Abstract
As an important pilot target for shale gas exploration and development in China, the Longmaxi Formation shale in the Dianqianbei Area is characterized by high content of nitrogen, which severely increases exploration risk. Accordingly, this study explores the genesis of shale gas reservoir and the mechanism of nitrogen enrichment through investigating shale gas compositions, isotope features, and geochemical characteristics of associated gases. The high-nitrogen shale gas reservoir in the Longmaxi Formation is demonstrated to be a typical dry gas reservoir. Specifically, the alkane carbon isotope reversal is ascribed to the secondary cracking of crude oil and the Rayleigh fractionation induced by the basalt mantle plume. Such a thermogenic oil-type gas reservoir is composed of both oil-cracking gas and kerogen-cracking gas. The normally high nitrogen content (18.05%–40.92%) is attributed to organic matter cracking and thermal ammoniation in the high-maturity stage. Specifically, the high heat flow effect of the Emeishan mantle plume exacerbates the thermal cracking of organic matter in the Longmaxi Formation shale, accompanied by nitrogen generation. In comparison, the abnormally high nitrogen content (86.79%–98.54%) is ascribed to the communication between the atmosphere and deep underground fluids by deep faults, which results in hydrocarbon loss and nitrogen intrusion, acting as the key factor for deconstruction of the primary shale gas reservoir. Results of this study not only enrich research on genetic mechanism of high-maturity N2 shale gas reservoirs, but also provide theoretical guidance for subsequent gas reservoir resource evaluation and well-drilling deployment in this area.
Highlights
Compared with conventional hydrocarbons, shale gas is essentially different in the occurrences, accumulation characteristics, and enrichment mechanisms under the low porosity and low permeability conditions (Wang and Li 2017; Wu et al 2018; Chen et al 2018a; Daniel 2018)
The Longmaxi Formation shale in the Dianqianbei Area is featured by over-high thermal maturity and high nitrogen content, which increases the difficulty and risk of shale gas exploration
According to experimental tests of the Longmaxi Formation shale samples from Wells Y2 and B1 (Table 1, Fig. 2a, b), the methane content greatly varies in the range of Sample no
Summary
Shale gas is essentially different in the occurrences, accumulation characteristics, and enrichment mechanisms under the low porosity and low permeability conditions (Wang and Li 2017; Wu et al 2018; Chen et al 2018a; Daniel 2018). The Longmaxi Formation shale in the Dianqianbei Area is featured by over-high thermal maturity and high nitrogen content, which increases the difficulty and risk of shale gas exploration. There are many key issues to be solved, including the genetic type of shale gas, the occurrence and enrichment law of shale gas, and the source of nitrogen in some high-nitrogen, low-carbon gas reservoirs. Elaboration of these issues will greatly promote the exploration and development process of shale gas in this area. The genetic mechanism of the high maturity shale gas reservoir is elaborated, accompanied by construction of a model illustrating the shale gas accumulation process in the Dianqianbei Area. Results of this study are expected to provide an important geological basis for subsequent exploration and development of shale gas reservoirs in this area
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
