Genetic Mechanism Of Dissolution Of Feldspars And Carbonate Minerals During Diagenesis And Its Impact On Reservoir Poroperm

Diagenetic fluids in clastic reservoirs of petroliferous basins,and mineral dissolution,mass transfer and precipitation of secondary minerals that are controlled by these geological fluids are at the cutting edge of research of diagenesis.And these physical and chemical reactions have significant impact on reservoir quality evolution.This dissertation focuses on clastic reservoirs in the middle part of the 3rdmember?Es₃²?to the 4thmember?Es4?of Shahejie Formation in the Shengtuo area,Dongying Sag,and clastic reservoirs in the 3rdmember of Shahejie Formation?Es3?in the Gaoliu area,Nanpu Sag.With utilization of sedimentary petrology,experimental geochemistry,simulation geochemistry and petroleum geology,phenomena on selective dissolution of feldspars and carbonate minerals,and heterogeneous precipitation of secondary minerals in clastic reservoirs were identified.Diagenetic events and key diagenetic fluids in Es₃²-Es4 reservoirs in the Shengtuo area and in Es3 reservoirs in the Gaoliu area were studied.Based on numerical simulations of fluid-rock reactions with constraints of thermodynamics,selective dissolution of feldspars in the presence of carbonate minerals by CO₂ and organic acids from decarboxylation of organic matter in relatively closed geochemical system in the Shengtuo area was proposed to be responsible for secondary porosity generation.Also,chemistry of dissolution of both feldspars and carbonate minerals by abundant CO₂ from meteoric water and decarboxylation of organic matter for secondary porosity development in relatively open geochemical system in the Gaoliu area was deciphered.With combination of numerical simulations restrained by mineral dissolution,mass transfer and secondary mineral precipitation and features of feldspar secondary pores and secondary minerals?authigenic clays and quartz cements?,the impact of mineral dissolution on reservoir poroperm and redistribution pattern of byproducts in open and closed geochemical systems was distinguished.Petrography evidences demonstrate four types of diagenetic assemblages of feldspars and carbonate minerals in clastic reservoirs,which are little feldspar dissolution with extensive carbonate precipitation?I?,extensive feldspar dissolution-little carbonate dissolution?II?,extensive feldspar dissolution-extensive carbonate dissolution?III?,and little feldspar dissolution-extensive carbonate dissolution?IV?,respectively.Type I and type II are common in Shengtuo reservoirs,while type III is developed in Gaoliu reservoirs.Mineral textures in clastic reservoirs suggest five types of mineral assemblages of leached feldspars,authigenic clays and quartz cements,which are weakly leached feldspars-few clays and quartz cements?I'?,weakly leached feldspars-massive authigenic clays and few quartz cements?II'?,extensive leached feldspars-massive authigenic clays and quartz cements?III'?,extensive leached feldspars-massive authigenic clays and few quartz cements?IV'?,and extensively leached feldspars-small amount of authigenic clays and quartz cements?V'?.Type I' and type III' are common in Shengtuo reservoirs,type IV' in Minfeng area.Type V'are common in most Gaoliu reservoirs,and type II' is identified in some Gaoliu reservoirs.Chemical calculations show that the equilibrium constants of K-feldspar leaching reactions are generally higher than that of calcite leaching reactions when these minerals are leached by CO₂ or CH₃ COOH in a K-feldspar-calcite-acid?CO₂ or CH₃COOH?-H2 O system.Numerical simulations demonstrate that in K-feldspar-calcite-acid-H2 O system,lower pH is required for occurrence of calcite dissolution.With the lower equilibrium constants and the lower pH,dissolution of calcite relies more strongly on the volume scale and aggressiveness of fluids in the fluid-rock system,leading to selective dissolution of feldspars in the presence of carbonate minerals for extended periods of time.Simulation results demonstrated that the increase of acid concentration and water volume promotes dissolution of both K-feldspar and calcite.Salt effect of ions promotes mineral dissolution,while common-ion effect inhibits mineral dissolution.Precipitation of illite instead of kaolinite can promote dissolution of more K-feldspars.The Es₃²-Es4 sub-members in the Shengtuo area are characterized by a few faults and moderate to strong fluid overpressure.Stable isotopic data of authigenic minerals and pore water chemistry suggest a relatively closed geochemical system in the Shengtuo area and little impact of meteoric water during the burial diagenetic process.In such a geochemical system,fermentative degradation of organic matter in mudstones in eodiagenetic stage provided carbon source for early carbonate cementation and led to extensive precipitation of carbonate minerals in sandstones at the margin of sandstone beds,forming mineral assemblage of little feldspar dissolution with extensive carbonate precipitation.In mesodiagenetic stage,the volume of pore fluids in Shengtuo reservoirs was limited without meteoric water supply,leading to suppression of carbonate dissolution.Organic CO₂ and organic acids from decarboxylation of the organic matter in mudstones were transported into reservoirs,and these acids selectively dissolved feldspars to generate large amount of secondary pores.The Es3 members in the Gaoliu area are commonly characterized by widely developed faults and normal fluid pressure,and weak to moderate fluid overpressure occurs only in some deeply buried local reservoirs.Stable isotopic data of authigenic minerals and pore water chemistry suggest a relatively open geochemical system in the Gaoliu area and significant impact of meteoric water during burial diagenesis process.In initial burial stage,early calcite minerals were precipitated with increase of temperature.During uplift and re-burial stage,large amounts of meteoric water penetrated into the Es3 member through the widely developed active faults.At the same time,decarboxylation of organic matter in mudstones could provide some acids.With large volume of water and acids from meteoric flux and organic matter,early carbonate cements and feldspar were dissolved to form large amount of secondary pores.With injection of CO₂-charged fluids into sandstone beds,numerical simulations with coupled mineral reactions and mass transfer and constraints of suitable geological factors suggest the development of dissolution zone,transitional zone and precipitation zone from upstream to downstream.In the dissolution zone,K-feldspar is dissolved with no secondary mineral precipitation;in the transitional zone,K-feldspar is dissolved with precipitation of kaolinite;in the precipitation zone,K-feldspar is dissolved with precipitation of both kaolinite and quartz.Higher temperature,lower fluid velocity,faster mineral reaction rates and higher ion concentration in injected fluids are inclined to shorten the dissolution zone and the transitional zone;while lower temperature,higher fluid velocity,slower mineral reaction rates and lower ion concentration in injected fluids tend to extend the dissolution zone and the transitional zone.In strata with few faults,mineral assemblage V' probably occurs insandstone beds close to the earth's surface characterized by low temperature and high water flow rates;mineral assemblage ?' may occur in shallow sandstone bed with moderate temperate and moderate water flow rates;mineral assemblage ?' probably occurs in buried sandstone with high temperature and low water flow rates.The existence of widely developed fracture system increases the possibility of development of mineral assemblage ?' and ?' in moderate to deeply buried sandstone beds.In the closed geochemical system characterized by high temperature and low water flow rates in the Shengtuo area,carbonate cements were not dissolved.Byproducts from leached feldspars were precipitated in situ,and secondary pores formed by feldspar dissolution are redistributional secondary pores and mineral dissolution did not improve reservoir poroperm.In the open geochemical system characterized by massive meteoric water supply,high temperature and high water flow rates in the Gaoliu area,feldspars and carbonate cements were dissolved extensively.Byproducts from leached feldspars were removed from sandstone reservoirs,and feldspar secondary pores are enhanced secondary pores.Mineral dissolution did improve reservoir poroperm significantly.