The discovery of a new unconventional oil and gas reservoir in the Sichuan Basin could reshape China’s energy landscape, according to research published in the *Journal of Geomechanics* (Dizhi lixue xuebao). Led by TANG Song, a geologist whose affiliation is not publicly listed, the study sheds light on a previously overlooked geological setting—the deep-water lagoon marlstones of the Middle Triassic Leikoupo Formation—that may hold significant untapped hydrocarbons.
The findings center on the Lei 3-2 sub-member, a previously under-explored interval in the Leikoupo Formation. Drilling in Well CT-1 revealed organic-rich marlstones with average total organic carbon (TOC) content of 1.16%, peaking at 1.78%, values that are unusually high for carbonate-rich rocks. “The co-occurrence of anoxic conditions and high paleo-productivity created a perfect storm for organic matter accumulation,” TANG noted in correspondence with the journal. This dual control mechanism—low oxygen and elevated nutrient input—allowed organic carbon to be preserved in exceptional concentrations.
What makes this reservoir particularly attractive is not just its richness, but its structure. High-resolution imaging using CT scans and scanning electron microscopy (SEM) revealed a complex pore network dominated by nano- and micro-scale pores and microfractures, with porosity exceeding 3%. These features form the “sweet spots” where hydrocarbons can be effectively stored and produced. The reservoir developed in a deep-water, gypsum-rich lagoon within an intracontinental sea, a setting previously considered marginal for oil and gas accumulation.
Equally important is the geological timing. The study links the formation of these organic-rich marlstones to a maximum flooding event during the Middle Triassic, when rising sea levels flooded large parts of the Upper Yangtze Craton. This event was accompanied by a monsoon-driven increase in weathering and nutrient runoff, further boosting productivity in the lagoon. Overlying regressive evaporites then acted as a seal, trapping hydrocarbons over geological time.
From a commercial perspective, the implications are substantial. The widespread distribution of these deep-water lagoons across the Sichuan Basin suggests a new unconventional play fairway within the Leikoupo Formation. Unlike traditional shale plays, which rely on organic-rich mudstones, this reservoir is a marlstone—harder, more brittle, and potentially more fracture-prone—offering new drilling and completion strategies. “This isn’t just another shale play,” said one industry geologist familiar with the region. “It’s a carbonate-dominated system with hybrid storage, which could open up new engineering approaches.”
The study also hints at a broader significance: these marlstones may not only serve as source rocks but could also feed conventional reservoirs in the same formation. This dual role could reduce exploration risk and accelerate appraisal programs.
As China seeks to diversify its energy mix and reduce reliance on imports, discoveries like this are critical. The Sichuan Basin already hosts some of the country’s most productive conventional gas fields. Now, with evidence of a viable unconventional play, the region could become a dual hydrocarbon hub—conventional and unconventional—within the same geological system.
The research, published in *Journal of Geomechanics*, adds to a growing body of evidence that intracontinental sags and carbonate platforms, long overlooked in favor of marine basins, may harbor significant unconventional resources. It challenges conventional wisdom and invites a rethink of exploration strategies in landlocked sedimentary basins.
For energy companies with acreage in the Sichuan Basin, the message is clear: the Lei 3-2 sub-member is no longer a curiosity—it’s a target.