The Relationship between Permian Sedimentary Facies and Oil and Gas Enrichment in Sichuan Basin

In recent years, the authors have in more detail studied a vast amount of the data on Permian sedimentary facies in Sichuan Basin, with the emphasis upon northeastern Sichuan, and compiled Permian sedimentary facies maps during various stages such as Qixia and Maokou stages of Early Permian and Longtan and Changxing stages of Late Permian. This paper deals with the features of Permian sedimentary facies, the limits of old land, sediment types, biological associations, the nature of the seas, transgression direction, Sichuan-Guizhou Movement and basalt eruption in the study area. It can be seen that there was a complete transgressive-regressive cycle during Qixia and Maokou stages of Early Permian, respectively. At the beginning of the cycles, relatively stable low-energy limestones and eyed limestones were laid down, while at the end of them, sparry red algal limestones were developed, indicating a high-energy beach facies under the influence of regression. These beach bodies were developed directionally and successively. Influenced especially by Sichuan-Guizhou Movement during the late Maokou stage, block faulting occurred frequently. Reef and beach facies tended to be located in the highs of the faulted blocks, whereas intraplatformal trench and intraplatformal basin facies where siliceous rocks were widely developed tended to be dispersed in the lows of them. During early Late Permian, strong block faulting led to the rise of the western side of the basin on he whole and then land followed. The well-defined facies belts in Longtan stage were arranged in the order from west to east: continental facies and littoral facies and intraplatformal shallow-marine facies. A great deal of basalt erupted in front of Kangdian Old land. Both basalts and diabases have also been recognized in some boreholes in eastern and northeastern Sichuan. Up to late Late Permian, the sedimentary basement descended and the sea area became larger at that time than in Longtan stage, thus creating favourable conditions for the formation of reef and beach facies to the east of the Huaying Mountains. The stage, therefore,has been regarded as a reef-building and beach-forming stage in Late Permian. On the basis of sedimentary facies and palaeostructural patterns, the conclusion has been drawn as follows. During Permian,Sichuan Basin was a epicontinental sea composed of huge shallow-sea carbonate platforms where both shallow-water and deep-water facies existed. Controlled by faulting, the sedimentary units were evidently differentiated. Reef-beach and trench-basin facies constituted a facies belt favourable to the enrichment of oil and gas. The discovery of tempestites, organic reefs and deep-water sediments have opened up a new prospect for the study of the Permian.