Abstract
Diagenesis is the main reason for tight reservoir lithology, complex pore-throat structure, strong heterogeneity, and variable occurrence and distribution characteristics of fluid in the pore-throat, which directly restricts the Chang 8 oil layer group in the Ordos Basin. Exploration and development. Based on the analysis of a large number of cores and thin slices, through the application of scanning electron microscopy, high-pressure mercury intrusion and other experimental testing methods, the diagenesis characteristics of the Chang 8 tight reservoir in the Hujianshan area of the Ordos Basin and the mechanism and process of tight porosity evolution are analyzed in depth. The relationship between different diagenetic facies belts, pore structures and their impact on reservoir quality is further explored. The results show that the Chang 8 reservoir in the Hujianshan area is in the mid-diagenetic stage A, which mainly experienced compaction-pressure dissolution, cementation, dissolution and other diagenesis. The compaction caused the loss of a large number of primary pores in the sandstone, which is the most important factor for the densification of the Chang 8 reservoir in the study area, followed by cementation of carbonate and clay minerals. The chlorite clay film formed in the early diagenesis retained the primary particles. The inter-pores maintain good reservoir physical properties, and are affected by the acidic diagenetic environment. The unstable feldspar minerals dissolve. The dissolution pores produced effectively improve the connectivity and permeability of the pores. The compaction reduces the porosity on average by 19.37% and the loss rate reaches 51.86%. The cementation reduces the porosity on average by 11.97% and the loss rate reaches 32.66%. The dissolution increases the porosity by 3.26% on average. In the process of pore evolution, the compaction of the early diagenetic stage A and the authigenic clay minerals and carbonate cementation in the middle diagenetic stage are the main reasons for the substantial decrease in porosity. The early diagenetic stage B is due to its short duration. Less porosity is lost. The secondary pores generated in the early diagenetic stage A are basically filled with microcrystalline calcite, while the organic acid dissolution in the middle diagenetic stage is limited. According to different diagenetic minerals and pore evolution characteristics, combined with logging response characteristics, the whole area is divided into four diagenetic facies. Among them, the residual chlorite-membrane intergranular-pore facies and the feldspar dissolution facies of chlorite film are the most favorable diagenetic facies. The findings of this study can help for better understanding of the diagenetic characteristics and quantitative evolution of pores, reveal the diagenetic-pore evolution rules of Chang 8 sandstone reservoirs in the study area, and provide a basis for reservoir evaluation and prediction in the study area.
Highlights
The tight sandstone reservoir has become a hot topic of research into the oil and gas resources (Zou et al 2012; Dai et al 2012)
The Ordos Basin, one of the major tight reservoirs in China, is endowed with gigantic volumes of oil and gas reserves resulting from the composite tightening processes involving sedimentation, diagenesis and tectonic
The original texture of the reservoir rocks was governed by sedimentary environments and sedimentary factors, sedimentary systems, microfacies, and petrographic properties of sediments affect the consolidation of sandstone reservoirs and the types of authigenic minerals(Gould et al 2010; Paxton et al 2002; Ajdukiewicz and Lander 2010; Dutton et al 2012)
Summary
The tight sandstone reservoir has become a hot topic of research into the oil and gas resources (Zou et al 2012; Dai et al 2012). Most of the research only briefly describes the sedimentary environment and diagenesis of the target reservoir, but fails to reach deeper into the differentiated diagenetic features of the reservoir and their influence on the pore evolution (Zou et al 2008). Such diagenetic features, as an immediate, comprehensive result of clastic compositions, natures of interstitial materials, pore-throat structures and evolutionary history, are of great significance to future exploration actions toward the target reservoir (Ceriani et al 2002; Fu et al, 2013; Zhu et al 1992; Chen et al 1994). The research level of the study area was changed
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