准噶尔盆地西北缘不同成因类型天然气来源及其分布规律

准噶尔盆地西北缘天然气具有多种成因类型,为了明确不同类型天然气的来源及其成藏规律,结合区域地质背景,运用有机地球化学分析和天然气藏解剖方法对其进行了研究。结果表明:该区天然气主要有油型气和煤型气两种端元类型:1油型天然气为湿气,类型为气顶气,与原油相伴生,并与原油同源(下二叠统风城组烃源岩);2煤型天然气为干气,主要来自以腐殖型母质为主的下二叠统佳木河组烃源岩,于古近纪晚期成藏。结论认为,不同类型天然气的分布具有序列性:1垂向上,中二叠统下乌尔禾组以油型气为主,佳木河组顶面不整合附近以混合气为主,佳木河组内幕以煤型气为主;2平面上,环玛湖凹陷分布的西北缘油气运移有利指向区,仅在西段克拉玛依—中拐地区分布有纯天然气藏,且在远离气源灶的方向上,依次分布油型气、混源气和煤型气;3天然气的有序分布是新近纪气源灶供气层位及分布范围、"断—盖"遮挡条件、"断—面"输导条件三者耦合的结果。

Sources and distribution patterns of natural gas of different genetic types at the northwestern margin of the Junggar Basin

Various genetic types of natural gas occur at the northwestern margin of the Junggar Basin. In order to make clear the sources and accumulation patterns of natural gas of different genetic types, geochemical analysis and gas reservoir dissection were performed in the context of regional geologic setting. The following results were obtained. There are mainly two end member types of natural gas in this area, namely oil type gas and coal type gas. As wet gas, the oil type gas occurs in the cap of oil reservoirs, and is associated with crude oil sourced from the Lower Permian Fengcheng Fm. In contrast, the coal type gas is dry gas sourced from the humic source rocks of the Lower Permian Jiamuhe Fm and accumulated in the Late Paleogene. The distribution of different types of natural gas show obvious sequential patterns. Vertically, the Lower Wuerhe Fm of the Middle Permian is dominated by oil type gas, reservoirs near the unconformity of top Jiamuhe Fm contains mainly mixed type gas, while the reservoirs in the inner part of the Jiamuhe Fm are dominated by coal type gas. Laterally, in the northwestern margin area around the Mahu Depression which is a favorable target area for hydrocarbons migration, pure natural gas reservoirs only occur in the Zhongguai area of western Karamay, and the oil type gas, mixed type gas and coal type gas are distributed successively in the direction far away from the Neogene hydrocarbon kitchen. The sequential distribution of natural gas is jointly controlled by three factors including gas source distribution, “fault caprock” sealing conditions and “fault surface” migration conditions.