| Calcite vein is one of the most common types in sedimentary basin,especailly widespread in the black shales which are rich in organic matters and carbonates.The formation of the calcite veins is closely connected with the diagenetic evolution,hydrocarbon generation,tectonic activity and fluid migration in mudstones.The research of the bedding-parallel calcite veins in the hydrocarbon source rocks which develop the horizontal laminae are received extensive attention.The bedding-parallel calcite veins as the most important veins in the hydrocarbon source rocks contain lots of information about the hydrocarbon source rock evolution.This paper take the calcite veins of the muddy hydrocarbon source rock in the Dongying Depression as the main study subject and apply core observation,thin section authentication,mineral analysis and element geochemistry analysis comprehensively.The formation of the calcite veins and their implications are determined by the research of the petrology of hydrocarbon source rocks,petrography of the calcite veins,the origin and migration mechanism of the calcium carbonate.The results show that the calcite veins in the hydrocarbon source rocks are lenticular or lamellose of various sizes.The crystals are perpendicular to the vein wall and the shapes of the crystals are fibrous.The fibres are straight and uniform in a regular arrangement and along the growth direction the width of the fibres shows no significant increase,which indicates that the growth is continuous and there is no growth competition between the fibres.The plane of the growth loccated on outer surfaces of the vein.Based on the arrangement of the solid inclusions within the vein we suggest our vein is formed by total constant growth rates at variable accretion sites.The source of the vein derived from the wall rocks and supplies to the fractures in a diffusion way between the laminaes.The results of the C-O isotope analysis show that the?13C of the bedding-parallel fibrous calcite veins of muddy hydrocarbon source rocks in the Dongying Depression is between2.75.4‰,and the average value is 4.0‰.The?18O is between 16.619.4‰,and the average value is 18.3‰.The figures reveal that the origin of the carbon of the bedding-parallel fibrous calcite veins of muddy hydrocarbon source rocks in the Dongying Depression is dominated by inorganic carbonates,and the organic matter contributed little to the carbon of the vein.The C isotpoes between the vein and carbonate laminae of the wall rock are highly positive correlated?R=0.71?,which indicates the formation of the veins is related to the wall rocks.The calcite vein is related to the evolution of the organic matter and the near-source precipitation of the fluid migrates in a diffusion way.The origin of the carbon in veins comes from the carbonate laminae rather than the organic matter in the wall rocks.The organic acid and CO2 which formed in the evolution of the organic matters provide conditions for dissolution of the carbonates in wall rocks and the dissolution is mainly caused by the organci acid.The origin of oxygen in veins come from the carbonate laminae and the water of the fluids.And the contribution of the water is about 7.62%which indicates the fluids experienced intense water-rock interaction.The origin of the water may the combination of the interlayer water of the montmorillonite and the water which formed during the evolution of the organic matters.The analyses of trace elements show that,calcite veins are rich in Sr,Ba and depletion in Mn.?REE is between 13.03×10-6 and 146.60×10-6,with an average of 49.37×10-6,LREE is between 12.32×10-6 and 136.97×10-6,with an average of 45.52×10-6,HREE is between 0.71×10-69.63×10-6,with an average of 3.85×10-6,the average of LREE/HREE is 13.84,?La/Yb?N ranges from 12.14 to 34.02,with an average of 19.52.The calcite veins are significantly rich in LREE and depletion in HREE,indicating the degree of the fractionation between the LREE and HREE is high.The?Eu of fibrous calcite veins vary within wide limits,showing generally weak negative Eu anomalies??Eu=0.071.64,with an average of 0.75?,?Ce changes stably and anomaly is not obvious??Ce=0.861.03,with an average of 0.93?.And wall rocks have obvious medium Eu anomaly??Eu=0.470.65,with an average of 0.6?,Ce anomaly is not obvious??Ce=0.870.93,with an average of 0.9?.The changes of the Eu in different calcite vein samples mainly depend on the extent of water-rock interaction during calcite precipitation and the redox environments of the fluid.So the fluid media of our calcite veins has the characteristics of Eu anomaly and is reductive on the whole.The hydraulic fracture caused in the evolution of the organic laminated mudtones is an important driving force for overpressure relief and fluid discharge,which has a great influence on the movement of the fluids within the mudstones.The horizontal fractures are caused by the vertical seepage of hydrocarbon fluids and then migrating laterally along the fractures.The presence of the horizontal fractures represents the hydrocarbon fluids will experience a lateral migration.And the formation of the veins has a great effect on the hydrocarbon generation efficiency,as well as the rate and direction of hydrocarbon migration.The open of the horizontal fractures provide space for the formation of the veins.During the growth of the veins,because of the fracture open rate is slower than the crystal growth rate,the force of crystallization controls the fracture expansion.The continuous growth of the fibres indicates that the open and close of the horizontal is not periodic.And the horizontal fractures remain open for some time,to make sure the continuous horizontal migration of the hydorcarbon fluids. |
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