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SUMMARY Cemented layers in the shallow lagoonal, intertidal, and supratidal sediments surrounding the Qatar Peninsula, Persian Gulf, contain high‐magnesian calcite and aragonite cements. The magnesian calcite appears to occur both as a primary precipitate and as a replacement of aragonite. These cements are believed to be forming today from sea‐derived waters. Radiocarbon dating sets an upper age limit of about 4,500 years. Two major types of cemented layer can be distinguished: beachrocks form variably cemented units about 1 m thick at the surface of freely draining exposed beaches: cemented sheets a few cm thick form beneath the surface of broad intertidal sand flats by the growth, hardening, and coalescence of small friable lumps near low‐tide level. Replacement of aragonite matrix, druse, and pellets by microcrystalline magnesian calcite is accomplished by dissolution and reprecipitation; detail is obliterated but the gross features of the original fabric are preserved. The process helps to explain the crystal fabrics of many older limestones.

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Summary A substantial sedimentary basin in southeast Co. Antrim is indicated by gravity measurements and has been explored by the Larne-2 geothermal test well which encountered 9395 ft of Permo-Triassic sediments without reaching their base. Porous sandstones near the top of the Sherwood Sandstone occur beneath thick impervious halite horizons in the Mercia Mudstone. Further potential reservoir rocks occur in Lower Permian sandstones below a 370 ft thick Zechstein salt at a depth of about 6000 ft. Potential traps in the form of tilted fault blocks have been indicated by reconnaissance Vibroseis. Suitable source rocks, the remaining essential ingredient for a petroleum prospect, are expected but have yet to be proved. A similar Permo-Triassic basin is indicated by gravity beneath Lough Neagh. Although covered by thick Tertiary basalts and Lough Neagh Clay, its western margin has been successfully mapped by reconnaissance Vibroseis. The Permo-Triassic here is thinner and is without salt. However, the underlying Carboniferous contains Westphalian and Namurian coals, and geochemical studies confirm their potential to produce gas where suitably buried. There is a good chance that similar Carboniferous strata with coals underlie the Permo-Trias of the Larne Basin of southeast Antrim and provide the source rock for an important gas province.