Research on the Geochemical Characteristics of Quaternary Biogenic Gas Source Rocks in the Linhe Depression of the Hetao Basin: A Case Study of Well LQY1

Abstract

ABSTRACT To characterize the characteristics of Quaternary biogenic gas source rocks in the Linhe Depression of the Hetao Basin, total organic carbon (TOC), Rock‐Eval pyrolysis, and gas chromatography–mass spectrometry (GC–MS) analyses were carried out on 15 samples from Well LQY1 to assess their gas generation potential, organic matter source, depositional environment, and microbial degradation. The results show that the vertical heterogeneity of organic matter abundance is remarkable: the middle interval has the optimal quality with a TOC value of 1.21% and an S 1 + S 2 value of 2.84 mg/g, followed by the lower and upper intervals. The organic matter is dominated by Type III (humic‐type) kerogen, with the hydrogen index (HI) varying from 30.62 mg/g to 325.85 mg/g, showing prominent gas‐generating superiority. The n‐alkanes of hydrocarbon source rocks take n‐C 31 as the main peak, and regular steranes present an inverted ‘L’‐shaped distribution with the abundance order of C 28 < C 27 < C 29 , which indicates that terrestrial higher plants are the dominant parent material input. The pristane/phytane (Pr/Ph) ratios ranging from 0.41 to 0.93 and gammacerane indices of 0.16–0.22 reflect a freshwater to brackish‐water depositional environment under reducing conditions. Obvious unresolved complex mixture (UCM) humps in saturated hydrocarbon chromatograms confirm the existence of microbial degradation. This study delineates the favourable intervals of Quaternary biogenic gas source rocks and provides a reliable basis for the optimization of regional exploration targets.