Study On The Interaction Mechanism Between Organic Matter And Vanadium In Black Rock Series

The black rock series is an important product of environmental abrupt changes during the geological history period.It not only contains abundant oil and gas resources,but also contains a large number of metal and non-metal mineral resources such as vanadium（V）,nickel（Ni）,molybdenum（Mo）,platinum group element（PGE）,and phosphorus（P）.The associated relationship and interaction between organic matter and vanadium in black rock series has been widely concerned by geologists.The black rock series developed in the Xiangxi area is relatively stable.In this paper,the black rock series of the Lower Cambrian Niutitang Formation in the Longbizui section and the Sancha section were taken as the research objects.The method of combining organic geochemistry and inorganic geochemistry is used to discuss the material source,tectonic background and sedimentary environment of black rock series,and an enrichment model of organic matter and vanadium was established.The effects of vanadium on the hydrocarbon generation process of organic matter were analyzed by means of physical simulation and numerical simulation,and the influence mechanism was summarized.The following achievements and understandings have been achieved:（1）The average TOC contents of the black rock series in the Longbizui and Sancha section of the study area are 6.7%and 8.09%,respectively.The organic matter abundance reaches the standard of good source rocks.The types of organic matter are mainly typeⅠandⅡ,the maturity of organic matter reaches high maturity,or even over-maturity,the organic matter mainly produces dry gas with low hydrocarbon generation potential indicators.The content of saturated hydrocarbons and aromatic hydrocarbons is relatively low,while the content of non-hydrocarbons and asphaltenes is relatively high.Biomarker compounds and organic micro-components indicate that the organic matter is mainly derived from planktonic algae,such as primitive sphagnosphaera,sphaerococcus tumefaciens,and reticulococcus,benthic algae,zooplankton such as sponges and bacteria.（2）Among the main elements of the black rock series,Si O₂ accounts for the highest with an average content of more than 60%,followed by Al₂O₃,and the rest of the elements are below 5%.The trace elements are characterized by the relative enrichment of V,Ni,Mo,Cd,Tl,Pb,U,and the relative loss of Cu,Sr,In;The total amount of rare earth elements（∑REE）is lower than the average content of North American shale and the Upper Crust.The rare earth elements are characterized by the relative enrichment of light rare earth elements and the relative loss of heavy rare earth elements.Elemental geochemical parameters indicate that the tectonic setting of the black rock series is the passive continental margin.The sources of diagenetic materials are mainly terrigenous clastics and hydrothermal fluids.The sedimentary environment is in warm and humid climate,medium to low salinity water,and oxygen-depleted to hypoxic redox conditions and semi-stagnation basin with moderate to strong stagnation.（3）The results of the Rock-Eval hydrocarbon generation simulation experiment and Gaussian software simulation show that the appropriate concentration of vanadium compounds can promote the hydrocarbon generation of organic matter.Vanadium in different valences has different effects.High-valent vanadium（Na VO₃）mainly reduces the dissociation energy of C-C and C-H bonds,while low-valent vanadium(VO²⁺)reduces the activation energy of hydrocarbon generation reaction by reducing the dissociation of C-C,C-H bonds and C-O bonds,thereby accelerating the thermal evolution of organic matter,the promotion effect is weakened at high concentration.（4）Through comprehensive geological conditions,a model and mechanism of enrichment of organic matter and vanadium have been established.The vanadium element is initially enriched in organisms and is co-deposited with the biological remains.During the process of decomposing biological remains,microorganisms change the p H and Eh in the environment to further enrich,migrate,deposit and diagenese vanadium.The clay minerals formed by organic matter and diagenesis have an adsorption and enrichment effect on vanadium,so that they exist in the adsorbed state in the clay minerals and organic matters or in the form of vanadium illite and vanadium mica in the clay minerals.A stable organic-metal complex（vanadium porphyrin）is formed in the process of affecting the hydrocarbon generation of organic matter,which accumulates in the reservoir with oil and gas migration and precipitation,and is finally fixed in the highly evolved products.