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The Geohistory and Morphology of the Pasca A Reef


The Pasca A gas field is hosted in a Mid to Late Miocene reef located offshore of Papua New Guinea in the Gulf of Papua. The Pasca A reef is one of several pinnacle reef gas discoveries in the Gulf of Papua and thus is representative of a petroleum system type hosting significant resource volumes. As is typical of other fields, the nature and geohistory of Pasca A is understood through the interpretation of seismic and well data. In the case of Pasca A this data comprises 2D semi regional lines, 3D seismic coverage over the field itself and four well penetrations. Also essential to understanding the nature of the subsurface is the examination of analogues from other fields and from modern depositional environments. In this aspect Pasca A field is far from typical – the Pasca A reef is an outlying pinnacle reef of the Great Barrier Reef system which has been buried by the deltaic clastics supplied by the fluvial systems running off of Papua New Guinea into the Gulf of Papua. So the modern analogues are right there! The Gulf of Papua pinnacle reefs (Pasca A, Pandora and Uramu) are directly related to nearby present day living reefs at the surface and the top seal and overburden clastics over the reefs are directly related to the Fly River delta system as it continues to advance today. The high quality 3D seismic data over Pasca field, coupled with the high velocity contrast between the shale top seal and the carbonate upper surface of the reservoir allows for exceptionally sharp imaging of the outer surface of the reef. This imaging reveals that Pasca A grew in cycles, and the cyclical growth layers are expressed as terraces on the reef’s outer surface. Accommodation for the upward growth of the reef was created by subsidence of the Gulf of Papua while the layered overprint of the growth was controlled by cyclical changes in sea level due to global eustacy. The layered nature of the reef relates to phases of carbonate growth in a variety of carbonate depositional facies alternating with phases of subaerial exposure resulting in carbonate dissolution and karstification. These processes are critical to the porosity systems of the field. The terraces observed on seismic on the outer surface of the reef should therefore be correlatable to features seen on logs of the wellbores. This correlation has been achieved to some extent, through the use of wellbore imaging logs and seismic inversion within the reservoir, but the establishment of a robust one-to-one correspondence between the terrace layers and the well logs remains challenging. With the exception of the upper few meters of the reef crest (the “drowning sequence”), the Pasca A reservoir section is entirely dolomitized. The situation of the Pasca A reef as a pinnacle reef with its base in deep water, coupled with good vertical permeability through the core of the reef, enabled the development of a thermal convection cell within the reef. Seawater drawn into the reef at depth circulated up through the interior of the reef and out near the crest of the reef providing magnesium for the early dolomitization of the reservoir rocks. The depositional features of the overlying Pliocene and Pleistocene clastic section are clearly imaged on seismic data and reveal a prograding deltaic mega sequence. The sealing over pressured shales over the Pasca field are deep water pro-deltaic sediments. These are overlain by the massive foresets of the advancing delta slope which are in turn overlain be shallow water deltaics. The current day depositional analogues for the Pasca overburden can be observed today from the basin to the east (deep water deposition), ranging to the delta slope immediately to the east to the Fly River Delta to the west (shallow water deposition). A remarkable aspect of the deltic sequence is that the great bulk of the Pasca A overburden has been deposited in the Pleistocene. Pasca A field is now buried at 2000 metres below the seabed but for all but the last 1.6 million years only a few hundred meters of sediment covered the field. As evidenced by the overall high porosity of the reservoir, this shallow depth of burial for most of the field history has undoubtedly shielded Pasca A from much of the diagenesis that it would have undergone had it been buried at 2000 metres for its entire 10 million year history.