The Influence of Late Triassic volcanism on the development of Chang 7 high grade hydrocarbon source rock in Ordos Basin

Based on field observation of a great number of drill cores,outcrops and rock profiles,it was found that thin-layered and laminated tuff was extensively developed in the high grade hydrocarbon source rock of Chang 7 Member of Yanchang Formation in the Ordos Basin.The thin-layer tuff is mainly intermediate to acidic in composition,with SiO2 contents varying between 49.37%and 72.45%.According to results of petrographic observation,the tuff can be divided into crystalline tuff,vitric tuff and crystalline-vitric tuff.Also,laminated tuff sometimes shows ordered structure in terms of both composition and grain size,which is characteristic of simultaneous volcanic eruption,atmospheric precipitation and subsequent sedimentation.Based on rock types,composition of both major and trace elements,zircon U-Pb dating results as well as features of spatial distribution of the tuff,it can be inferred that the tuff occurring in Chang 7 high grade hydrocarbon source rock in the basin may be resulted from the simultaneous volcanic eruption in the Qinling orogen.Collision between the Yangtze and North China blocks and consequent regional structural movements in the Qinling orogen were the major dynamic elements leading to the maximum lake flooding event at the Chang 7 period,which provides the basic geological conditions for the development of the high grade hydrocarbon source rock.Meanwhile,the frequent volcanic eruptions and hydrothermal activities that occurred simultaneously at the lake bottom would together facilitate large scale development of the source rock.