The Paso Anomaly: shallow gas contained within Lower Pleistocene glaciogenic deposits in the Central North Sea

The perception of shallow gas accumulations has changed from one of ‘drilling hazards’ to ‘potential resources’ over the last few years. The Paso Anomaly, identified on seismic at a depth of c . 550 m true vertical depth subsea (TVDss), is a high-amplitude soft reflector that exists in the shallow overburden of the Catcher Field Area in the Central North Sea. This anomaly has previously been avoided during drilling as it could host gas. Amplitude v. offset (AVO) analysis has been undertaken that suggests a class 3 response, inferring that the anomaly is likely to be indicative of a gas-filled sand. Seismic indicators of gas presence, gas migration and potential migration pathways have been evaluated, and gas composition was analysed with reference to formation evaluation and gas ratio logs. This led to the interpretation that the Paso Anomaly represents biogenically altered thermogenic gas. It is further interpreted that the gas has migrated from depth due to faulting associated with Zechstein-aged salt diapirism, and subsequently has been biogenically altered once trapped at shallow depths. By utilizing facies-based seismic characterization, this work interprets the depositional environment of the anomalously high reservoirs as glaciogenic. Regional mapping supports an approximate Lower Pleistocene age for the deposits, which consists of two units; a Lower Unit that includes mega-scale glacial lineations, De Geer moraines and an esker; and an Upper Unit that comprises a sandur plain. Construction of a regional palaeogeographical model demonstrates that these deposits are the depositional record of an ice stream on the eastern edge of the British and Irish Ice Sheet. This ice stream advanced in a northeasterly direction into the Central North Sea before retreating, which has implications for the direction of Early Pleistocene ice flow within this area. This work lays the foundations for the Paso shallow gas accumulation to be viewed as a possible energy resource rather than a shallow gas drilling hazard, with a region of the sandur plain interpreted to be a potentially developable resource.