New understanding of the sedimentary model of Lower Cambrian Longwangmiao Formation reservoirs in the Sichuan Basin

Abstract

Natural gas in the gas reservoirs of Lower Cambrian Longwangmiao Fm is mainly concentrated in the granular shoal facies in central Sichuan Basin, therefore the study on lithofacies paleogeography and the prediction on granular shoal distribution are the key to improving the success ratio of natural gas exploration in this area. In this paper, outcrop, drilling, seismic and logging data were studied comprehensively. Subsequently, the sedimentary facies and sedimentary model of Longwangmiao Formation in the Sichuan Basin and its adjacent areas were studied, and lithofacies paleogeographic maps were prepared based on previous research results. Besides, “three-shoal” sedimentary model of Longwangmiao Fm was established, the distribution range of three shoal belts was predicted and their development characteristics and main controls were analyzed, thereby pointing out the direction and field of natural gas exploration in the next stage. It is indicated from the research results that: first, the paleogeographic pattern of the Sichuan Basin in the Longwangmiao period was represented as a rimed carbonate rock platform, with the developments of sedimentary facies such as mixed tidal flat, restricted platform, open platform, platform margin and slope facies. Second, the “three-shoal” sedimentary model of Longwangmiao Fm was established. One of them is the granular shoal in platform margin facies and is distributed along the eastern margin of the Sichuan Basin. The other two are intra-platform granular shoals which were distributed on the east and west sides of lagoon subfacies, belonging to restricted platform facies and open platform facies, respectively. Third, the granular shoal of restricted platform facies is the main reservoir of Longwangmiao Fm gas reservoir in the Anyue Gas Field. It is distributed in central Sichuan, and its development is controlled by lagoons and Gaoshiti–Moxi Paleo-uplift. Fourth, the platform margin granular shoal is mainly distributed along the northern and eastern margins of the basin, and its development is controlled by the slope break of sedimentary paleogeomorphy. In conclusion, the granular shoal of restricted platform facies and shoal of platform margin facies are good reservoirs that they are the important fields of natural gas exploration. Furthermore, the new understanding to “three-shoal” sedimentary model and granular shoal distribution of Longwangmiao Fm is of great theoretical significance to the study on sedimentary systems of ancient strata.