Dissolution Mechanism And The Origin Of High Quality Reservoirs Of The Lower Paleozoic Carbonate Rocks In Jiyang And Huanghua Area

During the diagenetic process of carbonate rocks in petroliferous basin,dissolution is the key factor for the development of high-quality reservoirs.The studies on key issues such as the time of dissolution,the origin of diagenetic fluid,the diagenetic environment of dissolution are of great importance for identifying the genesis and distribution of high-quality reservoirs.This study is exemplified by the lower Paleozoic carbonate rocks in Jiyang-Huanghua area.And the objective of his study is to depict the reservoir characteristics and identify the diagenetic fluids based on a combination of core observation,impregnated thin section identification,cathodeluminescence image,SEM,EPMA,SIMS,LA-ICP-MS and fluid inclusion analysis.And the origin of high quality reservoirs will be studied with the constraint of diagenetic evolution controlled by tectonic evolution as well as dissolution mechanism of carbonate.It shows that the lower Paleozoic carbonate rocks in study area are relatively tight.Primary porosity is rare and secondary pores,vugs as well fissures predominate in the reservoirs.The buried hills with different cap rocks show different distribution law for reservoir types.When the buried hills are covered by the Mesozoic or Cenozoic,the reservoirs show vertical zonation.The areas closed to the upper unconformity are dominated by pore-vug-fissure type reservoirs,whereas the parts which are far away from the unconformity are dominated by fissures.The buried hills covered by the upper Paleozoic show lateral zonation.Within the fault zone,pore-vug-fissure type reservoirs and fissures predominate,and fissures gradually become important when it gets far from the faults.When the strata are composed of dolostone,intercrystalline pore-fissure and pore-vug-fissure type reservoirs will developed in the inner buried hill.Modified by different kinds of diagenetic fluids,such as syndepositional intergranular water,meteoric water,hydrothermal fluid,diagenesis converted water,compaction released water and hydrocarbon,the lower Paleozoic carbonate rocks are characterized by multistage-polymorphic type cementation and multistage-polycomponent dissolution.The tectonic and diagenetic evolution of different buried hills shows that the diagenetic evolution of the lower Paleozoic carbonate rocks are consistent before late Triassic,which are characterized by cementation casused by syndepositional intergranular water from Cambrian to middle Ordovician,epigenetic karstification and cementation resulting from meteoric water from late Ordovician to early Carboniferous and burial cementation originated from diagenesis converted water from late Carboniferous to middle Triassic.According to the diagenetic evolution differences since late Triassic,the reservoirs can be divided into three types: “late stage-exposure” type,“middle stage-exposure” type and “no middle to late-stage-exposure” type.“Late stage-exposure” type buried hills continuously uplifted from late Triassic to early Himalayan period,and became exposed to the air during late Yanshan to early Himalayan period,which was followed by burial diagenesis during late Himalayan period.The “middle stage-exposure” type buried hills continuously uplifted from late Triassic to middle Jurassic period,getting exposed to the air during early to middle Yanshan period,followed by burial diagenesis from late Yanshan period to Himalayan period.Although experiencing different stage of tectonic evolution since late Triassic,the “no middle to late-stage exposure” type reservoirs continuously kept in burial stage,underwent the modification of various diagenetic fluid such as diagenesis converted water,percolating meteoric water,asending hydrothermal fluid and compaction released water.The diagenetic evolution before late Trassic resulted in the densifieation of the reservoirs.The epikarstification and burial dissolution since Yanshan period are the key factors for the origin of high quality reservoirs in the lower Paleozoic carbonate rocks.The study based on the combination of diagenetic simulation and practical geological phenomenon show that the continuous flow of aggressive fluid under open to semiopen environment is the prerequisite for large scale dissolution of carbonate rocks.The development of fissure and intercrystalline pore in the rocks,which increase the contact area between mineral and fluid,can promote the rate of dissolution.It is the development of fissure and intercrystalline pore that promotes the forming of effective “spatial dissolution reservoirs”.The mineral component and the density of fissure and intercrystalline pore in the rocks together control the dissolution rate of carbonate rocks.In the condition of surface karstificaiton,limestones with fissures are most dissolvable.But in relatively closed burial condition,calcareous dolomite and dolomitic limestone with fissures and intercrystalline pores are more dissolvable.With an integration of diagenetic evolution and the dissolution mechanism of carbonates,the genesis of high quality reservoirs in study area can divided into exposure originated type and burial originated type.The exposure originated reservoirs correspond to “middle to late stage-exposure” buried hills.The transporting effect of fracture for meteoric water together with the enlarging-corrosion effect of meteoric water on wall rock lead to the formation of dissolution pores and caves.Karst reservoirs can develop in both limestone and dolostone,but will develop better in limestone.The following burial modification by aggressive fluids and timely oil-charging favor the preservation of karst reservoirs.The burial originated type reservoirs correspond to “no middle to late-stage exposure” type buried hills covered by the upper Paleozoic.The fault movement can lead to inter-breccia porosity and fissures in wall rock.Fractures and intercrystalline pores transport aggressive fluids such as percolating meteoric water,asending hydrothermal fluid and compaction released water,leading to burial dissolution of the rocks.The burial dissolution is characterized by selective dissolution,which leads to the prior dissolution of more dissolvable component such as calcite.High quality reservoirs resulted from burial dissolution mostly develop in calcareous dolomite and dolomitic limestone with fractures and intercrystalline pores.The dissolution in pure limestone and dolostone is relatively weak.