| Reservoir quality is a significant factor on influence oil-gas efficient exploration and development of tight sandstone reservoir.The microscopic pore-throat structure is the key factor to influence the hydrocarbon charging and migration,control the oiliness of the reservoir,determine the reservoir's storage and seepage capability,which has become a head hot and difficult issue in tight sandstone reservoir study.Due to the sedimentary and diagenesis,tight sandstone reservoirs are characterized by complex pore-throat structure,strong heterogeneity and large differences in microscopic pore-throat structures,which makes it difficult to evaluate the reservoir quality and oiliness.Therefore,in this study,taking the Triassic Chang 8 reservoir in the Maling area,Ordos Basin as an example,core observation and analyzed,the casting thin sections,scanning electron microscope(SEM),X-ray diffraction,cathodoluminescence,high pressure mercury intrusion and nuclear magnetic resonance(NMR)technology were used to quantitatively study the microscopic pore-throat structure characteristics of the tight sandstone reservoirs.Then,combining sedimentation and diagenesis,we analyzed the influence of pore-throat structure and its effects on macroscopic physical properties,storage and seepage capability and oil-gas charging of the reservoir.These can help us to further understand the mechanism of pore-throat structure for reservoir quality and oiliness of tight sandstone reservoirs,It's concluded that the main rock types of the Chang 8 member tight sandstones are lithic feldspar sandstone and feldspar lithic sandstone with low compositional maturity and moderate textural maturity.The storage space of the Chang 8 reservoir mainly developed residual intergranular pore,dissolution intragranular pore,interlayer and intercrystalline pore within clay aggregates.The shape of the throats is mostly harbor-shaped and lamellar curved-shaped with obvious curvature.The pore-throat system sizes vary from nano-to micro-scale and usually developed nano-scale pore-throat system.The pore-throat structure has the overall characteristics of a wide distribution range,poor sorting,high displacement and median pressure,high irreducible water saturation and etc,And it has become worse as reservoir's physical properties decreased.Sedimentation and diagenesis are the two major influencing factors of the pore-throat structure.The underwater distributary channels are mostly favorite to developed pore-throat structure in the sedimentary microfacies,which due to its hydrodynamics strongly and sand body developed well.Compaction is the main factor that destroys the pore-throat structure,followed by cementation and metasomatism.Dissolution has a significant effect on improving the reservoir storage capacity,while it has secondary effect on seepage capacity.The pore-throat that radius relatively large(>0.1?m)are mainly contributors to the reservoir seepage capacity in reservoirs with different petrophysical,in contrast,while radius relatively small mainly control the reservoir storage capacity and contribute less to permeability.With the decreased of physical properties,the proportion of pore throats that radius relatively small of the reservoirs increased.The reservoir that has large storage and seepage capacity and well quality has the characteristics of the small pore-throat sorting coefficient,large distortion,large median radius and high mobile fluid saturation.The oil-gas charging of reservoirs is obviously controlled by pore-throats size,content and their connectivity.In the reservoir,pores and throats jointly control oil-gas charging and the most favorable pore and throat volume radio is 1.3?2.0.The oil-gas charging is mainly controlled by radius size range from 0.077 to 1?m of pore-throat.And in these pore-throats,the larger proportion of connected pore throats,the more favorable it is for oil-gas charging and the higher oiliness of the reservoir. |
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