Influence Mechanism Of Multistage Oil Emplacements On Carbonate Cementation In The Fourth Member Of The Shahejie Formation In The North Of Minfeng Sag

The influence of oil emplacement on diagenesis is a topic of great concern and is still surrounded by controversy.In this research,the clastic reservoirs in the upper part of fourth Member of the Shahejie Formation in the north of Minfeng Sag were taken as the research object.Origin of carbonate cements and chronological relationship between oil emplacement and carbonate cementation were discussed by casting/fluorescent thin sections observation,cathode luminescence,homogenization of aqueous fluid inclusions,back-scattered observation,in situ analyses of Sr isotope and trace elements,analysis of C,O isotope after sampling by micro-drilling.The mechanism of the influence of multi-stage oil emplacement on carbonate cementation was discussed by combing with case study and physical simulation experiment.Calcite,dolomite,ferroan calcite and ankerite are discovered,but the ferroan calcite and ankerite are the main carbonate cements.The Ca sources of ferroan carbonate cements in the edge of sandstones were from the Ca²⁺released from smectite to illite and Ca²⁺in pore water in the adjacent mudstones.The Ca sources of ferroan carbonate cements in the middle of sandstones were from the Ca²⁺released from smectite to illite and carbonate minerals dissolution and Ca²⁺in pore water in the source rocks.The CO₂ released from organic maturation and the CO₃^2-released from dissolution of carbonate minerals in source rocks provide the C source of ferroan carbonate cements.Two stages of oil were emplaced in study area with yellow fluorescence and blue fluorescence,respectively.Chronological relationship between oil emplacement and carbonate cementation is shown as dolomite cementation-first stage oil emplacement-ferroan calcite and ankerite cementation-second stage oil emplacement.The microcosmic experimental research results of the influence of multi-stage oil emplacement on carbonate cementation showed that the oil migration paths would be skewed to the surfaces of oil-wetted calcite cements.The second stage of charged oil migrated along with the paths of first stage charged oil.The carbonate cementation was occurred in the water-filled pores and water films but stopped in the oil-filled pores as the result of heterogeneous distribution of oil and water.The research results of case study showed that Positive correlation is characterized between GOI index of yellow fluorescent oil inclusion(YGOI)and total content of carbonate debris and dolomite cements.Negative correlation is characterized between GOI index of yellow fluorescent oil inclusion(YGOI)and the content of ferroan carbonate cements.But the formation temperatures of ferroan carbonate cements in reservoirs with different YGOI are similar.Positive correlation is also characterized between GOI index of yellow fluorescent oil inclusion(YGOI)and GOI index of blue fluorescent oil inclusion(BGOI).In the processes of first stage of oil emplacement,the electrostatic force(between the earlier formed carbonate minerals and oil molecules)and the interaction force between oil molecules would cause the oil migration path to skew to the surfaces of carbonate minerals.The first stage charged oil and oil-wetted carbonate minerals would reduce the breakthrough pressure in the pore throat spaces,which facilitated the second stage of oil emplacement.The heterogeneous distribution of oil and water controlled by pore structure and wettability would cause the exogenous materials to continue to migrate in the water-filled connected pores and water films in the form of advection or diffusion and cause the following carbonate cementation,while the transport and cementation would stop in the oil-filled pores.Synergism between oil emplacement and carbonate cementation jointly control the formation of good reservoirs and the process of oil accumulation.