Classification of shale lithofacies from Ordovician Wufeng Formation to first section of first member of Silurian Longmaxi Formation, western Changning area, Sichuan Basin, and its significance

Abstract

Shale gas exploration has been successfully carried out in the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation of the Changning area of Sichuan Basin. To clarify the lithofacies differences, vertical and horizontal distribution signatures, reservoir development characteristics and influencing factors in the western part of Changning area, based on core X-ray diffraction test, scanning electron microscopic observation and various analytical test data, the shale lithofacies of the Wufeng Formation (O3w) to the first section of the first member of Longmaxi Formation (S1l11) were studied in detail. Seven lithofacies in the study area were recognized including siliceous shale, mixed siliceous shale, carbonate-rich siliceous shale, clay-rich siliceous shale, mixed shale, argillaceous/siliceous mixed shale and calcareous/siliceous shale. The siliceous shale facies developed at the bottom of S1l11 is the best shale lithofacies in the study area since its TOC content, porosity and gas-bearing capacity are all higher than those of others. The mixed siliceous shale, carbonate-rich siliceous shale, calcareous/siliceous mixed shale and argillaceous/siliceous mixed shale in O3w and the upper S1l11 are slightly poorer than siliceous shale in terms of reservoir conditions, which are the sub-optimal lithofacies in the study area. Comprehensive analyses showed that the formation of dominant shale lithofacies reservoirs in O3w and S1l11 may be controlled by two factors. One is the sedimentary environment rich in oxygen in the upper layer and hypoxic and reducing in the lower layer, which provided a large amount of organic-rich siliceous minerals. Secondly, abundant organic-rich siliceous minerals provided excellent pore types for reservoirs during late diagenesis. The results will provide lithographic support for the studies of shale gas enrichment zones in the western Changning area, and are conducive to the further exploration of marine shale gas in O3w and S1l11 in the Sichuan Basin.

Keywords

• shale lithofacies
• reservoir characteristics
• controlling factors
• longmaxi formation
• changning area
• sichuan basin