Seismic facies and sedimentary distribution characteristics of the Upper Carboniferous in the Ounan Depression, eastern Qaidam Basin

Abstract

Objective The Carboniferous system of the Ounan Depression in the eastern Qaidam Basin is a key area for oil and gas exploration in new regions and strata. Drilling and outcrops reveal significant differences in the sedimentary characteristics of the Upper Carboniferous strata across the plane. Previous research, mainly based on outcrop and drilling data, has focused on longitudinal sedimentary evolution and the overall sedimentary background, leaving the distribution characteristics of sedimentary facies unclear. This uncertainty affects the deployment of oil and gas exploration in the Upper Carboniferous of the eastern Qaidam Basin. Methods The article focuses on the Upper Carboniferous strata of the Ounan Depression. It predicts the distribution characteristics of sedimentary facies under the guidance of sedimentary model theory using a limited amount of drilling data and 2D seismic data. Utilizing 2D seismic data from the Ounan Depression, it also conducts a detailed study of the internal structure and external geometric features of seismic reflections using seismic facies identification technology. Results The comprehensive study identifies six types of seismic facies within the Upper Carboniferous of the Ounan Depression: parallel sheet, sub-parallel sheet, mounded, wedge-shaped, oblique foreset, and chaotic. A planar assemblage map of the seismic facies was developed. By integrating regional geological data, outcrops, well logs, and seismic data, and under the guidance of seismic sedimentology and continental shelf sedimentation theory, the relationship between seismic facies and sedimentary facies in the Upper Carboniferous of the Ounan Depression was established, and the distribution characteristics of sedimentary facies in the study area were predicted. Conclusion The results indicate significant differences between the seismic facies in the western and eastern regions of the Ounan Depression. Overall, the western part exhibits lower frequency and more continuous phases compared to the eastern region; the western region also has greater monolayer thickness and a more stable sedimentary environment. From east to west, the Ounan Depression develops a sequential fluvial-delta plain sedimentary system, delta front sedimentary system, and prodelta–deep water shelf sedimentary system. Significance The research findings provide a scientific basis for predicting favorable reservoir distribution areas and deploying exploration targets for Carboniferous oil and gas exploration in the eastern region of the Qaidam Basin.

Keywords

• seismic facies
• sedimentary facies
• ounan depression
• upper carboniferous
• qaidam basin