Revealing Deepwater Gas Maturity through Continuous Light Hydrocarbon Fingerprinting under Oil-Based Mud Conditions: Integrated Drilling Mud and Cuttings Headspace Gases Analysis

Summary Precise hydrocarbon maturity assessment is critical in deepwater exploration. Traditional methods rely on core and fluid sampling, incurring significant costs and technical constraints. Complex geological settings and drilling alterations complicate sample representativity, necessitating continuous, cost-effective maturity detection methods for oil-based mud (OBM) systems. This study assesses the compositional fidelity of drilling mud gas (conventional/Flair logging) and cuttings headspace gas (CHSG) in OBM systems through continuous sampling at Well Y52, located in the Baiyun Sag, Pearl River Mouth Basin. The target interval, exceeding 5000 m depth, exhibits thermal maturities up to 1.7% vitrinite reflectance (VR). Systematic integration of gas chromatography (GC), stable carbon isotope (δ13C) analysis, and VR measurements reveals maturation-driven trends in hydrocarbon composition. Analysis indicates methane δ13C and C1–C5 compositional profiles in mud gas retain diagnostic fidelity to subsurface reservoir conditions, though incomplete OBM degassing obscures quantitative C6–C7 characterization. Despite volatile loss artifacts and potential interference in CHSG samples, select light hydrocarbon parameters—including isobutane/normal butane ratios, quaternary/ternary isoheptanes 2,4-dimethylpentane/2,3-dimethylpentane, and aromatic hydrocarbon indices—preserve diagnostic utility for thermal maturity assessment. Under elevated thermal maturity, i-C4/n-C4 ratios and aromatic indices exhibit heightened sensitivity to thermal evolution relative to methane δ13C. Notably, these proxies exhibit vulnerability to secondary processes or limited sensitivity, necessitating cautious interpretation. Conventional empirical models linking methane δ13C and light hydrocarbon indices to VR display reduced applicability in offshore settings. This underscores the imperative for multiparameter frameworks integrating isotopic fractionation patterns, molecular thermal stability hierarchies, and source controls to optimize maturity calibration. The study advances methodological protocols for light hydrocarbon applications in complex offshore basins while delineating critical limitations in existing thermodynamic models.