Depositional System Of Chang 7 Oil Formation Of The Triassic Yanchang Formation In Northern Shaanxi Area

The Chang7 oil reservoir group of Yanchang Formation in northern Shaanxi area of Ordos basin is the main oil sources of the Mesozoic reservoirs of the basin. As oil-generated reservoirs, there are many researches on the reservoirs. The new drilling indicates that there are well developed reservoirs in the Chang7 oil reservoir group. They have favorable conditions to form self-generated and self-stored oil reservoirs.On the basis of geological and geophysical studies, the lithologic formations, sedimentary facies, sedimentary microfacies and favourable reservoir facies tract of the Chang7 oil reservoir group of Northern Shaanxi area are comprehensively studied in the paper, by using sedimentary facies research methods, taking the oil reservoir group as a mapping unit; and adopting the predominant facies mapping rules to prepare the individual well histogram and planar distribution map of the sedimentary facies. The sedimentary system types, characteristics, and sedimentary facies planar distribution of the Chang7 oil reservoir group of Northern Shaanxi area are summed up in the paper. Distribution features of the sands are predicted. The relationship between Chang7 sedimentary sands and the oil reservoirs is analyzed with combination of analysis data of the discovered Chang7 oil reservoirs. And the main results achieved and understanding can be summarized as follows:The strata of Yanchang Formation of the Ordos Basin are compartmentalized and contrasted in details. A set of regional marker beds are identified in the basin's Yanchang Formation. Lake strandline and planar distribution of Chang 7 oil reservoir group sedimentary system are confirmed. It is identified that Chang 7 oil reservoir group are composed of lake sedimentary systems and delta sedimentary systems.Accordance with the flow orientation of the paleo-channels and the enrichment law of heavy minerals and lanthanon (rare-earth elements), the source rock of Chang7 oil reservoir group in the study area is mainly from the northeast Daqingshan sources, the northwest Alxa paleo-land sources, and the southwest Qinling Mountains sources.The Chang7₃ Age was the peak period for the lake basin. The deep lake lines are in Xunyi - Changwu - Jingchuan - Zhenyuan - Huanxian - Majiatan - Wuqi - Zhidan -Huanglong area; the northeast Lakeshore line is in the Hengshan - Zichang -Yanchuan area. Petrofacies palaeogeographical distribution features of the Chang72 Age are basically in succession of the pattern of Chang7₃ Age. The lake water area in the basin had an overall declining trend. The corresponding delta plain facies tract was widened; The petrofacies palaeogeography of the Chang7₁ Age are basically in succession of the pattern of Chang7₂ Age. As a whole the lake water area in the basin had an obviously decreasing trend compared with that of Chang7₂ Age. The obvious feature of Chang7₁ Age is that the turbidite sands and front facies sands are thicker than those of Chang7₂ and Chang7₃ Ages.The Chang7 sedimentary system controls the distribution of the oil reservoirs. Early semideep lake - deep lake sediments supply the reservoir-forming with quality source rocks; the turbidite sands in the central lake provide quality reservoirs for oil and gas reservoir-forming.