Diagenetic Controls on Reservoir Characteristics of the Second Eocene Reservoir, Wafra Field, PZ
The Second Eocene is a dolomitized, heavy oil reservoir in the Wafra Field, located in the Partitioned Zone between Saudi Arabia and Kuwait. The reservoir lies at a subsea depth of ~1400 feet. Sediments within the reservoir were deposited in shallow subtidal to peritidal ramp environments. The lower portion of the reservoir consists of shallow subtidal sediments; the upper portion contains stacked subtidal-dominated cycles with thin peritidal capping beds. Stratigraphic and diagenetic studies were conducted during construction of a new reservoir characterization model to support field management decisions.
Pervasive dolomitization of the Second Eocene
reservoir altered original depositional fabrics and produced new pore systems. Dolostones were classified by diagenetic fabrics including crystal size and
pore types to more accurately characterize petrophysical trends. The major diagenetic trends in the Second Eocene are related to sequence stratigraphy.
The upper subtidal-peritidal portion of the reservoir is fine crystalline
(<5 to 30 µm) and largely preserves original depositional fabric. The
lower subtidal portion of the reservoir is coarser crystalline (50 to 100
µm) and generally lacks discernable depositional fabric. Fine
crystalline, fabric-preserving dolostone is dominated by micro, interparticle
and moldic porosity. The coarser crystalline, fabric-destructive dolostone is
dominated by intercrystalline, moldic and vuggy porosity and has generally
higher permeability than the fabric-preserving dolostone. The boundary between
fabric-preserving and fabric-destructive dolostones is immediately below the
stacked subtidal-peritidal cycles. The change from fine, fabric-preserving dolostone to coarser crystalline, fabric destructive dolostone is best
explained by dolomitization by seepage reflux of evaporated seawater from
restricted lagoons and ponds associated with tidal flats. The evaporated
seawater (brine) moved down due to its greater density. The brine was highly
supersaturated relative to dolomite near the surface causing rapid
precipitation and fine crystals. As the downward moving brines precipitated
dolomite, they became less supersaturated resulting in slower precipitation and
coarser crystalline dolomite. Production in the Second Eocene comes mainly from
the lower portion of the reservoir characterized by higher permeability,
fabric-destructive subtidal dolostone.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California