We present a study of benthic foraminiferal faunas from the outer continental shelf off Congo (tropical West Africa), with the aim to determine the impact of the discharge of oily drill cuttings on the sea floor environment, to judge the regenerating capacity of the benthic ecosystem, and to investigate the possibility to develop an environmental monitoring method for open marine continental shelf environments, based on benthic foraminifera. We studied the spatial distribution and microhabitats of living and dead foraminiferal faunas, sampled in April 2003, 4 years after the end of disposal activities, in the upper 3 cm of the sediment at 9 stations (about 180 m depth) offshore Congo, that were subject to various degrees of pollution by oily cuttings from 1993 until 1999. Our results describe a zonation of foraminiferal faunas in the 750 m around the former disposal sites. At the immediate vicinity of the discharge points (within 70 m), faunas are characterized by low foraminiferal densities. Faunas between 70 m and 250 m of the disposal sites have very high foraminiferal densities, with high percentages (about 80%) of opportunistic taxa such as Bulimina aculeata, Bulimina marginata, Textularia sagittula, Trifarina bradyi and Bolivina spp. Between 250 and 750 m from the disposal site, foraminiferal densities decrease, and the percentages of opportunistic species are lower (40-60% of indicator species). These results show that 4 years after the cessation of oily cutting disposal, strong environmental impact is limited to the 250 m around the disposal sites. We used these data to develop a quantitative pollution index, values of which are strongly correlated to distance to the disposal site. This foraminiferal index offers the possibility to quantify the impact of anthropogenic eutrophication in continental shelf environments, but its validity must be tested in other continental shelf environments.

In order to assess the possible environmental impact of oily cuttings discharged during oil exploration activities, we studied the benthic foraminiferal faunas in a five-station, 4-km-long sampling transect around a cuttings disposal site at about 670 m depth offshore Angola (W Africa), where drilling activities started 1.5 years before sampling. Living (Rose Bengal stained) and dead foraminiferal faunas were sampled in March 2006. The faunal patterns mirror the spatial distribution of hydrocarbons, which are dispersed into a southeastern direction. Four different areas can be distinguished on the basis of the investigated faunal parameters (density, diversity and species composition of the living fauna, and comparison with subrecent dead faunas). The fauna at station S31, 300 m SE of the oil cuttings disposal site, appears to be clearly impacted: the faunal density and diversity are maximal, but evenness is minimal. Taxa sensitive to organic enrichment, such as Uvigerina peregrina, Cancris auriculus and Cribrostomoides subglobosus, have largely disappeared, whereas the low-oxygen-resistant taxon Chilostomella oolina and opportunistic buliminids and bolivinids attain relatively high densities. At station S32, 500 m SE of the disposal site, environmental impact is still perceptible. The faunal density is slightly increased, and U. peregrina, apparently the most sensitive species, is still almost absent. The faunas found at 1 and 1.8 km SE of the disposal site are apparently no longer impacted by the drill mud disposal. Faunal density and diversity are low, and the faunal composition is typical for a mesotrophic to eutrophic upper slope environment. Finally, Station S35, 2 km NW of the disposal site, contains an intermediate fauna, where both the lowoxygen-resistant C. oolina and the more sensitive taxa (U. peregrina, C. auriculus and C. subglobosus) are present. All taxa live close to the sediment-water interface here, indicating a reduced oxygen penetration into the sediment. Since the hydrocarbon concentration is low at this station, it appears that the faunal characteristics are the consequence of a slightly different environmental setting, and not due to a contamination with drill cuttings. Our data underline the large potential of benthic foraminifera as bio-indicators of anthropogenic enrichment in open marine settings, such as caused by the disposal of oily drill cuttings. The foraminiferal faunas react essentially by a density increase of a number of tolerant and/or opportunistic taxa, and a progressive disappearance of more sensitive taxa in the most impacted area. Rather surprisingly, large-sized taxa appear to be more sensitive than small-sized foraminiferal taxa.

Résumé -Impact des activités de forage en mer chaude : capacités de recolonisation des fonds marins -Étant donné les contraintes importantes liées aux réglementations environnementales, le rejet des déblais de forage en mer doit être contrôlé. Il est par conséquent nécessaire de recueillir des informations afin de contribuer à la sélection de fluides de forage respectueux de l'environnement. Cette étude a été menée dans l'objectif d'évaluer, dans les conditions les plus réalistes possible, l'impact des déblais de forage, la capacité de régénération des écosystèmes et les processus de recolonisation. Dans une première phase, différents tests ont été réalisés sur plusieurs sortes de boues de forage et de déblais (contenant du gazole, deux huiles paraffiniques différentes et du glycol) : des tests de toxicité standards (OSPAR) et expérimentaux, des mesures de biodégradation en conditions aérobie et anaérobie, et des tests de lixiviation. Dans une seconde phase, les mesures les plus appropriées ont été appliquées dans le cadre d'une expérience sur un site de rejet de déblais au Congo (Afrique). Les opérations de rejet ont débuté en novembre 1993 et ont pris fin en avril 1999. Parallèlement à cette campagne de mesures, une analyse complète du benthos a été réalisée huit ans après le début des rejets. Les résultats préliminaires ont démontré que : -la zone de toxicité provoquant des modifications du benthos est inférieure à quelques centaines de mètres autour du site de rejet ; -la faune originale a été remplacée par une faune plus spécifique. Dans le contexte plus général des contraintes environnementales liées à l'industrie pétrolière, ce travail a démontré le confinement des zones d'impact, l'absence de risques significatifs de modification des éco-systèmes adjacents ainsi que l'application possible à d'autres sites de rejet de déblais. Abstract

