26 August 2025

Mosman Oil and Gas Limited

("Mosman" or the "Company")

US Helium Projects Update

Mosman Oil and Gas Limited (AIM: MSMN), advancing helium, hydrogen and hydrocarbon projects in the USA, is pleased to provide an operational update on its Colorado helium portfolio, comprising the Sagebrush and Coyote Wash projects.

Highlights

·      Contractor selection for the Sagebrush 3D seismic programme is well advanced, with field operations targeted to begin in early October.

·      Sproule's independent verification of Sagebrush is progressing, with economic and reservoir modelling now underway.

·      The permitting process at Coyote Wash continues to advance, with regulatory engagement and further discussions scheduled.

·      Preliminary design work for surface facilities has been reviewed, confirming no major technical barriers to future development.

Sagebrush - 3D Seismic

Mosman has reviewed bids for the planned 3D seismic survey at Sagebrush, which is expected to cover approximately 16 square miles. The programme is anticipated to commence in early October, subject to final contracting and mobilisation. The seismic data will provide a much higher-resolution picture of subsurface structures and potential helium and oil traps than currently available, significantly de-risking future drilling. By mapping fault blocks and closures in detail, Mosman aims to refine future drilling targets and optimise well placement, providing confidence in both helium and hydrocarbon prospectivity.

Sproule Independent Verification - Sagebrush

Sproule has now delivered an initial dataset covering engineering and resource inputs, which is being refined. Economic modelling and the final independent verification report are expected to follow. This work is an important step as Sproule's scope of work includes a comprehensive review of geological and engineering data, including historic gas sample analyses from the Sagebrush-1 well, which indicated a gas composition of 2.76% helium, 23% methane and other hydrocarbon gases. The independent assessment is expected to provide estimates of both contingent and prospective helium resources, further validating and refining Mosman's internal estimates to support future development and commercialisation efforts.

Coyote Wash - Permitting Progress

The permitting process continues to advance. Mosman has updated and re-submitted regulatory documentation following feedback from governmental agencies, and further discussions are scheduled in the coming weeks. This is expected to bring the project closer to final approvals under the Indian Mineral Development Agreement framework.

Surface Facilities

Mosman has also been assessing options for surface facilities required to process helium once production is established. Initial reviews of plant design and fabrication timelines have been encouraging, with no technical impediments identified. This early work is important to ensure that Mosman can move quickly from successful well testing into commercial helium sales, by aligning subsurface development with a practical and scalable surface solution.

Mosman Interim-Chief Executive, Howard McLaughlin, commented: "Across Sagebrush and Coyote Wash we are executing at pace. At Sagebrush, seismic acquisition planning is well-advanced and we are targeting an early-October start, while Sproule's independent verification is moving toward economics and final reporting. At Coyote Wash, the IMDA package has been updated and resubmitted, with our BIA meeting set for early September. In parallel, our facilities workstream is progressing with credible vendors. These steps build on the recent Tribal approval for Sagebrush 3D seismic and position Mosman for a material period of newsflow and value-driving activity."

Market Abuse Regulation (MAR) Disclosure

The information contained within this announcement is deemed by the Company to constitute inside information as stipulated under the Market Abuse Regulations (EU) No. 596/2014 ('MAR') which has been incorporated into UK law by the European Union (Withdrawal) Act 2018. Upon the publication of this announcement via Regulatory Information Service ('RIS'), this information is now considered to be in the public domain.

Enquiries:

Updates on the Company's activities are regularly posted on its website: www.mosmanoilandgas.com

Notes to editors

Mosman (AIM: MSMN) is a helium, hydrogen and hydrocarbon exploration, development, and production company with projects in the US and Australia. Mosman's strategic objectives remain consistent: to identify opportunities which will provide operating cash flow and have development upside, in conjunction with progressing exploration. The Company has several projects in the US, in addition to royalty interests in Australia.