Onshore Sector Research Articles

Environmental risk assessment and the environment agency

This paper explores the role of environmental risk assessment in environmental protection, as a tool to assist decision-making by both the environmental regulator and regulated industry, and as a means of supporting the relationship between the two parties. The use of risk assessment in process safety has been established over the last few decades, and is demonstrating benefit in many different applications. It is considered that similar benefits will also be found in environmental applications of risk assessment. Recent developments in environmental risk assessment within the Environment Agency will be described. The concepts of goal-setting and risk-based `Safety Cases' have been introduced into safety legislation affecting offshore industry and certain onshore sectors such as rail transport. The further development of these concepts to include environmental legislation will be discussed, and the part which industry may play in this process considered. There are potential benefits to adopting a risk-based goal-setting approach to environmental protection, but there are also limitations in the application of risk assessment to the environment which need to be overcome. Whereas safety risk relates to the occurrence of accidents, environmental risk may arise from both accidents and routine operations, involving potentially long term processes and complex environmental behaviour. Both the benefits and limitations will be identified, and a `way forward' outlined for progressing a risk-based approach to environmental protection.

RESERVOIR AND SOURCE‐ROCK CHARACTERISATION OF THE EARLY PALAEOZOIC INTERVAL IN THE PERIBALTIC SYNECLISE, NORTHERN POLAND

Detailed investigations have been conducted on samples from 2,750m of conventional cores from 18 wells penetrating the Cambrian siliciclastics in northern Poland. The results of sedimentological, petrological and geochemical analyses were combined with a computeraided regional evaluation of the Polish onshore sector of the Peribaltic Syneclise. Our results may help in the successful future identification of Cambrian prospects in northern Poland.The Middle Cambrian reservoir sandstones consist of quartzites in which low porosities (5% ‐ 8%) and permeabilities (5 to 24 mD) were preserved only in oil‐bearing structures in the more deeply‐buried, western part of the Peribaltic Syneclise. Isotopic studies suggest crystallisation temperatures for the pore‐plugging quartz cement of between 53d̀C and 78 d̀C. These temperatures were not reached in the shallower, eastern part of the basin, where porosities of up to 20% are present in the quartz arenites which constitute potential reservoir units.A 150‐m thick, organic‐lean (0.1‐l.0 wt. % TOC) Middle Cambrian shale below the quartzites, and an up to 20‐m thick, organic‐rich (4.5–10 wt. % TOC) Late Cambrian shale unit above the quartzites, are regarded as source rocks for the oil accumulations in the Cambrian reservoirs. The organic matter in these two shale units consists of abundant grey, lenticular particles and finely‐dispersed bituminite‐like constituents. The reflectance intensity of these bitumen particles is a useful parameter for evaluating the source rocks' maturity. Geochemical investigations have proved the (potential) source rocks to be immature in the shallower, eastern part of the Peribaltic Syneclise, and to have reached the “oil window ” in the more deeply‐buried western part of the basin.The extractable organic matter is dominated by saturated hydrocarbons. Late Cambrian extracts can be differentiated from Middle Cambrian samples by their higher content of aromatic compounds. Reservoired Cambrian oils and extracts from Cambrian source rocks could not be distinguished by triterpane “fingerprints ”. Both are characterised by a well‐developed sequence of tri‐ and pentacyclic compounds, with no variation in their relative abundances.Burial‐history modelling indicates that rapid subsidence of the Cambrian deposits to depths of more than 2,500m occurred during the Silurian in the western part of the basin. In the eastern part, however, a much more moderate burial history is inferred. Here, the Cambrian interval lies at depths of only 1,500 to 2,200m. An analysis of the basin's geothermal history indicates that porosity‐plugging quartz cementation in the Cambrian quartzites commenced in the western Peribaltic Syneclise during this rapid Silurian burial. In this western region, the source rocks reached the “oil window” during the Devonian. Hence, migration and accumulation of the generated hydrocarbons could preserve only the remaining pore space, which had already been drastically reduced. This genetic model implies that petroleum traps were formed before or during the Carboniferous.Because the thick Silurian marlstones acted as an efficient regional seal, the generated hydrocarbons must have been trapped somewhere in the underlying Early Palaeozoic sequence. Oil stains in Cambrian quartz arenites in the organic‐geochemically immature eastern part of the basin point to long‐distance, up‐dip migration towards the basin's eastern margin. A scarcity of structural traps and reduced shalyness of the eastwards‐coarsening, Early Palaeozoic sequence are adverse features, relative to a high degree of preservation of the hydrocarbons generated. The search for subtle diagenetic traps, however, may prove to be challenging.

Acoustic Impedance Studies in Triassic Reservoirs in the Netherlands - Applications to Development and Exploration: ABSTRACT

Simple and cost effective seismic forward modelling techniques have been used in conjunction with petrophysical and geological data to provide an integrated approach to understanding the seismic response of Triassic gas reservoirs onshore and offshore Netherlands. Analysis shows that for the Volpriehausen Sandstone in the offshore sector a relationship exists between reservoir acoustic impedance and porosity such that an increase in porosity leads to a decrease in acoustic impedance. Data can be sub-divided on the basis of fluid fill and cementation with trends for both gas and water cases. Regression analysis has defined the optimum relationship for each fluid case and these relationships have been used to predict the acoustic impedance profiles for a variety of reservoir scenarios. Modelling shows that the highest seismic amplitudes and the greatest relative amplitude variation with fluid fill are related to high porosity reservoir. In the onshore sector, analysis for the Roet Sandstone has shown that even small scale variations in reservoir properties can be recorded within the detail of the seismic response. Results from seismic forward modelling compare with amplitude variations observed in real data and suggest that, within the limitations of the dataset and methodology, the technique can be used to predictmore » reservoir attributes from the seismic response. So far, the technique has been sucessfully applied to both exploration and field development projects.« less