Lithofacies Paleogeography of the Cambrian in the Central Ordos Basin

The Cambrian System in the Ordos basin is a set of marine sedimentary formations dominated by carbonate rocks and interbedded with a small amount of clastic rocks. In recent years, the Cambrian System in this area has been considered to have great exploration potential, but so far no large reservoirs have been discovered. Previous research has mainly focused on the eastern, western, and southern parts, while research on the central part has been severely lacking, which has hindered breakthroughs in oil and gas exploration in the region. In this study, single–well divisions were conducted on 56 drill cores and 13 outcrop sections, and based on the characteristics of 120 thin sections and rock combinations, two sedimentary facies were identified: slope facies and platform facies. Based on this, inter–well comparisons were carried out, and two cross–well sections running through the east-west and north-south of the basin were delineated with the center of the basin as the focus, to explore the sedimentary thickness and facies changes in different periods. The study suggests that the Cambrian in the Ordos basin was formed under a transgression background, and after the Xijian period, seawater advanced from the western and southern parts of the basin towards the interior. By the Zhangxia period, the sea had reached its peak and most of the basin was submerged by seawater, leaving only sporadic low–lying ancient land. During the sedimentation period of the Sanshanzi Formation, the sea transgressed to regress. This study also reconstructed the paleogeographic pattern of “one uplift and four depressions” and restored the tectonic and sedimentary evolution process of the Cambrian period in the central part of the basin.