The Palaeogene sequence of the Outer Moray Firth in the UK sector of the North Sea consists of a series of stacked submarine-fans and associated shelf deltaic deposits. The sequence was derived from the elevated Shetland Platform to the northwest. The clastic material was transported southeastwards along the axis of the Witch Ground Graben towards the basin low in the Central Graben. Towards the top of the Palaeogene submarine-fan slope deposits, there is a disturbed sequence, Oligocene in age, which is interpreted to have been deformed by Miocene age extensional faulting. A semi-regional study of Quadrants 15, 16, 21 and 22 has revealed that the faults, which cut the Eocene to Lower Miocene section, terminate at the Middle Miocene unconformity. Fault activity coincided with a Middle Miocene tilting event indicated by the onlap of Upper Miocene sequences higher on the shelf slope. This tilting event acted as a trigger for extensional fault movement. Detailed study of the extensional fault geometries illustrates the complex nature of the deformation. The dominant fault trend is northeast-southwest, commonly with the downthrown side towards the northwest. The faults do not exhibit a marked listric geometry nor do they have a common detachment horizon. A common feature is the decrease in observed brittle deformation with depth. Analogue sandbox modelling illustrates the development of extensional faults associated with the gravitational collapse of a tilted sequence. Deformation within the sandbox models is dominated by non-rigid block rotation. The Miocene-aged faulting in the Outer Moray Firth is interpreted to be a result of the gravitational collapse of the Palaeogene slope sequence. Fault dip towards the shelf was probably controlled by shear stresses within the deforming sequence and a downslope resistance to deformation. A non-rigid block rotation model is proposed as a possible mechanism for the deformation. A basal detachment is not a requirement of this model.
A numerical basin modelling approach was used to investigate the hydrodynamics and development of overpressures in a Jurassic Prospect in the Greater Gorgon area of the Barrow Sub-basin. Abnormally large fluid pressures have been encountered by numerous exploratory wells in the Carnarvon Basin; however, the mechanisms responsible for overpressuring were uncertain. To better evaluate the risk of encountering overpressures while drilling Jurassic targets in the area, quantitative basin modelling was conducted. Groundwater flow, heat transfer, and hydrocarbon generation were simulated along two geologic cross-sections. Model results for the fluid pressure history of the Jurassic Prospect were constrained by hydrodynamic data from specific wells in the region. A series of modelling experiments was used to determine the relative significance of compaction disequilibrium, tectonic uplift, organic maturation and permeability on overpressure generation. Results indicate that compaction disequilibrium and the permeability of shale layers are the dominant controls on overpressures, while organic maturation does not contribute a significant amount to the pressure anomaly. Quantitative basin modelling applied to pressure prediction provides critical insight needed prior to drilling and well construction.
The economic consequence of exploitation in areas with an unspecified risk of abnormal pressure profiles range from increased drilling costs to unrealised prospect potential. Optimised planning practices will impact on not only the costs of drilling but also on the quality of the reservoir evaluation and productivity assessment.Numerous exploratory wells in the Carnarvon Basin have encountered unprognosed high pore pressures in the past, resulting in increased concerns about well control and safety, and as a consequence higher drilling costs. High pressures encountered at Parker–1ST1, Forrest–1AST1, Venture–1ST1, and Venture–2ST1, in particular, caused a high level of concern for moderately deep drilling targets. As a result of the diversity of potential overpressure mechanisms, there is a variety of opinion on the risk of encountering abnormal pressure in particular areas, and no standard way to capture and incorporate this information into planning and drilling decisions.At the end of 1999 a project was defined to evaluate the risk of overpressure of a prospect in this area. This project had the primary focus of evaluating the risk of encountering high pore pressures when drilling an exploratory well. This risk assessment was directed towards a decision point for well design, which had cost implications in anticipation of drillingIn the process of technical assessment of abnormal pressure in this prospect, it was important to include all of the necessary data and technical practices, which could contribute to an understanding of the prediction. The application of a cooperative multi-functional team and a software infrastructure (Juniper) was instrumental in allowing this process to take place. A visual decision tree with risk assessment allowed for input from all contributors and for the inter-relationship of the inputs. This comprised the methodology for assessment.The methodology highlighted, explicitly, areas of high uncertainty where evidence neither for nor against overpressure was available. These areas became the focus of technical work, as they had potential for significant impact on the risk assessment. A sub-team extensively reviewed the pressure tests, sonic logs, borehole influxes and mud weights of nearby wells. The results of the well study had a bearing on the choice of analogue, the geological model, and pressure prediction. A geological modelling sub-process was carried out to test the significance of compaction disequilibrium, organic maturation and permeability on overpressure generation. A geophysical sub-process was initiated to complement the pressure modelling risk element. Surface seismic data was depth processed to relate velocities to trends obtained from the well study.The methodology was found to be a highly effective technique for recording the decision processes and as a tool for interdisciplinary communication in a cooperative and non-threatening environment. The outcome of the study was to highlight reduced risk of overpressure in the prospect as perceived by all parties (geoscientists, drilling engineers, management and joint venture parties). The common view on risk prompted a reassessment of the risk profiles on all the related wells in the program and allowed revision of the drilling programs and significant cost savings relative to the original forecasts for the program.
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