Early Karstic
Porosity Development or Late Burial Related Corrosion in a Middle East Giant
Field (Natih Fm., Oman): Implications
for Reservoir Properties Distribution
Taberner, Conxita1, Volker C.
Vahrenkamp2, Cathy Hollis3, Mateu Esteban4,
Alia Bahry2, M. Rejas5, Mike Caputi6, Abhijit
Mookerjee7, Yaduo Huang6, Peter Eadington8 (1)
Shell International Exploration and Production B.V, Rijswijk (ZH), Netherlands
(2) Petroleum Development Oman, Muscat, Oman (3) Shell International E&P,
Rijswijk, Netherlands (4) Carbonates International Iberia, S.L, Caimari,
Mallorca E 07314, Spain (5) Institute of Earth Sciences, CSIC, Barcelona, Spain
(6) Shell International Exploration and Production, Rijswijk (ZH), Netherlands
(7) Petroleum Development Oman, (8) CSIRO Petroleum, Kensington, WA, Australia
Porosity and permeability distribution in
Natih Fm. carbonate reservoirs (North Oman) has traditionally been
interpreted as resulting from recurrent subaerial exposure at the top of
several reservoir layers and major exposure at the top of the Natih Fm. In this
scenario, reservoir properties should follow top to bottom distribution
patterns, layer conform or disruptive. This interpretation was mostly based on
limited available diagenetic information and was model driven, as core recovery
was poor especially in high production intervals. A significant shift in the
interpretation of the reservoir properties distribution was proposed during the
last few years based on better core recovery during a recent coring campaign:
1) early diagenesis related to subaerial exposure actually caused cementation
at the top of reservoir units; while 2) extensive porosity was created below
cemented cycle tops later during the circulation of late burial corrosive
fluids. The stacking of small-scale cycles composed of bed-bound fractures in
cemented layers and extensive leaching right below these layers has created
high permeability zones with significant impact on reservoir behavior. This
finding is corroborated by a variety of evidence from cores, petrographic
observation and detailed geochemical analysis. Key factors for the shift in the
conceptual model are: 1) continuous core recovery, 2) multi-disciplinary core
workshops and 3) integration of results (sedimentologists, petrophysicists,
reservoir engineers) and groups (PDO, SIEP, Universities and vendors). Main
target of ongoing diagenetic work is to constrain parental fluids and plumbing
systems to lead to a better prediction of porosity and permeability
distribution in the reservoir.