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S U M M A RY:A 2 km thick sequence of subaqueous basaltic rocks--the Hillion-Erquy Volcanic Formation (HEVF)--forms the lower part of the late Proterozoic Brioverian supracrustal sequence around the Baie de St. Brieuc, N Brittany. Basic dykes cutting the adjacent Pre-Brioverian crystalline basement (the Pentevrian) are interpreted as feeders to the volcanic sequence. As a result of the end Precambrian-early Palaeozoic Cadomian orogenic event, the basic volcanics and feeder dykes experienced a regional metamorphism varying from prehnite-pumpellyite facies in the east to upper greenschist (or amphibolite) in the west of the Baie de St. Brieuc region.The compositions of the basic rocks are evolved, ranging from basalts to basaltic andesites. The suite is subalkaline and tholeiitic in character. Both major and trace element discriminants indicate the suite to be of continental flood basalt affinity. Incompatible element diagrams show evidence for a certain degree of crustal contamination during the evolution of the suite.It is proposed that the HEVF was erupted in a rapidly subsiding ensialic basin, formed during a phase of crustal extension. Volcanism ceased abruptly and the basin rapidly filled with sediment.A brief review is given of several other broadly similar basic sequences occurring in other parts of N Brittany. Their chemistry is compared with that of the HEVF and the implications for a geotectonic reconstruction of the region in late Proterozoic times discussed.

An episode of magmatism, which may represent the last igneous phase attributable to the Cadomian orogenic cycle, is found in southern Jersey, Channel Islands. A zone, some 3 km wide, of intensive dyke injection, the Jersey Main Dyke Swarm, cuts across post-orogenic calc-alkaline plutons, though showing a close temporal link with their emplacement.The nature of the dyke swarm magmatism is dominantly but not exclusively basic. A clear acid-basic bimodality is present with very little intermediate material. Although subjected to varying degrees of alteration under low-grade metamorphic conditions, the primary nature of the rocks is often preserved. All those having a basaltic composition are olivine-normative, but show a high-K calc-alkaline character. Fairly high LREE enrichments are present. MORB-normalized spidergrams confirm that the magmatism was of a mature calc-alkaline type, similar to that generated at active continental margins such as the Andes.The presence of the swarm indicates a period of late post-orogenic crustal extension sub-parallel to the NE-SW strike of the proposed Cadomian active continental margin in the Armorican Massif. It could represent injection of the last remnants of magma from an already decayed subduction zone system into a newly cratonized environment.

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During early Ordovician times volcanic and high-level intrusive activity occurred at numerous centres across North Pembrokeshire. Previous work suggested a geochemical transition in this activity from calcalkaline through to tholeiitic with time. However, new data indicate more extensive calc-alkaline magmatism and that both magma types were coeval. The origin of the calcalkaline magmas remains equivocal, but the tholeiitic magmas appear to have been derived from a source similar to N-type MORB, variably modified by supra-subduction zone fluids, combined with some fractionation during ascent. These data are consistent with emplacement in a supra-subduction zone marginal basin.

Samples collected from the coarse basal portions of mid-Cretaceous volcaniclastic turbidites from the Mariana and Pigafetta basins are remarkably similar in terms of the petrographic and chemical features of their igneous clasts and bulk rock composition. Clasts of magmatic origin are dominated by glassy vesicular shards, variably phyric, holocrystalline basalts, and crystal fragments (olivine, clinopyroxene, plagioclase, amphibole, and biotite). The composition of the pyroxenes and amphiboles are typical of those found in differentiated hydrous alkali basalts. The bulk chemical composition of the volcaniclastites (based on stable incompatible elements and their ratios in highly vitric samples) is characteristic of alkali basalts found in within-plate oceanic eruptive environments. Miocene volcaniclastites from Site 802 are broadly similar to the Cretaceous samples in terms of clast type and bulk composition, and have also been derived from an oceanic alkali basalt source. The chemistry of the Miocene volcaniclastites differ, however, in having distinctive Zr/Y and Zr/Nb ratios and a more restricted chemical composition. The magmatic products of nearly emergent seamounts within the western Pacific basins appears to have been dominated by alkali basalt volcanism during the mid-Cretaceous and also the Miocene.The highly vitric nature of the Cretaceous and Miocene volcaniclastites, together with the morphology and vesicularity of their shards, suggests that they are the reworked (via mass flow) products of hyaloclastite accumulations produced in a shallow-water eruptive environment, such as that adjacent to nearly emergent seamounts or ocean islands. The association of ooids, reefal debris, and, in rare cases, woody material with the volcaniclastites supports their shallow-water derivation.

Conventional techniques for Plug and Abandonment (P&A) operations usually involve multiple trips downhole to mill the casing and underream the section. This paper describes new tools and methodologies developed to reduce rig time in Plug and Abandonment operations, specifically section milling and underreaming.The service provider has been working with an operator in its on-going Brent Field abandonment campaign, during which over 140 wells will be abandoned. The scope of work requires section milling a window in the 9-5/8" casing, and then underreaming the open hole to a half inch diameter over the original wellbore size. This is to allow placement of an abandonment cement plug against the original formation. The complete operation requires displacing the wellbore to milling fluid, section milling and then underreaming, all of which have traditionally been performed in separate runs.In response to these operational/economic challenges, a P&A system was developed using an innovative Ball Activated Drilling Valve (BADV) and a unique High Ratio Underreamer (HRU). This combination of tools made it possible to execute all operations in a single trip. The system allows the operator to mill and underream a window in a single trip; this will allow placing a rock-to-rock abandonment barrier plug in the required position.While tripping in hole, the BADV is open to the annulus which allows the string to self-fill, so no extra time is required top filling the pipe. Once the Bottom-hole Assembly (BHA) is at the section milling depth, the wellbore is displaced to milling fluid through the open BADV. Once displacement is complete and fluid rheology is suitable to ensure good hole cleaning of metal swarf during milling, a ball is dropped to shut the valve. Cut-out and section milling are then performed to achieve the required window length, once this has been competed the HRU tool is activated by drop ball and the open-hole section underreamed.A field test plan was presented and discussed with the offshore supervisors during pre-job briefing and they were introduced to the tool's functionality in the workshop. The first run with the combined section mill, HRU and BADV assembly took place on Brent and saved over 24 hours rig time. The system was then successfully trialed on seven challenging 9-5/8Љ casing milling jobs which resulted in total savings of over six days rig time. In addition, there was a reduction in the safety exposure to the crew by handling only a single BHA.Over the course of the campaign, the P&A team made several changes to the abandonment process enabling the operator to eliminate dedicated cleanout trips between section milling and underreaming runs. This streamlining process successfully combined operations that previously required four trips to complete down to one. Additional design changes/modifications were made during field testing and performance continually monitored to ensure the field test objectives were met and improvements to the tool/system reliability were being made. Product Engineers and Fie...