Interbedded shale formation of the 7th member of the Yanchang Formation in the Ordos Basin: Petroleum accumulation patterns and controlling factors

doi:10.13745/j.esf.sf.2022.10.17

Abstract

Abstract:

Recent breakthroughs in shale petroleum exploration in China demonstrated good exploration prospects, and Chang-7 member of the Yanchang Formation in the Ordos Basin is a major exploration area. To study in more detail the petroleum distribution patterns and accumulation controlling factors in the main production interval and interbedded shale formation of Chang-7 member, we conducted multi-scale investigation by means of reservoir profiling, core and thin section observation, reservoir rock evaluation, rock pyrolysis and field emission scanning electron microscopy, combined with microprobe analysis and physical modeling of oil charge and migration. According to the results, petroleum distribution in the interbedded shale formation is highly anisotropic at the play, oil-layer and intra-layer levels. At the play level oil accumulation is mainly controlled by source rock quality and quantity; whilst within the oil layer accumulation is controlled primarily by the reservoir/source-rock assemblage and also by the reservoir rock properties, while reservoir anisotropy due to diagenetic effects is the main cause of oil saturation heterogeneity within the oil layer. Mechanistic studies further reveal the control mechanisms of different factors on oil accumulation in the interbedded shale formation. The source rock controls oil accumulation at the play level as it provides both the driving force for hydrocarbon migration and the source of hydrocarbon while short-distance secondary migration of hydrocarbon occurs before accumulation. Oil accumulation at the oil-layer level is primarily controlled by reservoir and source-rock assemblage as the migration driving force decreases with increasing distance between oil layer and source rock. Besides, the reservoir rock properties, due to their exponential relationship with migration resistance at petroleum charging, also control the petroleum accumulation at the oil-layer level and within the intra-layer space. Whilst fractures enhance migration efficiency by reducing the threshold pressure gradient in interbedded sandstone, which promotes petroleum accumulation in the shale formation.

Key words: petroleum in shale formation, distribution, multiple spatial scales, heterogeneity, controlling factors of petroleum accumulation

CLC Number:

P618.130.2

PANG Zhenglian, TAO Shizhen, ZHANG Qin, BAI Bin, LIN Senhu, ZHANG Tianshu, CHEN Yanyan, FAN Jianwei, SUN Feifei. Interbedded shale formation of the 7th member of the Yanchang Formation in the Ordos Basin: Petroleum accumulation patterns and controlling factors[J]. Earth Science Frontiers, 2023, 30(4): 152-163.

Figures/Tables 13

Fig.1 Distribution of petroleum enrichment zones (red) and source rock distribution (blue) in Chang-7 member, Ordos Basin. Part of the data adapted from [5,27⇓⇓-30].

Fig.2 Anisotropic petroleum accumulation in Chang-7 member in different plays across the basin

Fig.3 Composite diagram showing stratigraphy column, core photos and thin sections for typical interbedded sanstone in well Z233

Fig.4 Scatter diagram showing the spread of reservoir-property data for interbedded sandstone in northeastern (blue) and central (red) plays

Fig.5 Relation between oil-layer/sandstone-layer thickness ratio and source-rock thickness in hydrocarbon reservoirs of central play

Fig.6 Relation between oil saturation and porosity in interbedded sandstone from two areas of northeast play

Fig.7 Relation between (S1+S2) value and porosity of sandstone (in proximity to source-rock) in analysis wells

Fig.8 Microscopic characteristics of interbedded sandstone

Fig.9 Microscopic characteristics of interbedded sandstone

Table 1 Sandstone sample information and key results from the physical simulation experiments on oil charging and migration in interbedded sandstone

序号	井号	深度/m	岩性	孔隙度/%	渗透率/mD	启动压力梯度/(MPa·cm^-1)
1	A83	2 190.1	粉细砂岩	6.6	0.099	0.719 99
2	A83	2 184.5	细砂岩	7.6	0.090	0.730 27
3	A83	2 194.4	中细砂岩	7.8	0.126	0.713 00
4	B15	1 949.1	粉细砂岩	11.9	0.217	0.106 57
5	C75	2 076.3	粉细砂岩	9.0	0.065	0.225 25
6	H261	2 152.2	粉细砂岩	9.5	0.219	0.215 74
7	H261	2 195.0	粉细砂岩	12.2	0.515	0.000 50
8	Z233	1 771.5	粉细砂岩	11.1	0.130	0.125 05

Fig.10 Percolation characteristics of interbedded sandstone with different reservoir properties

Fig.11 Relation between permeability and threshold pressure gradient (physical simulation experiment result)

Fig.12 Petroleum migration and accumulation phases delineated based on the key results (top panel) of physical simulation of petroleum migration and accumulation in fractured interbedded sandstone from well C75

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