Genesis of the tight reservoirs in the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag

doi:10.13745/j.esf.sf.2020.5.19

Abstract

Abstract:

The Bonan Subsag is an important oil and gas-rich region of the Jiyang Depression, Bohai Bay Basin. The deeply buried tight glutenite reservoir in the northern Bonan Subsag is significant for hydrocarbon recovery from the upper fourth member of the Shahejie Formation, and understanding its genesis is the basis of exploring new reservoirs in this area. Here, using various techniques, including drilling, thin section analysis, scanning electronic microscope, physical property measurement, mercury penetration and nuclear magnetic resonance, we show that the tight reservoirs can be recognized in two kinds of fan delta deposits and they were influenced significantly by differential depositions. The fan large delta formed in the early stage hosts an extensive deposition of well separated coarse and fine-grained sandstones due to strong mechanical differentiation, and showed a relatively good correlation between porosity and permeability. Whereas the small fan delta formed in the later stage is composed of poorly separated mix-grained sandstones that accumulated rapidly near the source of clasts, and there was no apparent correlation between porosity and permeability for this reservoir. The source sediment should contain sufficient amount of hard and soluble rock fragments which can effectively improve reservoir property. We show that the tight reservoirs are mainly composed of lithic arkose and feldspathic litharenite and generally immature both texturally and compositionally. Commonly observed were dissolved and primary pores, and flaky or curved lamellar throats, with poor pore connectivity. Five types of pore structures resulted in strong reservoir heterogeneity. We further show that compaction and cementation were important factors for destroying the reservoir properties of the upper fourth member of the Shahejie Formation in the northern Bonan Subsag. Cementation was multitype, multistage and had strong destructive impacts, and the degree of lillite cementation, specially, had a significant influence on the reservoir. Dissolution, fracturing and overpressure were beneficial for improving reservoir properties, however, their overall influence was limited. We then discuss the genesis of tight reservoirs in the upper fourth member of the Shahejie Formation, northern Bonan Subsag, and reveal that the coarse-grained sandstone, containing roughly equal amounts of quartz, feldspar and lithic fragments and a little illite, was beneficial for making good reservoirs in the study area. Besides porosity and permeability, pore size and pore to throat size ratio are key parameters in reservoir evaluation. As it has been demonstrated, higher porosity and permeability, larger pore size and smaller pore to throat size ratio can result in better reservoirs.

Key words: tight reservoir, glutenite, genesis, upper fourth member of Shahejie Formation, Bonan Subsag, Zhanhua Sag

CLC Number:

WU Dong, LIU Xiantai, DU Yushan, ZHU Xiaomin, JIANG Long, GENG Bin, CHENG Ziyan, GUO Shibo. Genesis of the tight reservoirs in the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag[J]. Earth Science Frontiers, 2021, 28(1): 202-213.

Figures/Tables 10

Fig.1 Tectonic boundaries of the northern Bonan Subsag and generalized stratigraphic column for the Bonan Subsag, Zhanhua Sag. Modified from [11,12,13].

Fig.2 Sandstone classification triangle diagram for the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag. Modified from [11].

Fig.3 Reservoir properties of the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag

Fig.4 Pore throat and diagenetic characteristics of glutenite reservoir in the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag

Fig.5 Injection-Hg capillary pressure curve characteristics of the glutenite reservoir in the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag

Fig.6 Sedimentary microfacies distribution of the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag. Modified from [23].

Fig.7 Relationship between the detrital content and reservoir property of the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag

Fig.8 Relationship between clay mineral content and reservoir property of the upper fourth member of Shahejie Formation in the northern Bonan Subsag, Zhanhua Sag

Fig.9 Relationship between porosity and pressure coefficient of the upper fourth member of Shahejie Formation in the northern Bonan Subsag, Zhanhua Sag. Adapted from [29].

Fig.10 Distributions of pore and throat sizes for different samples from the upper fourth member of Shahejie Formation in the northern Bonan Subsag, Zhanhua Sag

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