Gas invasion alteration and molecular fractionation in deep petroleum systems: insights from HTHP quantitative simulation

Abstract

Gas invasion is a key geological–geochemical process that induces compositional fractionation and phase transitions in deep petroleum accumulations. Previous studies have relied heavily on geochemical interpretations of natural oil and gas samples, resulting in an incomplete understanding of the dynamic gas invasion process and its fractionation mechanisms. Here, we designed and conducted a high-temperature high-pressure (HTHP) multi-cycle quantitative gas invasion fractionation simulation to monitor continuously progressive gas invasion alteration on black oil under ultra-deep conditions (140 °C, 75 MPa). Molecular composition changes in residual and carry-over oils at different gas invasion intensities were analyzed using GC, GC–MS and GC × GC-TOFMS. Results show that n-alkanes in residual oils exhibit a systematic backward shift of the peak carbon (from C7 to C10) and selective loss of low-carbon-number components (<C10). Aromaticity (toluene/n-heptane) increases from 0.27 to 0.42, whereas paraffinicity (n-heptane/methylcyclohexane) decreases from 1.71 to 1.34, revealing selective fractionation between alkanes and aromatics during gas invasion. Most conventional biomarker parameters (e.g., Ts/(Ts + Tm), Pr/Ph) remain stable, whereas some terpenoid ratios (e.g., C24 tetracyclic terpane/C26 tricyclic terpane ratio) exhibit systematic opposite trends between residual and carry-over oils. Diamondoids are progressively preferentially enriched in carry-over oils, with markedly different concentration change rates among individual isomers. This simulation verifies “differential dissolution-graded fractionation” as the core mechanism for gas invasion fractionation in ultra-deep strata, governed by differences in mass transfer rates and compound-specific fractionation rates during gas invasion. Accordingly, indicators based on stable/sensitive biomarker parameter pairs and diamondoid isomer ratios were established for gas invasion intensity evaluation. This study provides both a new perspective and novel tracing tools for understanding continuous gas invasion alteration on pre-existing oil pools under HTHP conditions and holds great significance for quantitative evaluation of secondary alteration in ultra-deep petroleum systems.