Publication Date:
2011-03-01
Description:
Analysis of subsurface pressure data from Taranaki Basin using direct (e.g., repeat formation tester) and indirect measurements (drilling parameters and wireline log data such as sonic and resistivity) indicates the presence of three pressure zones: a near-hydrostatic regime (zone A) that extends across the entire basin and to varying depths; an underlying overpressured regime (zone B), with pressures approximately 1100 psi (7.584 MPa) above hydrostatic, that extends throughout the Manaia graben and north along the eastern basin margin at depths of 1900 to 4100 m (6234–13,451 ft); and a third regime (zone C), with approximately 2100 psi (14.479 MPa) overpressure, that directly underlies zone A and zone B in different parts of the basin (although well penetrations are limited). The primary cause of overpressure is interpreted to be disequilibrium compaction preserved in upper Eocene and Oligocene marine shales. In parts of the basin, hydrocarbon generation (and in particular cracking to gas at high maturities) is interpreted to contribute to overpressures. The overpressures drain laterally and vertically into permeable units. Intervening transition zones (seals) comprise lithologic boundaries, diagenetic zones, and fault planes. Oligocene carbonates, although commonly thin, provide an effective barrier to vertical hydraulic communication over much of the basin. The Manaia graben is a partially closed system, with overpressures retained by a complex combination of a top shale seal overlying a regional sequence boundary, lithologic barriers within fault compartments, fault planes, and subcropping sequences; episodic fault breach enables vertical transfer of fluids from zone B to zone A in a dynamic fault valve process. To date, all oil reserves have been found in zone A, a large proportion of gas-condensate reserves are within zone B, and no commercial reserves have been established within zone C. The spatial definition of these zones and the appropriate pressure regime is important for well design, drilling safety, determining hydrocarbon column heights and gas expansion factors, and for exploration migration analysis. Regional analysis of pressure regimes can identify subsurface barriers and seals. Faults, in particular, are key elements in fluid migration and the focusing of liquids at abrupt pressure transitions. The strength of fault planes and diagenetic zones is the likely control on dynamic fluid release. Zone C has been very lightly explored and may represent a potential for large dry-gas accumulations; the zone may be sealed by a diagenetic zone crosscutting lithologic boundaries (conventional mapping horizons). Mark Webster is gas exploration manager at Genesis Energy. Since graduating from Victoria University in Wellington in 1982, he has worked for a variety of companies in exploration programs in New Zealand, Australia, Thailand, Philippines, Malaysia, and Canada, including roles as chief geologist at Petrocorp Exploration and exploration manager at Santos and TAG Oil. He is a member of the Petroleum Exploration Society of Australia and has been a member of AAPG since 1982. Stephen O'Connor is technical manager at GeoPressure Technology (Ikon Science Group), supervising pressure projects around the world, including Europe, Black Sea, West Africa, and Trinidad. He specializes in producing pore and fracture pressure profiles using direct pressure measurements, petrophysical logs, and seismic velocity data for use in well planning, as well as both field and regional studies. He holds a B.S. degree in geological sciences from Leeds University and an M.S. degree from Reading University (Sedimentology). Bitrus Pindar is currently a petroleum geoscientist at Geopressure Technology Limited. Before joining GeoPressure Technology, he served as a regional geologist at the Nigerian Geological Survey Agency, Abuja. He holds a B.S. degree in geology from the University of Maiduguri and an M.S. degree in petroleum geoscience from Imperial College London. He is a member of AAPG and Petroleum Exploration Society of Great Britain. Richard Swarbrick holds a Ph.D. in sedimentology/tectonics from Cambridge University and commenced his career in 1979 when he joined Mobil, with assignments in the United Kingdom and the United States. He joined Durham University in 1989 and was a principal investigator for a multidisciplinary research group funded by 17 oil and gas companies. During that period, he developed training courses in subsurface pressures and founded the company GeoPressure Technology. He is an honorary professor at Durham University and has been an AAPG member since 1982.
Print ISSN:
0149-1423
Electronic ISSN:
1943-2674
Topics:
Geosciences
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