Abstract The management and outcomes of the volcanic crisis on Montserrat, which began with the onset of activity at the Soufrière Hills Volcano (SHV) on 18 July 1995, might have been very different without the scientific precedents set by the Mount St Helens eruption, USA, on 18 May 1980, and the research advances that followed. This narrative is intended to show the steps taken by health scientists in response to the unfolding developments at the volcano to characterize the hazard presented by the volcanic ash and to devise mitigation measures to prevent the development of irreversible lung disease in the island population. Initial assessments of the health risk for silicosis were deterministic and based on industry exposure limits derived from published epidemiological and clinical studies of workers exposed to dusts containing free crystalline silica. However, by 2003, new research findings on the ash enabled the risk to be updated with a probabilistic approach incorporating the expertise of scientists from a wide range of disciplines including toxicology, volcanology and statistical modelling. The main outcome has been to provide reassurance to the islanders and policy makers that the chances of developing silicosis on Montserrat are very small given the preventive measures that were adopted during 1995–2010 and the change in style of the eruption.

Abstract Improved matching of reservoir simulation model results to production data in gas fields allows improved reservoir management and optimization of hydrocarbon production. In high net:gross aeolian sandstones this requires the full range of scales of permeability heterogeneity to be represented in a model. This necessitates the modelling and appropriate upscaling of rock properties, including the millimetre-scale lamina effects within the aeolian dune slipface facies, the contrast between dune apron and slipface facies and the impact of first- and second-order bounding surfaces within the dune complexes and the super-bounding surfaces that separate them. The Neptune Gas Field lies on the northwest margin of the Southern North Sea gas basin and in Block 47/4b and extends into Blocks 47/5a and 42/29. Gas is reservoired in the Lower Leman Formation which in the Neptune Field, comprises a succession of stacked dry aeolian sandstones and thin, wet interdune deposits. Permeability varies significantly at the lamina and bedform scale and these variations impact the performance of production wells in the field. A deterministic reservoir simulation model was constructed prior to development drilling, with permeability determined from porosity-permeability transforms based on core analysis data. The collection of continuous bottom-hole pressure (BHP) data from three of the development wells showed that this model failed to match accurately the rate of pressure decline whilst flowing, and over-predicted the build-up pressure when the wells were shut in. Review of the sedimentological information revealed that heterogeneity in the aeolian sands was being lost in the modelling of permeability and the coarse upscaling. To test whether the poor dynamic history match was due to the deterministic model not incorporating the permeability heterogeneity, a fine-scale geocellular model was built to characterize the expected geological heterogeneity and then upscaled for flow simulation. To accommodate the range in scales of heterogeneity present, the modelling was undertaken in two principal steps. First, the lamina-scale effects were modelled to determine average flow properties perpendicular and parallel to dune migration directions. The results of the lamina-scale heterogeneity modelling were then incorporated into petrophysical models of dune bodies that were characterized by low permeability dune apron facies and high permeability base of dune slipface facies that grade upwards into moderate permeability slipface facies. Upscaling of the stochastic permeability grid captured the impact of the fine-scale heterogenetity and gave BHP responses at the development wells in better agreement with the production data than any deterministic permeability distribution modelled. The stochastic rock property distributions have, thus, been adopted as the base case in all future history match and reservoir performance prediction studies.

ABSTRACT The most important source rocks that generate hydrocarbons in the eastern Caribbean basins are Cenozoic in age. Geochemical analyses of rocks and oils from the Greater Antilles region, Barbados Accretionary Prism, Tobago Trough, Cariaco Basin, Urumaco Trough, Falcon Basin, and Guajira Basin prove the existence of Paleogene and Neogene source rocks that have generated hydrocarbons through time. Along the Greater Antilles region, the middle Miocene Sombrerito Formation contains immature oil-prone source rocks, whereas in the vicinity of the Virgin Islands, Eocene and Oligocene strata have gas potential. Oil fingerprinting of Azua-San Juan Basin oils suggests the existence of a Miocene to Pliocene carbonate source rock deposited in a possible lacustrine environment. The Eocene Joe’s River Formation and Scotland Group within the Barbados Accretionary Prism contain source rocks, which are oil- and gas-prone, respectively. These sequences could have generated the Barbados Island’s oils. In the southern part of the Tobago Trough, an immature middle Miocene section with fair to good Type III kerogen source rock has been identified. The Miocene, Oligocene, and Eocene sequences drilled in the Cariaco Basin contain source rock intervals, which could generate mainly gas and minor amount of oil. In the Falcon Basin, the Eocene and Miocene sequences have fair to good potential to generate gas and minor oil. The Miocene in the Guajira Basin contains a fair to good gas-generating source rock, and the large gas accumulations in the basin were generated mainly by microbial activity from this Miocene sequence. The existence of Cretaceous oil-prone source rocks in the Caribbean region is quite uncertain because of limited well penetrations, poor outcrops, and discontinuous seismic coverage. Consequently, their aerial extent, thickness, and distribution of organic facies are very limited. To date, there are no evidences to document the presence of the Upper Cretaceous La Luna and its equivalents within the Caribbean plate.

