四川盆地西北部栖霞组天然气差异成藏过程

四川盆地西北部中二叠统栖霞组天然气勘探近期获得重大突破，揭示出一个难得的冲断带-盆地系统不同构造区差异成藏典型实例。为及时总结成藏规律以供其他同类地质背景地区参考，也为下步勘探部署提供依据，本文主要从包裹体地球化学特征角度，结合其他成藏地质条件，开展了差异成藏过程研究。结果表明，以矿山梁、河湾场为代表的冲断带栖霞组储层充填大量沥青以及酸性流体沉淀矿物石英，经历了两期油气充注与一期破坏，含气性差。相比而言，以双鱼石为代表的冲断前锋带与以九龙山为代表的前缘隆起带储层沥青均欠发育，且以碱性流体沉淀矿物方解石充填为主，前者经历了两期油气充注，后者只经历了一期油气充注，含气性均相对较好。稳定保持是研究区大气藏形成的主要原因，冲断带前锋与前缘隆起随造山带推覆挤压应力减弱，虽然深大断裂欠发育导致深部下寒武统烃源岩供烃受限，但可以形成中二叠统腐泥腐殖混合型烃源岩的自生自储，稳定的构造环境有利于油气成藏后的稳定保持。盆山结合部凹陷区的深层—超深层油气勘探工作值得加以关注。

Differential gas accumulation process of the Middle Permian Qixia Formation,Northwestern Sichuan Basin

A great breakthrough has been made lately for natural gas exploration of the Middle Permian Qixia Formation, Northwestern Sichuan Basin, revealing a typical and unique case of differential hydrocarbon accumulation among different structural zones of the fold-thrust belt-basin system.Based on the analysis of geochemical characteristics of inclusions and geological conditions, we conducted the investigation of differential accumulation process, being of referential meaning to areas of similar geological setting, and of instructional significance to further exploration deployment in the study area.The results show that the Qixia Formation reservoirs in the fold-thrust belt represented by Kuangshanliang and Hewanchang areas are filled with plenty of bitumens and the quartz cements precipitated from acidic fluid following two-stage hydrocarbon charging and one-stage damage, leading to poor gas storage potential.In contrast, the reservoirs in the frontal zone of fold-thrust belt typically shown by Shuangyushi area and in the forebulge zone represented by Jiulongshan area, are filled with limited bitumen and quartz, but massive calcite cements precipitated from alkaline fluids.The former underwent two stages of hydrocarbon charging and one stage of hydrocarbon adjustment, while the latter only went through one-stage hydrocarbon charging, resulting in relatively good gas storage potential.Stable preservation is a critical factor controlling large gas accumulation formation.The underdevelopment of deeply-rooted large faults resulted in limited hydrocarbon charging from the deep Lower Cambrian source rocks under the impact of week compression stress in the frontal zone of fold-thrust belt and forebulge zone.However, the Middle Permian source rocks of sapropelic-humic types are favorable for the formation of self-sourced indigenous play.The stable tectonic setting is conducive to reservoir preservation.In all, the deep-ultra deep hydrocarbon exploration in sags of basin-mountain junction belts is worthy of more attention.