Testing origin of reservoir quality difference of tight sandstones in the Yanchang Formation, Ordos Basin, China. (March 2022)

• Record Type: Journal Article
• Authors: Zhang, Fan; Jiang, Zhenxue; Xiao, Hanmin; Hu, Bin; Chen, Peng; Tang, Xianglu; Sun, Wei; Zhu, Lin; Wang, Qianyou
• Is Part Of: Marine and petroleum geology. Volume 137(2022)
• Publication Date: 2022-03
• Subjects: Lacustrine sediments -- Delta front sediments -- Pore throat structure -- Tight sandstone -- Yanchang formation; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2021.105507
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: The Chang 7 and Chang 8 members of the Upper Triassic Yanchang formation are high-yielding tight sandstone oil reservoirs deposited in different sedimentary environments in the Ordos Basin. Lacustrine sediments are dominant in Chang 7 members, while delta front sediments are dominant in Chang 8 members. The causes of reservoir quality differences in the different sedimentary environments are not clear, which renders tight-oil exploration and development difficult. In this study, X-ray diffraction analysis (XRD), optical microscopy, scanning electron microscope (SEM), mercury injection tests and oil-water relative permeability tests were used to study the origin of reservoir quality differences between the Chang 7 and 8 reservoirs. The results show the following: 1) the Chang 8 and Chang 7 reservoirs are dominated by primary intergranular pores and dissolution pores, respectively. 2) Grain-coating chlorite is present in the Chang 8 reservoir, which is known to enhance the ability of reservoirs to resist compaction and prevent the growth of quartz. However, the reservoir pore space could be preserved only when the abundance of grain-coating chlorite is below the threshold of 4 vol%. 3) Carbonate cementation occurs pore-throat spaces, and the porosity of the reservoir tends to improve with carbonate content increase in the range of 0–10 vol%. This phenomenon is caused by carbonate minerals formed in different times. 4) The strong heterogeneity of pore sizes is the reason for rapid rise of capillary force of pore mercury saturation curve in Chang 8 reservoir. The throat volume controls the mercury saturation of the pore throats. The Chang 8 reservoir exhibit larger throat volume and higher permeability than Chang 7 reservoir. 5) The Chang 7 reservoir has small pore radius and poor connectivity. Therefore, it is difficult for water to displace the oil phase. As a result, the water phase permeability increases rapidly, the residual oil saturation is high, and the water productivity of the reservoir increases rapidly. In conclusion, the reservoir quality of Chang 8 Member is better than that of Chang 7 Member, and this can be attributed to the following factors: i) Chang 8 reservoirs have larger grain sizes and more abundant grain-coating chlorite, ii) the low content of illite and carbonate cement reduces reservoir cementation capacity and iii) the pore connectivity is better, because the throat radius and effective throat volume are larger. The results provide guidance for the efficient development of tight oil in these reservoirs. Highlights: Pore-throat types, size distribution and seepage ability are different of reservoir with different sedimentary environments. The compaction and cementation decrease reservoir quality, while the dissolution has no obvious effect on permeability. The difference of pore sizes in different types is the reason for the rapid rise of pore mercury saturation curve. Rreservoir quality difference can be attributed to hydrodynamic environment, mineral type and content, pore-throat structure.