准噶尔盆地南缘天然气地球化学与成因研究

Natural gas in southern Junggar Basin in northwest China: Geochemistry and origin

准噶尔盆地南缘地区是我国天然气勘探和研究长期关注的一个热点,天然气成因过往一直认为是以侏罗纪煤系为主,但鉴于区内尚有其他多套潜在烃源岩系,因此是否存在其他来源与成因天然气并不很清楚,限制了成藏规律认识与勘探部署。通过较系统的天然气地球化学分析,包括组分、烷烃系列碳同位素、轻烃等,结合地质地球化学背景,取得了新认识,发现研究区的天然气除了已有认识到的侏罗系成因外,还有其他2种成因类型,分别为可能的三叠系/二叠系来源高—过成熟煤型气,以及古近系来源低—中等成熟油型气,而白垩系油型气可能是因为混入量相对较低而未能显现,但理应存在。这些不同类型的天然气在地球化学特征上可以显著区分,且分布具有明显的地区规律性差异,遵循"源控论",即受控于烃源岩的分布。据此,提出研究区多种成因类型的天然气为可能的规模成藏提供了物质基础,侏罗系天然气是相对最为现实的勘探目标,霍-玛-吐背斜带是下步天然气勘探的重点目标区。这些认识还可供我国中西部前陆盆地和其他具有相似地质背景的盆地进行天然气勘探和研究时类比参考。

The southern Junggar Basin has been an important area for natural gas exploration and research in China for decades, where gas has long been believed to be sourced mainly from Jurassic coal-bearing sequence. However, as multiple sets of source sequences besides the Jurassic strata were found to occur in the study area, the gas may have other sources. Therefore the previous study of gas origin limits the understanding of gas formation and accumulation and the formulation of exploration strategy. Here we address the issue of gas origin via relatively comprehensive investigation of gas geochemistry and gas geology, based on chemical compositions, carbon isotopes and light hydrocarbons. Our analytical results yield new understandings. Two other types of gases, besides the commonly-known Jurassic-derived gas, were coal-type gas sourced from Triassic and/or Permian source newly found, including （1） highly to post mature rocks, and （2） lowly to moderate mature oil-type gas sourced from Paleogene source rock. In contrast, the Cretaceous-sourced gas has not been revealed, which is likely due to a relatively low concentration in the mixed gases. These three main types of gases vary apparently in geochemistry and thus can be easily identified. The distribution of these multiple types of gases follows the theory of ＂source-control＂, i.e., being controlled by the distribution and maturity of hydrocarbon source rocks. As such, natural gases in the study area with variable origins provide the material base for possible large-scale gas accumulation, and the gases sourced from Jurassic sequence are the most realistic exploration targets, while the Huo-Ma-Tu anticline belt is the key target area. These results can also be referred in the natural gas exploration and research in foreland basins in Central-West China and in other basins with similar geological settings.