Lithological characteristics and logging identification methods of the Chang73 menber in Jiyuan area, Ordos Basin

Abstract

The exploration of shale oil is influenced by the type and spatial distribution of sweet spots in the reservoir. Various types of deep-water fine-grained sedimentary rocks are developed in the Chang7₃ member of the Ordos Basin. By accurately characterizing the lithology, the distribution patterns and control factors of different lithologies are clarified, which is of great significance for the exploration of sweet spots in shale oil reservoirs in the Chang7₃ member. Drilling coring is the most accurate method for describing lithology, but the cost of coring is high, and the number of coring wells and sections is limited. The vertical resolution of logging curves is high, which can continuously and in situ reflect the characteristics of formation lithology, thus compensating for the problem of insufficient core wells. Based on the differences in electrical properties, physical properties, and radioactivity of different rock types in logging responses, first, by observing and describing 17 core wells inside the basin, it was identified that the Chang7₃ member mainly developed fine sandstone, siltstone, muddy siltstone, sandy mudstone, mudstone, and black shale. The logging data values of different rock types were statistically analyzed, and the results showed that the natural gamma and acoustic time difference logging curves had a more obvious response to rock types in different sedimentary facies zones. Secondly, by using the intersection diagram method, different logging classification standards for different rock types were identified. Through comparison and verification with core wells, the conformity rate reached over 80%. Finally, 130 wells in the study area were selected for lithology identification, and the identification results were applied to the plan and profile maps. It was found that the distribution of lithology is significantly controlled by the sedimentary system and sedimentary facies, and the lithology is orderly distributed from the delta front (fine sandstone, siltstone) to semi deep lakes (mudstone) and deep lakes (shale).