Abstract
The tight sandstone reservoirs of the Permian Shihezi and Shanxi Formation with strong heterogeneity constitute the main producing zone of the eastern Sulige gas field. The process of differential densification results in various reservoir qualities. Mineral composition, structural characteristic, pore system, and diagenesis were investigated with analyses of well logs, thin sections, porosity, and horizontal permeability of the core plugs; environmental scanning electron microscopy (ESEM); nuclear magnetic resonance (NMR); X-ray computed tomography (X-CT); and fluid inclusion homogenization temperature. The results show that lithic sandstone reservoirs experienced complex and various diagenetic evolutions. Eight types of densification modes can be divided according to the diagenesis paths; these modes represent lithofacies with different densification times and reservoir qualities. Intense mechanical compaction is the main reason for the formation of lithofacies 1, 2, and 5. Lithofacies 4, 6, and 7 formed due to intense cementation, increasing the impermeability of the diagenetic system. The primary pore space in lithofacies 3 is preserved due to the overpressure and chlorite coatings. The dissolution and weak cementation of lithofacies 8 contribute to reservoir development. The middle-lower part of braided channel lags and channel bars, the middle part of meandering riverbed lags, and the middle part of point bars are favourable for reservoir development.
Highlights
Since the mid-19th century, scholars have carried out numerous studies on diagenesis, diagenetic facies, and diagenetic reservoir facies as well as the diagenetic evolution sequence problem and other problems related to the densification mechanism of sandstone reservoirs [1,2,3]
According to the quantitative analyses of nearly 350 sandstone thin sections obtained from 30 coring wells, the lithologies of the He 4, He 8, Shan 1, and Shan 2 members are dominated by lithic sandstone, followed by feldspar lithic sandstone
The XRD tests show that the clay minerals are dominated by kaolinite and chlorite, which mainly appears in shallow sandstone (Figure 5(a)); an increase in the burial depth of the sandstone increases the content of illite (Figure 5(b))
Summary
Since the mid-19th century, scholars have carried out numerous studies on diagenesis, diagenetic facies, and diagenetic reservoir facies as well as the diagenetic evolution sequence problem and other problems related to the densification mechanism of sandstone reservoirs [1,2,3]. Diagenetic evolution is an extremely complex physicochemical process that is the result of a long period of fluid-rock interaction [4,5,6]. The processes of continuous, dynamic, and superimposed diagenesis in reservoirs are influenced by various factors and complex evolution processes [7,8,9]. Gu et al [11] suggested that there were three evolution pathways: predominant compaction and quartz overgrowth, predominantly chlorite coatings, and predominantly early calcite cementation
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