Topseal integrity assessment using seal properties and leakage phenomena

We present a novel seismic interpretational workflow applied to a large gas-chimney located above the Marjoram Gas Field, offshore NW Borneo. The gas-chimney is hosted by a clay-dominated sealing sequence characterised by meter-scale silt beds. The sealing sequence is affected by a complex network of normal faults. The gas chimney, interpretable as vertical anomaly cluster, is composed of several discrete amplitude anomalies and is the result of gas leakage from the crest of a ∼500 m thick carbonate pinnacle-reef below. By analysing the seal properties and the anomalies composing the vertical anomaly cluster we argue that the leakage is consistent with migration of gas either along faults or using faults via cross-fault-plane juxtapositions. We show that this process is focussed within a narrow pathway rather than spread over the fault network. Mode I fault opening in response to the large capillary pressure exerted by the long gas column of the subjacent Marjoram Gas Field is postulated to be the cause of leakage

Re-evaluation of mobile shale occurrences on seismic sections of the Champion and Baram deltas, offshore Brunei

3D seismic data in the Baram and Champion delta provinces offshore Brunei show that regions thought to be occupied entirely by chaotic seismic data and conventionally interpreted as shale diapirs, are regions of dimmed, but coherent reflectivity. Such data indicate shale diapir masses are not present, instead dimming can be attributed to sediment intrusive complexes, overpressured fluids and gas clouds, or processing artefacts. In this way significant delta structures are masked on 2D seismic data, which are important to interpret delta tectonic evolution. The Middle Miocene-Recent Champion and Baram deltaic provinces are characterized by typical gravity tectonics-related structures. However, being situated on an active margin they are also affected by episodic development of contractional structures, which are located on older reactive shale bulges and result in inversion of motion on some growth faults. The emplacement of shale pipes, gas clouds and intrusive complexes is generally relatively late (Pliocene) in comparison with the underlying reactive diapirs (Late Miocene) and their emplacement events may be separated in time by several million years. Late overpressured systems may be related to phases of pore fluid pressure increase during or following periods of inversion tectonics, which resulted in phases of enhanced fluid migration in the basin, where fluids were either expelled laterally oceanwards, or vertically