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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
January 12-15, 2000
San Diego, California
SCHOLLE, P. A., and D. ULMER-SCHOLLE, Southern Methodist University, Dallas, TX; and M. W. JEPPESEN and L. SIMONSEN, Maersk Oil Qatar AS, Doha, Qatar
Block
5, Offshore Qatar
Block
5 offshore Qatar are characterized by extensive secondary porosity, including
biomoulds, leached intergranular calcite cements (resurrecting primary
porosity), solution-enlarged fractures, and centimeter-sized vugs. Extensive
secondary porosity, ascribed to early meteoric diagenesis, has been described
from the UAE and Oman. Core observations, stable isotope geochemistry,
and petrography, however, indicate that the
Block
5 reservoirs were deposited
in deeper-shelf settings that were not exposed to meteoric diagenesis.
Early diagenesis was dominated by marine cementation, burial diagenetic
aragonite dissolution or neomorphism, extensive calcite cementation, localized
multi-stage dolomitization, and substantial small-scale fracturing. Early
calcite cements contain single-phase water-filled inclusions, indicating
formation at temperatures of max. 35<deg>C. The main phase of porosity
development post-dates early diagenesis and appears to be related to large-scale
throughput of highly corrosive pore fluids. Extensive corrosion of earlier
cements, development of solution-enlarged fractures and vugs, and the presence
of two-phase fluid inclusions in cements in secondary pores support a late
origin of this porosity. Primary fluid inclusions were formed at 55-60<deg>C
(slightly higher than present-day ambient temperatures); secondary inclusions
reveal the short-lived presence of much hotter (120-140<deg>C) waters.
Abundant hydrocarbon-filled inclusions in some cements indicate leaching
by hot brines that may have just preceded or accompanied hydrocarbon migration.
Such fluids may have been episodically expelled from nearby overthrust
belts (Zagros or Oman Mtns).