Original morphology reconstruction and lithologic assemblage prediction for buried volcanic edifices in a basin: A case study of the LK 7-A structure in the Bodong swell, Bohai Bay Basin

The original morphologies of volcanic edifices in a basin are often poorly preserved due to weathering and denudation, leading to inaccurate predictions of their lithology. The morphological characteristics of volcanic edifices are indicative of their lithologic assemblages, so it’s of great significance for reconstructing the original morphologies of buried volcanic edifices in hydrocarbon exploration of volcanic rocks. In this study, we investigate multiple buried remnant volcanic edifices in the LK 7-A structure of the Bodong swell within the Bohai Bay Basin. The denudation thicknesses of these volcanic edifices are quantified using the structural trend analysis and the balanced cross-section technique. Subsequently, their original morphologies are reconstructed through inversion. Models for discriminating volcanic edifice types are established based on the parameters of modern volcanic edifices, such as volume, diameter, slope, and height. Using these models, the types of volcanic edifices reconstructed in the basin are determined. After validation and correction using lithological data acquired through drilling, the models are employed to predict the lithologic assemblages of volcanic edifices in areas with limited or no well control. The results indicate that the original Mesozoic volcanic edifices in the LK 7-A structure are dominated by composite volcanoes and lava domes. Among these, the No. 1 volcanic edifice represents a composite volcano consisting of interbedded rocks of intermediate-acidic explosive and effusive facies, with a lithologic assemblage composed of trachyandesite-rhyolite-andesite interbeds. These discrimination results show good agreement with drilling data from well LK7-A-X. The No. 6 volcanic edifice is predicted to be a lava dome formed by the extrusion of high-viscosity acidic magmas, with its lithology inferred to be dominated by acidic explosive and effusive facies. Overall, the lithologic assemblages of buried volcanic edifices in a basin can be accurately predicted using (1) the combination of the structural trend method and balanced cross-section technique for volcanic edifice reconstruction, (2) the prediction model based on the correlation between volcanic edifice types and lithologic assemblages, and (3) five characteristic parameters (i.e., basement diameter, height, aspect ratio, slope, and volume) of volcanic landforms.