Since the onset of offshore drilling, the drill cuttings and their disposal have received much attention in the oil and gas industry. Numerous studies performed in the laboratory on the foreseeable fate of cuttings have led to the search for new formulations with the objective of achieving lower toxicity and better intrinsic biodegradability. The study of the marine environmental impact associated with the discharge of OBM (oil base muds) drill cuttings has already given rise to numerous works and surveys related to the North Sea, but it is clear that knowledge is still limited. Further information is required on the monitoring of sediments polluted with OBM cuttings, especially in warm sea conditions. On this topic, a research project was conducted in 2002 and 2003 in Congo (Africa) in order to study the modes of regeneration of the benthic fauna and the recolonization of the seabed after the discharge of OBM cuttings. Previous sampling and analyses were performed in 1995 (after the first step of drilling) and 2000 (one year and a half after the stop of cuttings discharge). In 2002, a systematic study of the abundance and diversity of the benthic fauna was conducted. In addition, a methodology based on classical physical chemical analyses on sediment, tests of toxicity, biodegradability and lixiviation was applied, in order to explain the evolution of the recolonization by the benthic fauna (SPE 77471). In the more general context of the environmental constraints of oil industry, our work demonstrated the containment of the impacted zones, the absence of significant risks of modification of the adjacent ecosystems and the possible application to other discharge sites. We present here the results related to the last phase of sampling carried out in 2003. It is noteworthy that the whole results confirm the main evolutions observed in the previous survey. Introduction Although the study of the impact associated with the discharge of drill cuttings impregnated with fluid has already given rise to numerous works related to the North Sea [1, 2], it is clear that knowledge is still limited. Field studies have been done to answer to an accurate representation of the deposition of cuttings on the seabed. But, pertinent biological and ecotoxicological parameters have to be related to the dose rate of cuttings for an accurate assessment of the effect of cuttings accumulations on the marine environment. Further information is therefore required on the monitoring of cuttings discharged today and on the fate of old cuttings, especially in warm sea conditions. In order to study the modes of regeneration of the benthic fauna and of re-colonization of the seabed after the discharge of cuttings, a project was conducted between 2000 and 2003 with different objectives:To validate an experimental protocol for monitoring re-colonizationTo assess the real impact of cuttings contaminated by oil based drilling fluidsTo assess the capacity for regeneration of the environment as a function of variations in the amounts of contaminating material. This project was performed in two steps. The first phase was dedicated to the study of oily cuttings from an eco-toxicological point of view (chemical analysis of hydrocarbons, aquatic and sediment toxicity, aerobic and anaerobic biodegradation, lixiviation). The main objective was to validate and select the most suitable methods for the continuation of the project [3]. The second phase concerned the study of the impact of drill cuttings piles on a real site of discharge under warm sea conditions (Congo, Africa). The site is located on the offshore N'Kossa field, some 60 km off Pointe Noire, under a water depth of 180 m. Cuttings discharge (essentially Low Toxicity Mineral oil Based Fluid cuttings) began in November 1993 and ended in April 1999 and generated around 10,000 m3 of cuttings. A baseline study was undertaken after the first step of drilling in 1995, and was followed by 3 monitoring surveys in November 2000, March 2002 and April 2003 (Fig. 1). Results for 2000 and 2002 were presented and discussed in previous SPE papers [3, 4]. We present in this paper the last results from the 2003 survey.

During E&P offshore activities, and in particular during drilling operations, the living conditions of the flora and fauna on the seabed may be disturbed. The paper describes the study made for assessing the actual impact of the discharged cuttings, the regenerating capacity of the ecosystem and the medium and long-term recolonization processes, based on in situ measurements. A previous study undertaken on cuttings recently discharged offshore Congo measured kinetics of space and time restoration of the drilling site. To validate the results we carried out a similar study dedicated on aged cuttings offshore Gabon. Usual process for monitoring biological effects of cuttings includes several approaches such as benthic macrofauna (> 1mm size) studies, physicochemical analysis of sediment and ecotoxicological tests. One of the key features of our study was to add, to this process, a study of the benthic meiofauna (0.1 to 1mm size). Experiments conducted demonstrated:very low concentrations of hydrocarbons stemming out from the cuttings,a very low toxicity of sediment,no benthic macrofauna community alteration,no change in the communities of actual and fossil foraminifera (meiofauna) and,last but not least, that the study of the benthic meiofauna improves the quality of the assessment made. Introduction In order to study the way the benthic fauna regenerates and ofrecolonises the seabed after the discharge of cuttings in a warm water context, a project was conducted in 2 phases between 2003 and 2004 off Congo with different objectives:To validate an experimental protocol for monitoring re-colonization in the context of aged cuttings. We focused on the impact of oily cuttings,To assess the real impact of cuttings contaminated by oil based drilling fluids and the capacity of regeneration for regeneration of the environment overas a function of time. [1–5] Off Congo, the last survey was realized 4 years after the end of cuttings discharge. To validate the Congo results, we carried out a similar study but on this study related to a site of aged dumped oil based cuttings (12 years after cessation of the disposal activities) offshore Gabon. Description The site is located off Gabon at the offshore Anguille Marine field, some 50 km off Port Ggentil, underunder a water depth of 30 m. Cuttings discharge began in 1986 and ended in 1992 but in 2003 an additional well was drilled with highly biogegradable base oil. We present in this paper the results from the survey performed in June 2004.