Study On The Difference Among Deep Sandstone Reservoirs At Different Structures In Foreland Basin

In the current work, the reservoirs of frontal uplift belts in the Triassic of Akkol upheaval in Tarim basin and the forth member of Xujiahe formation in Xinchang tectonic belts in Sichuan basin were investigated, and analyzed with theories of the sedimentary geology, and reservoir geology, etc. The samples were screened via methods of thin section, scanning electron microscopy, cathode luminescence, Carbon-Oxygen isotope, physical property analysis, mercury intrusion method, etc. The relationship among tectonic evolution, reservoir sedimentary, reservoir petrologic and diagenesis was systematically analyzed, while factors of structure evolution, compaction, burial time, burial heat effect, authigenic mineral and precipitation mechanism were evaluated to set up an evolution model of sandstone reservoir’s porosity at different structures in foreland basin.Analysis on reservoir sedimentology suggests that the Triassic system of Akkol upheaval in Tarim basin locates in frontal uplift belts of Kuqa foreland basin, and formed by non-continuity bury, which consists of a slow shallow-bury in early phase and a fast deep-bury in the following phase; The sedimentary system can be categorized as braided river delta-lacustrine deposit system, while the reservoir sand deposits on sedimentary miscrofacies of the front underwater distributary channel and estuary dam of the braided river delta. The Xinchang tectonic belts, which locate in frontal depression belts of foreland basin in the western Sichuan, were formed in a continuous deposit process. The deposit process consists of fast early-phase bury and lone time deep bury. The current burial depth is shallower than the ancient burial depth because of deposit lift. There are similar characters between the two forementioned belts.Analysis on reservoir petrologic indicates that the reservoir of Triassic system of Akkol upheaval is consisted of major developed feldspar lithic sandstone, minor plastic lithic, and minor matrix. In the reservoir, the cements are porous cementation with kaolinite and analcite, and can be easily sorted with point contact and high psephicity. The reservoir underwent a long time early diagenetic stage, then the mid diagenetic stage in neogene period, and now phase A of mid diagenetic stage, in which one can find a weak diagenesis, weak compaction and mid-strong dissolution. In Xinchang tectonic belts, the reservoir of the forth member of Xujiahe formation is consisted of major developed lithic sandstone and lithic quartz sandstone with low feldspar and high plastic lithic stone. In the reservoir, the cements are mainly calcite and siliceous cements, which can be easily sorted with low psephicity and linear contact, as well as pore-contact formation. The reservoir underwent a short time early diagenetic stage, then a long time mid diagenetic stage, and now phase B of mid diagenetic stage. Characters of strong diagenesis, strong-very strong compaction and mid-dissolution were found. Among the two reservoir, the Triassic system of Akkol upheaval favors to the formation of high quality reservoirs.Analysis on reservoir characters demonstrates that the reservoir of Triassic system of Akkol upheaval consists of interparticle pores, which include residual primary interparticle pores and interpartical dissolution pores. The results of capillary pressure analysis suggest that there are low displacement pressure and high mercury intrusion saturation with platforms in the curves, indicating that there are pores with wide throat, high permeability and ordered pore structure in reservoirs. As to the physical property of reservoir, there are high pore volumes with a function of porosity-permeability, suggesting that the reservoir can be categorized as a mesoporous mid-high permeable reservoir. But in Xinchang tectonic belts, the reservoir mainly consists of developed interparticle dissolution pores. Capillary pressure analysis indicates that the reservoir’s displacement pressurthere is high,but it’s mercury intrusion saturation is low,without platforms in the curves, suggesting that there are the pores with thin throat, low permeability and non-ordered pore structure in reservoirs. Therefore, the reservoir can be categorized as an ultra-low porous and permeable reservoir.In the current work, the pore evolution model of deep sandstone reservoirs at different structures in foreland basin was proposed with factors of compaction, burial time, burial heat effect, precipitation of minerals, developmental mechanism of secondary pore, etc. The model of the reservoir of Triassic system of Akkol upheaval can be categorized into pores evolution with non-continuous burial formation conditions, in which there is an early diagenesis with long time shallow bury and short time deep bury, as well as low the heat effect of rock formation and low compaction; the primary pores well developed, and organic matter was fast transformed to reservoir with high organic acid, which brings about dissolution of feldspar with strong acid fluids in open system; the secondary pores well developed with porosity near 19.5%-21.0%, while the deep reservoir developed that the reservoir is high quality. The model of the 4th member of Xujiahe formation can be categorized into diagenesis evolution model with continuous burial formation conditions, in which there is an early bury diagenesis with long time deep bury as well as high heat effect of rock formation and strong compaction-densification, resulting low primary porosity. The reservoir evolution can be attributed to feldspar dissolution with low acidity in a closed system, while the dissolution of potash feldspar was slightly promoted by transformation of developed kaolinite intoillite. As a result, there is a tight reservoir with a final porosity near 4%-9%.