There exist three sets of quality shale that developed in the Dalong, Wujiangping, and Maokou Formations during the mid-upper Permian, which are important replacements for marine shale gas exploration or the Wufeng Formation-Longmaxi Formation. Based on the relationships between important geological events built on an isochronous stratigraphic framework, sedimentary structures, paleoenvironments, and ancient living organisms, the influence of major geological events, such as middle-late Permian upwelling and volcanic activity, on the development of organic-rich shale in northeastern Sichuan was studied.
It was concluded that the 3rd member of the Maokou-Dalong Formation can be divided into five four-level sequences, among which the systems of TST1, TST3, TST4, and TST5 are favorable for black organic-rich siliceous shale development, and the geological response characteristics of Middle and Late Permian volcanic activity and upwelling events were clarified.
A high-quality shale development model with the combined action of volcanic activity, upwelling, and other geological events was established. It was clear that SqPm-2 represents the initial stage underwent by tectonic extension. The upwelling brought abundant soluble silicon and other nutrients that are favorable for the rapid breeding of organisms such as diatoms, siliceous sponges, and radiolarians. Belonging to a typical coupled developing mode of upwelling and organisms, the shale has high carbon (>10.0%) and silicon (>70.0%) content, whereas the thickness is relatively thin, a typical “thin and high-quality” characteristic, which represents a favorable new layer for shale gas exploration in the Puguang area. SqPw-2 is the state that underwent rapid extension, the tephra that carries abundant nutrients is favorable for organic matter accumulation, belonging to the coupled developing mode of volcanic activity and organisms. The TOC of shale is >4% and siliceous minerals >50.0%, but it is relatively thin, which is favorable for further exploration and expansion. SqPd-1~SqPd-2 is the flourishing stage whereby the ocean trough came into being. The base subsided, and upwelling and hydrothermalism enabled siliceous organisms to flourish. A great deal of organic matter is maintained in deep anoxic environments, belonging to a coupled development mode of upwelling and thermal fluids. The high-quality shale layer is relatively thick (>30 m), which offers good exploration potential and is a favorable stratum for the next step of large-scale shale gas storage and production.