The use of dipmeter logs in the structural interpretation and palaeocurrent analysis of Morecambe Fields, East Irish Sea Basin

Abstract

Dipmeter logs have been processed in a variety of ways as an aid to understanding the geology of the North and South Morecambe Gas Fields. They have yielded high quality structural information which has contributed to understanding the structural configuration and tectonic history of these fields. Several faults have been identified, cutting well paths, and dipmeter analysis has allowed the correct orientation and direction of throw to be determined. In one well, the deviated well path has intersected each of two listric faults in two separate places, allowing unusually precise definition of these fault planes. Throughout the Ormskirk Sandstone Formation, the predominant palaeocurrent direction has been found to be towards the west or southwest. The most consistent palaeocurrent orientations were measured in intervals of planar cross-stratified channel-fill facies sandstones, but similar results, with greater scatter were observed in ephemeral channel sandstones facies. These consistent palaeocurrent results suggest that the East Irish Sea Basin formed a distinct depo-centre during Ormskirk sandstone times. The implication is that during the deposition of the Ormskirk Sandstone, palaeocurrents were controlled by a regional dip towards the Keys Fault. There are changes in palaeoflow from the base of the Ormskirk Sandstone Formation to the top (a vertical thickness of 800 ft), also changes from the southern to the northern ends of the field. In the underlying St Bees Sandstone Formation, consistent palaeocurrent directions proved more difficult to obtain, since major channel facies sandstones are rarer. No angular unconformity was observed between the St Bees and Ormskirk Sandstone formations. Within the Ormskirk Sandstone interval, however, very consistent, 1° to 2° changes in structural dip occur at the same stratigraphic levels in several wells and indicate small-scale tectonic rotations during the deposition of these sandstones. These are probably related to tectonic reactivation which has been invoked in previous literature to explain periodic rejuvenation of the depositional system during deposition of the reservoir sandstones. The fact that the observation of such small-scale adjustments can be repeated in several wells is an indication of the degree of precision of the results obtained from the dipmeter logs in these wells.