Chemical Composition of Gases as a Geochemical Tracer of Natural Gas Migration

Key words: 
• natural gas migration /
• natural gas generation /
• chemical composi tional fractionation

Abstract: The chemical compositions of gases released from sedimentary rocks by acidolysis, canned head-space gases of rocks and commercial natural gases, which were collected from different areas of various oil/gas bearing basins in China, were analyzed. Combined with natural gas generation and geological evolution of Ordos basin, Tarim basin, Ying-Qun basin and Huanghua depression, the chemical composition change resulted from natural gas migration was discussed in this paper. As a result of the obvious fractionation in the chemical composition during gas migration through porous sedimentary rock, the evidence that the molecular of methane will preferentially migrate to heavier gaseous hydrocarbons and normal butane to isobutane was recognized. 1. acidolysis gases released from sendimentary rock The compositional ratios of C₁/C₂⁺ and C₁/C total of the gases released from sedimentary rocks by acidolysis, which were collected from 3 080～3 560 m section of well Chenchuan-1 in Ordos basin, decrease with increasing of the buried depth (as shown in Fig. 1 ). The organic matter in this section is from condensate to over-mature and three gas producing layers are found in brown sandstone with TOC values less than 0.3%. However, above two ratios resulted from thermal gas generation from organic matter should tend to increase with depth corresponding to its thermal maturation. This suggests that the molecular methane preferentially transfer to heavier gaseous hydrocarbons during gas migration. 2. canned head-space gases The methane concentration and heavier gaseous hydrocarbons concentration of canned head-space gases from reservoir rocks are far greater than those from source rocks (Fig. 2a), which are collected from 3 600～4 438 m section of well Chenchuan-1. And the ratio of C₁/(C₂+C₃) is much bigger in the reservoir rock than source rock and becomes bigger and bigger with the depth (Fig. 2b). C₁and C₂⁺ concentration in the source rock of 6th to 10th section of Majiagou Ⅴ group are obviously larger than those of Majiagou Ⅰ group to Ⅳ group and the C₁/(C₂⁺C₃) ratio becomes bigger with buried depth, indicating that natural gases generated in the 6th to 10th section of Majiagou Ⅴ group is migrated and trapped in the reservoir. 3. commercial natural gases The natural gases with the same origin identified by δ¹³C₁ andδ¹³C₂ values are trapped in different reservoirs, which are distributed in Donghetan reservoir of Tarim basin, Dongfang1-1 and Luodong15-1 reservoirs of Ying-Qun basin and Qikou sag of Huanghua depression. Their chemically compositional change of C₁/C ₂⁺, C₁/C total and C total gaseous hydrocarbons /C total non-hydrocarbons in the vertical also reveals that the molecular of methane preferentially migrate to heavier gaseous hydrocarbons and hydrocarbons to non-hydrocarbons. As a result of migrational fractionation, the natural gas with much more methane than heavier hydrocarbons is trapped in relatively shallow reservoirs.