ABSTRACT The Barbados Island is the topographically highest part of the Barbados Ridge and the only subaerially exposed part of the accretionary prism. Oil production from the Woodbourne field and the presence of migrated petroleum in outcropping rocks prove the existence of a working petroleum system. Detailed geochemical analyses suggest that petroleum onshore Barbados was derived from a Cretaceous deep marine shale source rock deposited under oxic-to-dysoxic conditions with varying contributions of marine and land plant-derived organic matter. This petroleum can be categorized into two groups. Group A petroleum was generated and expelled at the early oil window (0.72%Rc–0.77%Rc) from an interval of the shale source rock containing predominantly marine organic matter. By contrast, petroleum in group B was generated at the peak of the oil window (0.87%Rc–0.94%Rc) from a more proximal interval of the same source rock containing comparatively higher input of terrestrial-derived organic matter. These observations suggest the existence of the shale source rock at different maturities within a possible multiple-stacked source rock system. Reservoirs at the Woodbourne field have received at least two charges of hydrocarbons recognizable at present day. A first filling event is interpreted to have charged the reservoirs with the lower maturity petroleum (group A) after the middle Miocene uplift of the Barbados Ridge. Oils in reservoirs above 1000 m (3280 ft) depth are heavily biodegraded, whereas more deeply buried oils are moderately biodegraded. The second more recent charge consists exclusively of light hydrocarbons ( n -C3 to n -C9) expelled from the source rock at maturity levels similar to those determined for group B petroleum (0.86%Ro–1.05%Ro). These light hydrocarbons probably separated from their parental oils and migrated upward through faults or partially leaking seals, reaching reservoirs that contain early mature degraded oils above and below 1000 m (3280 ft). This charging event is inferred to be triggered by the Pliocene–Pleistocene tectonic event. A comparison using facies-sensitive biomarkers indicates that Barbados petroleum was not derived from carbonate facies typical of the La Luna Formation or its eastern equivalent, the Querecual Formation. This comparison shows that Barbados petroleum was sourced by clastic facies similar to those sourcing petroleum in the northern part of the Eastern Venezuelan basin, Gulf of Paria, and Trinidad. It implies that Barbados-type source rocks were deposited over vast areas along the Cretaceous passive margin of northern South America. These organic-carbon rich strata were probably incorporated into the western margin of the Barbados prism during the early stages of accretion (Paleocene–Eocene). Although speculative, the aforementioned observations suggest that significant petroleum potential may be present within the Barbados accretionary prism, but also further south within several offshore basins of Venezuela, Trinidad, and Tobago.

We present a study of older rocks exposed on Grenada and the Grenadine Islands of the southern Lesser Antilles arc platform (SLAAP) with a view toward understanding the structural and stratigraphic evolution of the platform that led to the modern (and Neogene) magmatic arc. The modern SLAAP is an east-tilted half horst that probably developed early in Miocene time by uplift along a zone of north-striking normal faults relative to both sea level and the crust of the adjacent Grenada Basin. The uplift was completed before Neogene magmas erupted at the surface of the SLAAP beginning at about 12 Ma. Twenty-six units of older rocks, defined as those that preceded local Neogene magmatism, have been identified by us and previous workers in the SLAAP. Our contributions are new stratigraphic and structural analyses and microfossil and radiometric dating. The age range of older rocks is early middle Eocene to middle Miocene. The most stratigraphic information comes from Carriacou, which contains newly defined formations of Eocene and Oligocene age that are thrust above markedly different late Eocene facies. The thrust is covered by Miocene, possibly late Oligocene, strata below which there is a lacuna possibly as long as 10 m.y. The oldest rocks of the SLAAP are the middle Eocene Mayreau Basalt. Earlier claims of a Cretaceous age of rocks on Union Island are in error; we show them to be Eocene. Assuming they were all deposited contiguously, the older rocks units can be grouped and interpreted as follows: I, middle Eocene pillow basalt of spreading origin and pelagic cover; II, late middle Eocene–middle Miocene deep basinal sediments comprising arc-volcanigenic turbidite and hemipelagite, together with minor intrusive basalt; III, early(?) and middle Miocene local carbonate platform. Group I basalts are correlated with crust of the Grenada Basin, and the Eocene and Oligocene sediments of I and II are equivalent to the deep strata of the Grenada Basin. No magmatic arc rocks of Paleogene age are recognized in the SLAAP. Older rocks of the SLAAP are deformed by folding, thrusting, and foliation development, and by Neogene normal faulting and intrusion. The principal deformation was a horizontal contraction of northerly bearing in late Oligocene and (or) early Miocene time before the uplift of the half horst. A later horizontal contraction of westerly bearing occurred in the middle Miocene after the horst had developed. The northerly contraction in the SLAAP is interpreted to have arisen by accretion of Grenada Basin cover and slices of shallow basement during a brief episode of subduction of the oceanic crust of the Grenada Basin relatively southward below the (?) Tobago terrane. The accretionary prism of the SLAAP is inferred to have been continuous with the accretionary belt of the Southern Grenada Basin Deformation Front that extends west-southwest to a point west of Margarita. A Paleogene magmatic arc was the source of copious volcanigenic and carbonate clastic sediment to the SLAAP basin in late Middle Eocene and Oligocene times. This arc was not in the SLAAP and has not been directly located. We infer its locus is south of the Grenada Basin, striking west-southwest from Grenada; its original orientation is uncertain, but scanty paleomagnetic data provide no evidence for large rotation. The Neogene magmatic arc of the southern Lesser Antilles is not a clone of the Paleogene arc. It developed transverse to the Paleogene arc and SLAAP accretionary prism, presumably by a major reconfiguration within the Caribbean-American plate boundary zone. The large change in arc trend is thought to be due to collision between the Paleogene arc system and continental South America.