Lithofacies Paleogeography Characteristics of Ordovician Wulalike Formation in the western margin of Ordos Basin

Key words: 
• Ordos Basin /
• Wulalike Formation /
• marine shale /
• lithofacies palaeogeography /
• depositional model

Abstract: [Objective] The western part of the Ordos Basin was located in the Qilian Sea area in the Middle-Late Ordovician Wulalike period, where a thick layer of black marine shale was developed. In recent years, many exploration wells in the Ordos Basin have produced industrial oil and gas flow from the Wulalike Formation, which demonstrates a good potential for exploration and development of marine shale oil and gas. The study of the Ordovician paleogeography in the western part of the basin is conducive to clarifying the distribution of hydrocarbon source rocks and reservoirs, as well as the next exploration direction of marine shale gas. [Methods] Through a large amount of field outcrops, drilling cores, rock thin sections, and elemental geochemistry data, this paper comprehensively studies the petrological characteristics, sedimentary facies types, and sedimentary evolution of the Wulalike Formation. [Results] The results show that: (1) Wulalike formation includes various types of rocks such as siliceous shale, calcareous shale, clay shale, mixed shale, and granular limestone, among which laminated siliceous shale was identified as the most favorable lithofacies for shale gas exploration; (2) Based on the analysis of sedimentary facies, five sedimentary facies types were identified from west to east: basin facies, broad sea shelf facies, slope facies, and carbonate platform facies; (3) During the Wulalike period, the western part of the Ordos Basin experienced multiple transgressive-regressive processes, with the water gradually becoming shallower and the sedimentary environment transitioning from deep water to shallow water. [Conclusion] In conclusion, a carbonate platform margin sedimentary model of the Wulalike period in the western Ordos Basin was established, and it was proposed that the broad sea shelf, slope, and granular shoal are favorable exploration facies for natural gas in the next step.