Abstract
Deeply buried sandstones in the Jurassic, Toutunhe Formation, are a crucial exploration target in the Junggar Basin, NW China, whereas, reservoir-forming process of sandstones in the Toutunhe Formation remain unknown. Focused on the tight sandstone of the Toutunhe Formation, the impacts of diagenesis and hydrocarbon charging on sandstone reservoir-forming process were clarified based on the comprehensive analysis of sedimentary characteristics, petrography, petrophysical characteristics, and fluid inclusion analysis. Three diagenetic facies developed in the Toutunhe sandstone reservoirs, including carbonate cemented facies (CCF), matrix-caused tightly compacted facies (MTCF), and weakly diagenetic reformed facies (WDF). Except the WDF, the CCF and the MTCF entered the tight state in 18 Ma and 9 Ma, respectively. There was only one hydrocarbon emplacing event in sandstone reservoir of the Toutunhe Formation, charging in 13 Ma to 8 Ma. Meanwhile, the source rock started to expel hydrocarbons and buoyancy drove the hydrocarbon via the Aika fault belt to migrate into sandstone reservoirs in the Toutunhe Formation. During the end of the Neogene, the paleo-oil reservoir in the Toutunhe Formation was destructed and hydrocarbons migrated to the sandstone reservoirs in the Ziniquanzi Formation; some paleo-oil reservoirs survived in the WDF. The burial pattern and change of reservoir wettability were major controlling factors of the sandstone reservoir-forming process. The buried pattern of the Toutunhe Formation in the western section of the southern Junggar Basin was “slow and shallow burial at early stage and rapid and deep burial at late stage”. Hence, pore capillary pressure was extremely low due to limited diagenetic reformation (average pore capillary pressures were 0.26 MPa). At the same time, high content of chlorite coating increased the lipophilicity of reservoirs. Therefore, hydrocarbons preferably charged into the WDF with low matrix content (average 4.09%), high content of detrital quartz (average 28.75%), high content of chlorite films (average 2.2%), and lower pore capillary pressures (average 0.03 MPa). The above conditions were favorable for oil and gas enrichment.
Highlights
The formation mechanism of sandstone reservoirs mainly involves two aspects, namely the reservoir rock formation mechanism and the hydrocarbon emplacementaccumulation mechanism
The weakly diagenetic reformed facies (WDF) features a high content of detrital quartz and zeolite and the development features a high c environments mostly presents itself as the basal cementation of calcite, and the contentofofshore-lake, ferrocalcit and ferrodolomite in the Toutunhe Formation is less than that in the Badaowan Formation (Figure 4d–g)
The results show that the compaction-induced porosity creased the compaction resistance of sandstone loss was about 2% to 62.5% for the Toutunhe sandstone in the Kayindike area (Figure 3b)
Summary
The formation mechanism of sandstone reservoirs mainly involves two aspects, namely the reservoir rock formation mechanism and the hydrocarbon emplacementaccumulation mechanism. Previous studies have indicated the obvious control of the sedimentary environment on the Jurassic reservoir rocks in the Sikeshu Sag [28,29,30,31] They generally fail to consider the effects of diagenetic modification and hydrocarbon emplacement on reservoir quality. It is of urgent importance to clearly describe the reservoir-forming mechanism of the Toutunhe sandstone in the Kayindike area to clarify whether or not the reservoir quality and exploration potential of the northern Sikeshu Sag are analogous to those of the southern part. This research targets Toutunhe Formation sandstone in the Kayindike area of the Sikeshu Sag, the South Junggar, and the objective is threefold: (1) to illustrate diagenetic processes in different facies and discuss the reasons for sandstone densification; (2) to investigate the temporal sequences of sandstone densification and hydrocarbon emplacement;. (3) to reveal the formation process of the Toutunhe sandstone reservoir
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