Integrated identification of sequence boundary through outcrop, seismic with logging and geochemistry: A case of Jurassic in northeastern margin of Ordos basin

Jurassic strata is mainly rock series containing coal and uranium in Ordos basin. The recognition of Jurassic sequence interfaces were mainly relied on the field outcrop, logging lithology of unconformity surface or the sedimentary discontinuity. However, it is tend to experience human recognition of randomness in the sequence interfaces recognition without macroscopic characteristics. Therefore, based on routine identification work such as outcrop profile and electrical curve of nearly 1 000 boreholes, combined with vertical changes of two-dimensional seismic profile and chemical index of alteration(CIA), three long-term cycle sequence interfaces(TSB1-TSB3) and seven medium-term cycle sequence interfaces(SB1-SB7) were finally identified in Jurassic period. The vertical change of CIA generally reflects that the paleoclimate experienced a transition process from warm-humid(dry-wet) alternation to drought-semi-drought from Yan'an to Zhiluo period, which can be used to identify the interfaces. Comprehensive identification of sequence interfaces can not only improve the accuracy,but also endow key sequence interfaces with paleo sedimentary environment attributes, and reduces the impact of human experience influence. Thus provide reliable basis for the division and correlation of Jurassic sequence strata in the whole basin.