic carbons 19 are primarily controlled by thermal maturity. Similar observations have also been made for shales from other basins basin 17 , 23 , 28 . However, the cracking mechanism of different structural parameters of kerogen can vary because of the difference in their thermal stability.…”

“…To determine the sensitivity of different kerogen structural parameters with thermal maturation, linear correlation plots were made using the structural parameters of kerogen and calculated vitrinite reflectance (Figs 1 and 2 of Supplementary Information). It has been previously shown that maturation leads to a decrease in aliphatic and an increase in aromatic carbon chains 17 , 23 , 26 , 28 . However, the sensitivity and reactivity of carbon chains within the different aliphatic and aromatic carbon fractions are still not understood.…”

“…3 ), indicates there was little or no contribution from these structural components to the total hydrocarbons generation potential, contradicting the observations of Longbottom et al . 28 . Therefore, we propose a new equation for determining the hydrocarbon potential that is based on a multiple regression model using the fractions of structural parameters: mobile and quaternary alkyl group, mobile methyl group and immobile alkyl chains: where MM is mobile methyl chains (-CH 3 groups), MA is mobile alkyl chains (CnH 2 n groups, n >1) and IA is immobile alkyl chains (CnH 2 n groups, n >1).…”

“…However, the evolution of different molecular components of kerogen on thermal maturation and the primary contributors to HC generation are still not well understood, especially for mature shales. A recent study by Agrawal and Sharma, 2018 19 indicated that kerogen structural parameters used in previous models for determining HC generative potential 28 and thermal maturity 26 , 32 could over or under estimate these values.…”

Organic Geochemical and Petrographic Characteristics of the Coal Measure Source Rocks of Pinghu Formation in the Xihu Sag of the East China Sea Shelf Basin: Implications for Coal Measure Gas Potential

“…the organic matter, as it contributes to hydrocarbon generation potential, is of the major interest in exploration and gas production (Davis et al, 2007;Gross et al, 2015;Zhang et al, 2018). Various techniques have been used to characterize the organic matter (Petersen and Nytoft, 2006;Alsaab et al, 2007;Longbottom et al, 2016). Rock Eval pyrolysis is one of the most basic organic geochemical methods for the characterization of total organic matter potential, organic type and thermal maturity (Bertrand, 1984;Dayal et al, 2014).…”