Formation Mechanism Of Carbonate Cements And Its Influence On Reservoir Quality In Shahejie Formation,Raoyang Sag

The reservoirs of the Shahejie Formation of the Paleogene in Raoyang Sag have deeper burial and carbonate cements are more developed.The forming stages of carbonate cements of 800 flakes of 22 explorations in the Shahejie Formation of the study area are divided by thin-section identification,scanning electron microscopy and cathodoluminescence analysis.Also according to coexisting minerals study,carbon and oxygen isotope analysis,and homogeneous temperature of fluid inclusions of each period of carbonate cements,the forming mechanism of the carbonate cements is studied.The impact of matter on reservoir quality has mainly achieved the following achievements:?1?The main rock types of the Shahejie formation are feldspar sandstone and lithic feldspathic sandstone.The advantages of quartz grain development in the reservoir are obvious.At the same time,the unstable component feldspar is more developed than the rock debris,and the Shahejie Formation reservoir sandstone shows low maturity.The sandstone grain size is mainly dominated by medium-micro sands,and the grain separation is mainly based on good and medium.The roundness of the sandstone is developed from sub circle,sub-sub-sub circle,sub circle-sub-edge and sub-edge.The content is low and there is little difference between the layers.?2?The types of reservoir space in the Shahejie formation mainly include primary pores,secondary pores,and microfractures.The primary pores include primary intergranular pores,primary pores,and a small number of micropores.The secondary pores are mainly intergranular pores,mold hole,intragranular hole and intercrystalline hole.The porosity of the reservoir is low,the average porosity is about10%.In general,it is a low-pore,ultra-low pore grade;the permeability is low-permeability,ultra-low-permeability,ultra-low-permeability,and the permeability is very strong.There are distributions from hypertonic to dense layers,and the porosity and permeability are positively correlated.?3?The authigenic minerals in the Shahejie Formation reservoir rocks are mainly calcite,dolomite,iron-calcite,iron-dolomite,siliceous,anhydrite,pyrite,and kaolinite.Carbonate cement is very developed,especially Calcite cement is the most developed,dolomite is second,iron calcite and iron dolomite are relatively few,and other clay minerals and siliceous cement are also developed.?4?There are totally three stages of carbonate cements developed in the study area:the first stage was dominated by granitic calcite,belonging to mud-and-powder grades,which was formed in the early diagenetic stage before rocks was fully compacted,accompanied by a small amount of pyrite cement;the second stage is mainly granular fine-grained,calcite and dolomite are developed in forms of plaques and interlocking,and clay film often formed between this stage and early stage cement;the third stage is columnar or granular fine-grained,with calcite,dolomite,iron calcite and iron dolomite as the main components,and these components filled in dissolved pores such as feldspar and metasomatic early stage carbonate.?5?The homogeneous temperature of inclusions in the first stage of carbonate cements developed in the Shahejie Formation reservoir was 57⁸3.5°C,with an average of 71.75°C.It is in the A₁ stage of diagenetic rocks,and the formation time was 30.31²7.53Ma,before the first large scale hydrocarbon filling.The homogeneous temperature distribution of carbonate cement inclusions in the second stage ranged from 89¹15.5°C,with an average of 109.04°C.It was in the A₂ stage of diagenesis,and the formation time was 14.21Ma¹1.43Ma.Before the second stage of hydrocarbon charging.The third phase carbonate cement inclusions was between 121.2¹29.8°C,with an average of 125.5°C.It was in the B phase of diagenesis with a formation time of 6.42³.33Ma.It was in the second stage of the middle stage of hydrocarbon filling.The diagenetic environment gradually transitions from weak acidity to alkaline environment.?6?According to the carbon and oxygen isotope composition of calcite cement in the Shahejie Formation reservoir restores the ancient salinity Z value both less than120,which is the reason for the continuous salinization of high salinity in freshwater.The paleotemperate is estimated to be 65¹29.9°C with an average of 102.8°C,mainly concentrated at 100¹10°C,and the fluid inclusions have a uniform temperature distribution range of 100¹40°C and an average of 115°C,indicating that the calcite cement formed in the A stage of the diagenesis.Decarboxylation of organic acids produces large amounts of CO₂,the chemical equilibrium shift towards the direction of generation of carbonate,and precipitates as carbonates,which means the carbon is sourced from organic carbon.?7?The controlling factors for reservoir development in the Shahejie Formation include sedimentation,diagenesis,tectonism,hydrocarbon charging and abnormal pressure.Sedimentation influences the maturity of components and the composition of rock skeletons through source,the higher the content of unstable components,the worse the physical properties;compaction and cementation are destructive diagenesis,while dissolution is a constructive diagenesis;The role of double-sided features,squeeze the pores to densify the role,tensile effect of cracks communicate pores,increasing the permeability;hydrocarbon charging play a positive role in the reservoir,acid fluids not only the erosion of debris particles,Moreover,hydrocarbon charging inhibited the development of carbonate cement;under the overpressure system,reservoirs in Raoyang Depression developed well.?8?The carbonate cements in the Shahejie formation were formed from the early diagenesis stage to the middle diagenetic stage.Cementation reduction is the main reduction method in the study area.The early carbonate cementation increased the compressive strength of the sand body and thus suppressed the compaction to a certain extent.Therefore,the early cementation plays a positive role in improving the reservoir quality.However,carbonate cements in the middle and later stages fill the pores formed by the dissolution of feldspar and rock fragments,reduce the porosity and permeability of the reservoir,and have a negative impact on reservoir quality.