地质科技通报 (May 2023)
Pore characteristics and its controlling factors in the Middle Jurassic tight sandstone reservoirs of the Shengbei Sag, Turpan-Hami Basin
Abstract
The Middle Jurassic sandstone reservoirs of the Turpan-Hami Basin are the main targets for unconventional tight oil and gas exploration and development.To better understanding the main controlling factors of pore characteristics in the Middle Jurassic tight sandstones of the Shengbei Sag, Turpan-Hami Basin, a comprehensive investigation referring to lithology, diagenesis, physical properties, and pore structure was conducted on samples obtained from 8 wells. The results indicate that the Middle Jurassic tight sandstone reservoir characterized by low porosity and ultralow permeability primarily contains feldspar lithic sandstones, followed by lithic sandstones.These sandstone reservoirs suffered strong compaction and complicated mineral cementation, replacement, and dissolution. These sandstone reservoirs are dominated by the secondary pores generated by feldspar dissolution, with some residual interparticle pores, quartz dissolution pores, clay mineral interlayer pores, and microfractures.Pore throat of 5-50 nm most widely appears in these reservoirs, while the porosity and permeability of these reservoirs mainly depend on the pore throats of 50 nm-1 μm and 100-800 μm, as indicated by the good positive correlations between pore throats and volumes. Both porosity and permeability correlate positively with the quartz and feldspar contents but negatively with the clay and carbonate contents. These correlations were supposed to be caused by two factors: ①strong compaction led to the loss of most interparticle pores but kept the residual interparticle pores associated with rigid quartz, and caused the occurrence of microfractures and dissolution pores within the quartz; the migration of hydrocarbon fluids containing organic acids into the Middle Jurassic reservoirs resulted in the significant dissolution of feldspar and generation of secondary dissolution pores. This process promoted theoccurrence of pore throats of 50 nm-1 μm and 100-800 μm and improved the porosity and permeability. ②The primary interparticle pores and secondary microfractures were filled with authigenic clay or carbonate cement; the replacement of feldspar by calcite disturbed the positive effects of feldspar dissolution on the porosity and permeability, which reduced the occurrence of 50 nm-1 μm and 100-800 μm pore throats and the physical properties of these reservoirs.Therefore, the occurrence of pores was closely related to the early sedimentary environments and the later diagenesis after deposition.More importantly, mechanical compaction, feldspar dissolution, and authigenic mineral cementation played crucial roles in regulating the occurrence of pores and the physical properties of these sandstone reservoirs in the study area.This study should be helpful in predicting the favorable exploration areas of tight oil and gas in the Middle Jurassic sandstone reservoirs in the Shengbei Sag, Turpan-Hami Basin.
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