近源成藏——来自鄂尔多斯盆地延长组湖盆东部“边缘”延长组6段原油的地球化学证据

延安以东和以北的鄂尔多斯盆地东北部地区曾被认为位处三叠纪延长组7段（长7段）沉积期古湖盆的"边缘",一些学者认为该区延长组烃源岩不发育,因而其油藏原油系湖盆中心生成的原油经长距离侧向运移而来;但也有研究认为,包括盆地边缘在内的整个鄂尔多斯盆地致密油藏均为近源成藏。为明确盆地东部地区延长组原油来源,对三叠纪湖盆"东缘"七里村油田的主力油层延长组6段（长6段）原油开展了原油地球化学和油源对比研究。七里村油田长6段原油具有高饱和烃、高饱芳比、低非烃和低沥青质的特征,正构烷烃呈前高单峰型,主峰碳为C₁₉,生物标志化合物分析显示长6段原油为同源成熟原油,生油母质以藻类等低等水生生物为主,混有陆源高等植物,母源沉积环境为偏还原性的淡水湖泊。七里村油田长6段原油与本地区长7段黑色泥页岩和暗色泥岩2种烃源岩均具有明显亲缘关系,而与志丹、富县等湖盆中心地区的长7段烃源岩在族组成、生物标志化合物和稳定碳同位素特征上均存在明显区别。综合分析认为七里村油田长6段原油并非湖盆中心长7段优质烃源岩所生油气经长距离运移而来,而主要为原位长7段烃源岩生成的原油经垂向运移和短距离侧向运移在长6段等储层中聚集成藏,属于近源成藏。

Near-source hydrocarbon accumulation:geochemical evidence of lacustrine crude oil from the Member 6 of Yanchang Formation,eastern margin of Ordos Basin

The northeastern part of Ordos Basin in the east and north of Yan'an was once considered to be located in the "margin" of the Member 7 of Triassic Yanchang Formation ancient lacustrine basin. Some scholars believe that the source rocks in Yanchang Formation are not developed in this area, where the oil is generated in the center of lacustrine basin and then migrated laterally from there over long distances; however, some studies have suggested that the tight oil reservoirs in the entire Ordos Basin, including the edge of the basin, are all formed by near-source accumulation. To clarify the source of oil from Yanchang Formation in the eastern part of the basin, a study is performed on the geochemistry of crude oil and oil-source correlation in the main reservoir Member 6 of Yanchang Formation of the Qilicun oilfield in the "eastern margin" of the Triassic Lacustrine basin. The Member 6 of Yanchang Formation crude oil in the Qilicun oilfield is characterized by high saturated hydrocarbon content, high saturated hydrocarbon/aromatic hydrocarbon ratio, low non-hydrocarbon and asphaltene contents. The n-alkanes show the front high unimodal type, with the main peak carbon of C₁₉. The analysis of biomarkers shows that the Member 6 of Yanchang Formation crude oil is cognate mature oil. The kerogen is mainly composed of low-grade aquatic organisms such as algae, mixed with terrestrial higher plants, and the sedimentation environment of kerogen is a partially reducing freshwater lake. The Member 6 of Yanchang Formation crude oil shows an obviously genetic relationship with the black shale and dark mudstone of Member 7 of Yanchang Formation in Qilicun oilfield, but is quite different from the Member 7 of Yanchang Formation source rocks in the central lacustrine basin zones such as Zhidan and Fuxian area in the characteristics of group composition, biomarker and stable carbon isotope. The comprehensive analysis suggests that the Member 6 of Yanchang Formation crude oil in the Qilicun oilfield is not that migrating over long distances from the high-quality source rocks of Member 7 of Yanchang Formation in the center of the lacustrine basin; it is mainly generated from the Member 7 of Yanchang Formation source rocks in the area. After vertical migration and lateral migration over short distances, it accumulates in the Member 6 of Yanchang Formation and other reservoirs, formed by near-source accumulation.