Abstract

Subsurface data collected in 2011 in the Mad Dog North appraisal wellbores (GC0738_1) show that the area is geologically unlike anything seen in the field previously. Reservoir sands are generally thinner, with a lower net-to-gross ratio. Two different oil water contacts (OWCs) are estimated, neither of which agree with previously defined Mad Dog OWCs. Petrophysical analyses calculate lower porosity and permeability, and higher shale volume. Measured and apparent water salinities are abnormally high, varying within sands, and between sands and wellbores. In situ reservoir pressures were higher than those predicted by the reservoir simulator, indicating that the north region is separated from the producing hydrocarbon accumulations by barriers that vary in strength from strong baffles to complete seals.

The compartmentalization at Mad Dog North was initially considered sub-seismic. Recently acquired and reprocessed seismic data improved the resolution of the image such that possible compartmentalizing baffles can be interpreted and correlated with the dynamic evidence of differential reservoir depletion. Had the GC0738 well been drilled during the exploration or early appraisal phase of field development, we might have ended the appraisal after one wellbore and considered the entire area to be in perfect communication. The four Mad Dog North appraisal wellbores appear to be contained within an area of disturbed reflectivity that is broadly scoop-shaped, with arcuate, listric internal features. As such, we currently interpret the feature as a sedimentary slump that developed shortly after deposition. We propose the slump was emplaced by collapse to the north, down the flank of a bathymetric high created by the slowly rising Mad Dog salt diapir during a period of low sedimentation and isostatic adjustment, after the primary reservoir turbidite deposition events had ceased. Slumps of this magnitude have not been identified elsewhere in the southern Green Canyon Lower Miocene reservoirs. None of the previous 24 reservoir penetrations have encountered this level of compartmentalization on the inter-wellbore scale, however, the ability to identify compartmentalization is enhanced by the significant amounts of production and concomitant reservoir depletion since the field came online in 2005.