Distribution characteristics and geological significance of the thickest Aeronian bentonite bed in the Middle-Upper Yangtze Region, China

Abstract

Founded on the data deriving out of the outcrops and key exploratory wells, the distribution characteristics of the thickest bentonite layer of the Aeronian in the Middle-Upper Yangtze Region as well as its unique role in the geological evaluation of shale gas and prediction of shale gas “sweet layer” has been researched altogether. There are three preliminary conclusions that have been reached: (1) Firstly, the thickest bentonite layer of the Aeronian is widely and stably distributed in the Middle-Upper Yangtze Region with a thickness of 5–40 cm. It is easily identifiable in most of the exploration areas for its logging features that appear to be distinctive. It has general GR values of 188–235 cps (or API) and appears as a peak on the gamma log in the southern and eastern Sichuan Basin. Nonetheless, there is no evident GR value peak or even takes on trough feature in central and northeastern Sichuan Basin and northern Middle Yangtze Region. (2) Secondly, one may consider the bentonite layer as the key marker bed for segmental partition and regional correlation of the Longmaxi Formation. In the field, initially looking for the appearance of demirastrites or the first gamma curve trough below the bentonite layer is an effective way to identify the upper boundary of Rhuddanian. (3) Lastly, the emergence of the thickest bentonite layer in the Aeronian has a profound influence on the formation and distribution of high-quality shale of the Wufeng-Longmaxi Formation. Its appearance is an important symbol in regards to foreland basin folding and the ever-increasing sedimentary rate. Meanwhile, it also represents a transition from organic-rich and silica-rich shale to clay-rich and calcium-rich shale all within the Longmaxi Formation. Therefore, the organic-rich and silica-rich shales were mainly formed prior the appearance of the bentonite layer. The above is a control effect that must be taken into account when predicting the spatial distribution of the “sweet spot” layer in the Wufeng-Longmaxi Formations. In most parts of the Middle-Upper Yangtze Region, the thickest bentonite layer in the Aeronian can be regarded as the upper boundary of the high-quality reservoir. Keywords: Yangtze region, Aeronian, Bentonite, Sedimentary response, Deposition rate, High-quality shale