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延安地区下古生界碳酸盐岩主体位于古岩溶斜坡,勘探证实含气性良好,但天然气的成藏过程和机理研究相对薄弱。主要采用盆地数值模拟技术,恢复出关键地质时期的古流体动力特征,分析天然气运移方向,总结上、下古生界输导体系的配置关系,根据马家沟组微观流体活动记录,如包裹体的均一温度、盐度反映的成岩环境和活动期次,及与上古生界储层的对比,探讨了下古生界天然气的成藏过程和机理。研究认为,下古生界天然气主要来自上古生界。三叠纪末,山西组发生第一期油气充注,因规模较小且向下运移动力不足,未能在马家沟组充注。中、晚侏罗世,山西组发生第二期油气充注,在向下运移动力的驱动下,沿着砂体-裂缝输导体向下运移到风化壳,由古沟槽处和岩溶高地铝土岩缺失部位进入马家沟组,在储集性能较好的古地貌构造高点以及沟槽附近聚集成藏。早白垩世,上古生界烃源岩进入生、排烃高峰期,山西组发生第三期油气充注,是天然气向下运移的主要时期,除在第二期油气充注的区域外,在东部酸性流体活动形成的次生孔隙中也可聚集成藏。晚白垩世,烃源岩生气能力降低,天然气进入运聚调整阶段。总体上,延安地区西部、中部,因长期接受上古生界天然气的运移而发生充注,并在马家沟组内发生侧向运移,容易形成较大规模聚集。
The Lower Paleozoic carbonate rocks in Yan’an area are located in ancient karst slopes, and exploration proved that the gas-bearing properties are good. However, the research on the process and mechanism of natural gas accumulation is relatively weak. Mainly using basin numerical simulation technology to recover the paleo-hydrodynamic characteristics during the key geological period, analyzing the migration direction of natural gas, and summarizing the disposition relationship of the upper and lower Paleozoic transport systems. According to the micro-fluid activity records of the Majiagou Formation, The homogenization temperature of body, the diagenetic and active periods reflected by salinity, and the comparison with the Upper Paleozoic reservoirs, discussed the process and mechanism of gas accumulation in the Lower Paleozoic. Research suggests that the lower Paleozoic natural gas comes mainly from the Paleozoic. At the end of the Triassic period, the first phase of hydrocarbon accumulation occurred in the Shanxi Formation. Due to its small size and insufficient downward mobility, it failed to charge the Majiagou Formation. In the middle and late Jurassic, the second period of hydrocarbon accumulation occurred in the Shanxi Formation. Driven by the downward migration motions, the migmatites migrated downward to the weathering crust along the sand body and fracture conductor, The missing part of highland bauxite enters the Majiagou Formation and accumulates in the vicinity of the trenches with good reservoir properties. In the Early Cretaceous, the Upper Paleozoic source rocks entered the peak stage of hydrocarbon generation and expulsion. The third period of hydrocarbon accumulation occurred in Shanxi Formation, which is the main period of downward migration of natural gas. Except for the second phase of hydrocarbon accumulation, Accumulation can also accumulate in the secondary pores formed by the activity of the eastern acidic fluid. During the Late Cretaceous, the gas generation ability of source rocks decreased and natural gas entered the stage of adjustment and adjustment of transportation and assembly. In general, the western and central Yan’an areas were recharged by the long-term transport of natural gas in the Upper Paleozoic and migrated laterally within the Majiagou Formation, resulting in a large-scale accumulation.