Title
Thermal Convection Dolomitization Affected by the Emeishan Large Igneous Province: A case from the Qixia Formation, Middle Permian, Northwestern Sichuan Basin, China
Presentation Type
Event
Start Date
27-4-2019 10:50 AM
End Date
27-4-2019 11:29 AM
Abstract
In the northwestern Sichuan Basin, the thick dolomite of the Qixia Formation in the Middle Permian is a high-quality natural gas reservoir which has achieved great commercial exploitation. Four types of dolomites, matrix dolomites (Md1, Md2 and Md3) and saddle dolomite cements (Cd) were identified by petrographic and geochemical methods. Md1 dolomite occurs very fine(<60μm)，and the original structure remains remarkably, with many peloids visible clearly. Md2 and Md3 are crystalline dolomites, which are planar-e to planar-s crystals with 120-600μm and planar-s to nonplanar crystals with 350-800μm respectively, with ghost outlines of the original structure. Cd dolomite mainly fills the dissolution vugs and/or fractures (>600μm), showing strong sweeping extinction, which is much less in the study area. Through fields and wells observation, Md2 and Md3 are commonly distributed in layers on a certain scale. Md1 only developed at the bottom of the Qixia Formation. The higher δ18O values (-6.32 to -4.02‰VPDB) and 87Sr/86Sr ratios (0.707371-0.707852) indicate that the Md1 dolomites were formed in penecontemporaneous seawater. Almost the same Mn/Sr ratios, δ12C values, REE distribution patterns, weak negative anomaly of δEu, and 87Sr/86Sr ratios (0.7073-0.7081) suggest that Md2, Md3, and Cd dolomite were all formed at shallow burial stage from formation fluids resembling Permian seawater. However, the δ18O values of Md2 (-6.92 to -4.27‰VPDB), Md3 (-7.92 to -6.55‰VPDB), and Cd dolomite (-7.37 to -6.60‰VPDB) increase in turn, indicating that the forming temperatures increased gradually. Although the northwestern Sichuan area is far away from the Emeishan large igneous province, the volcanic basalt eruption at the end of the Middle Permian still brought about abnormal geothermal temperatures, provided abnormal heat sources underneath the formation. At this time, the Qixia Formation deposited in a shallow burial period in the platform margin shoal facies, whose porosity was still good. Due to the effect of high geothermal temperature and the shelter effect of dense argillaceous limestone at the bottom of the overlying Maokou Formation, warm seawater in the formation and cold water from out of slope-basin formed rapid convection exchange in the high-porosity shoal facies, making the water-rock reaction rapidly and thoroughly, and then formed a certain amount of layered dolomites. It is considered that the dolomites in the Qixia Formation in northwestern Sichuan Basin is mainly thermal convection dolomites in shallow burial stage, and the abnormal geothermal, high-porosity condition and thermal convection and circulation of geofluids were the key factors for dolomitization.
Thermal Convection Dolomitization Affected by the Emeishan Large Igneous Province: A case from the Qixia Formation, Middle Permian, Northwestern Sichuan Basin, China
In the northwestern Sichuan Basin, the thick dolomite of the Qixia Formation in the Middle Permian is a high-quality natural gas reservoir which has achieved great commercial exploitation. Four types of dolomites, matrix dolomites (Md1, Md2 and Md3) and saddle dolomite cements (Cd) were identified by petrographic and geochemical methods. Md1 dolomite occurs very fine(<60μm)，and the original structure remains remarkably, with many peloids visible clearly. Md2 and Md3 are crystalline dolomites, which are planar-e to planar-s crystals with 120-600μm and planar-s to nonplanar crystals with 350-800μm respectively, with ghost outlines of the original structure. Cd dolomite mainly fills the dissolution vugs and/or fractures (>600μm), showing strong sweeping extinction, which is much less in the study area. Through fields and wells observation, Md2 and Md3 are commonly distributed in layers on a certain scale. Md1 only developed at the bottom of the Qixia Formation. The higher δ18O values (-6.32 to -4.02‰VPDB) and 87Sr/86Sr ratios (0.707371-0.707852) indicate that the Md1 dolomites were formed in penecontemporaneous seawater. Almost the same Mn/Sr ratios, δ12C values, REE distribution patterns, weak negative anomaly of δEu, and 87Sr/86Sr ratios (0.7073-0.7081) suggest that Md2, Md3, and Cd dolomite were all formed at shallow burial stage from formation fluids resembling Permian seawater. However, the δ18O values of Md2 (-6.92 to -4.27‰VPDB), Md3 (-7.92 to -6.55‰VPDB), and Cd dolomite (-7.37 to -6.60‰VPDB) increase in turn, indicating that the forming temperatures increased gradually. Although the northwestern Sichuan area is far away from the Emeishan large igneous province, the volcanic basalt eruption at the end of the Middle Permian still brought about abnormal geothermal temperatures, provided abnormal heat sources underneath the formation. At this time, the Qixia Formation deposited in a shallow burial period in the platform margin shoal facies, whose porosity was still good. Due to the effect of high geothermal temperature and the shelter effect of dense argillaceous limestone at the bottom of the overlying Maokou Formation, warm seawater in the formation and cold water from out of slope-basin formed rapid convection exchange in the high-porosity shoal facies, making the water-rock reaction rapidly and thoroughly, and then formed a certain amount of layered dolomites. It is considered that the dolomites in the Qixia Formation in northwestern Sichuan Basin is mainly thermal convection dolomites in shallow burial stage, and the abnormal geothermal, high-porosity condition and thermal convection and circulation of geofluids were the key factors for dolomitization.