--> Characteristics of Shale Lithofacies Evolution in the Wufeng-Longmaxi Formation and Its Controls on Shale Gas Productivity

AAPG ACE 2018

Datapages, Inc.Print this page

Characteristics of Shale Lithofacies Evolution in the Wufeng-Longmaxi Formation and Its Controls on Shale Gas Productivity

Abstract

Fuling Shale Field realized the world’s largest shale gas commercial discovery in Sichuan Basin, China besides the tremendous success achieved in North America. The main producing layers are the organic rich shales in the Upper Ordovician Wufeng formation and Lower Silurian Longmaxi formation. Previous studies assumed the Wufeng-Longmaxi formation as one stratum unit when predicting the distribution of the deep-water shelf facies, suggesting the shale formation in the south having larger thickness and higher quality than that in the southeast. But paradoxically, the current shale gas development in the southeast of Sichuan Basin is much superior. In this study, differences of shale facies are distinguished along the vertical and horizontal directions of the Sichuan Basin, which leads to a diverse material basis for shale gas. With attributes including deep-water shelf environment, fine-grain and thin layers, high-quality shales are studied through fine division of stratigraphic units. It is revealed that Wufeng-Longmaxi Formation underwent at least two transgression-regression cycles. Diversity and evolution of mineral composition and total organic content are elucidated. Drilling data gathered from two directions, parallel and normal to the deep-water shelf extension, are compared indicating that shale facies differ horizontally in different areas ranging from the south of Sichuan Basin to the southeast and the northeast. Meanwhile, from the deep to the shallow depth, the depocenter shows a lateral migration which is from east to west and from north to south. Correspondingly, the vertical variation of mineral content is quite different. By mapping multiple layers with great scrutiny, distribution of various lithofacies is clarified and various mineral contents show different relationship with TOC. Therefore, regionally the northeast area is identified as the most favorable lithofacies with siliceous shale in both the Wufeng and the Longmaxi formation; the south area is classified as favorable with siliceous shale in the Wufeng formation and carbonate-bearing shale in the Longmaxi formation; in the surrounding areas of Sichuan Basin, carbonate-bearing minerals and clay minerals in the Longmaxi formation are most common, so these areas are relatively less favorable; further close to the craton with shallow shelf facies, mudstone and silty mudstone are dominant leading to a worse material basis for shale gas generation.