Abstract
Light hydrocarbons (C1-C9) offer a specific opportunity to study petroleum generation mechanisms. However, a significant amount of light hydrocarbons evaporate during sample collection, preservation, and preparation. This study aims to identify the origin of light hydrocarbons preserved in the closed pores of Jurassic shale strata from the Sichuan Basin where evaporative losses have been minimized. Twenty-one samples of various maturity levels and lithofacies were investigated using a new approach based on online decrepitation-gas chromatography. Light hydrocarbons were released from the closed pores of finely stratified mudstone, limestone, and siltstone. Among these, gaseous hydrocarbons in the C1-C5 range were utilized to discriminate between petroleum generated by two opposing reactions, i.e., free radical and carbenium ion cracking. Accordingly, in situ petroleum can be divided into three types. (1) Free radical cracking type: a predominance of methane and straight-chain alkanes in the gaseous hydrocarbon range; (2) Carbenium ion cracking type: a deficiency of methane and straight-chain alkanes in gaseous hydrocarbons; (3) Mixed cracking type: their gaseous hydrocarbons exhibit intermediate compositions between those of free radical and carbenium ion cracking types. Differences in the chemical composition of the gaseous hydrocarbons, which can be used to discriminate between the two generation mechanisms, were further supported by variations in the mineral composition, e.g., calcite and mixed-layer clay minerals are characteristic of free radical reaction and carbenium ion reaction, respectively. The present classification diagrams for gaseous hydrocarbons are based on the Jurassic shale strata of the Sichuan Basin. This novel approach for determining the whole range of hydrocarbons from closed pores shows promising prospects for deciphering the origin of light hydrocarbons and thus may be extended to other regions of interest.
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