Sedimentary Geochemistry Of Laiyang Group In Late Mesozoic Rift Basin In The Offshore Of Shandong Province

The set of Laiyang Group’s sedimentary strata in the Late Mesozoic rift basin in the offshore of Shandong Province is different to the sedimentary characteristics in its adjacent area.There are some typical sections in Lingshan Island,Dingziwan and Taolin that are appropriate to investigate.In this study,our main research area is Lingshan Island,the element geochemistry,organic geochemistry and sedimentary analysis method will be applied to analyze the lithology and tectonic setting of provenance,sedimentary environment of this basin and evaluate the source rock potential of deepwater turbidite.Slump,turbidite,debris flow,hybrid event beds(HEBs)were recognized in the gravity flow of Laiyang period in Linghan island,and HEBs have been found to occur mainly in the outer and marginal parts of deepwater systems.The gravity flow sandstone contains a large amount of carbonate minerals,mediate-acid volcanic rock fragments and metamorphic rock fragments,e.g.quartzite.Large plant debris are also abundant in some sandstones.Iron nodules are widely distributed in sandstones,iron materials can be identified in many thin sections and pyrite bands can be found in some layers,which can act as indicators for dysoxia sedimentary environment.Some ratios,e.g.Ti/Zr,Cr/Zr,and discrimination diagrams,e.g.Co/Th-La/Sc,La-Th-Sc have been applied to analyze the characteristics and tectonic setting of provenance,in addition,the lithology of conglomerate was helpful in the provenance analysis.The results indicate that the provenances are mainly acidic volcanic rocks,and the mature polycyclic quartzose detritus also contribute.The tectonic setting of provenances is continental island arc or active continental margin.Element geochemistry and organic geochemistry are also used to analyze the paleoclimate and sedimentary environment.The climate was dry and hot in Laiyang Period.The rift basin belongs to marine basin,which is different to the coeval sedimentary environment of Jiaolai Basin.The salinity of sedimentary water was medium-high,and there might be an increasing trend from east to west.Lingshan Island might be located in the outer and marginal parts of deepwater systems,the water was relatively deep and it might be in the dysoxia-anoxia environment.The TOCs(total organic content)of deepwater turbidites in Lingshan Island are high,but they vary.The TOCs of Td are stable,which mainly depend on the ratio of silt-sized materials,while the organic matter content of turbidite sandstone are greatly affected by the content of large plant remains,the Ta which contains large plant debris always belongs to excellent source rock.The HI,n-alkanes index and micro-organic quantitative analysis indicate the organic matter in turbidites is dominated by terrestrial organic matter,the organic matter belongs to type III and the deepwater turbidites have gas-prone potential,which can result from the fact that the terrestrial organic matter is resisted compared with marine organic matter.Even though the organic matter content of deepwater turbidites is high,the source rock potential varies in different turbidite systems,different periods,different architectural elements of turbidite systems and turbidites with different grain sizes.Therefore,it would be risky to treat deepwater turbidites as targets of hydrocarbon exploration without other effective hydrocarbon source rocks,but it can work as an important hydrocarbon source rocks supplement.