Reservoir Geology of the Morecambe Field

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Abstract
Summary: The Morecambe Gas Field, located in the Irish Sea Basin, came on stream on 1 January 1985 and is to be used as a seasonal storage facility. Structurally the field is a northwest-southeast trending faulted anticline divided into two parts by a narrow southwest-northeast graben system. Gas is produced from the upper part of the Lower Triassic Sherwood Sandstone Group. The reservoir sandstones were deposited predominantly by the action of low sinuosity braided streams during periods of high stage flow. Facies associations recognised from core and wireline log data include: Channel Sandstones, Secondary and Ephemeral Channel Sandstones, Sheetflood Sandstones, Floodplain fine member and Channel abandonment deposits. Reservoir quality is dependent on grain size, shape, sorting, facies control and the distribution and abundance of authigenic cements. Quartz and dolomite cements are a major factor in causing porosity reduction, whilst authigenic illite controls effective permeability. Platy illite only occurs below an identifiable datum believed to represent a ‘paleo’ gas-water contact. Below the illite datum permeability is impaired by up to one hundred fold. A change in illite crystal morphology in the lower parts of the reservoir results in a basal zone which exhibits improved quality. The main producing horizons are stacked sandbodies within the Keuper Waterstones and Keuper Sandstone Frodsham Member. On the basis of facies modelling a three dimensional reservoir model has been developed for simulation purposes. A simplified three layer model which incorporates the influence of authigenic illite has also been erected and will control future field development. A phased development programme is planned for the field over the next ten years. The first stage is currently underway, utilising slant drilling technology to economically drain the reservoir. Maximum gas production from the field will be 1200 million cubic feet per day.