AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Reservoir Characterization of the Paleocene/Eocene First Eocene Dolomite Reservoir at Wafra Field, Partitioned Zone (Saudi Arabia and Kuwait)
(1) Chevron Energy Technology Company, Houston, TX.
(2) Saudi Arabian Chevron, Houston, TX.
The First Eocene reservoir is estimated to hold more than 18 billion barrels of 14-22 API, high sulfur oil of which 5-10% may be produced by primary development. Consequently, steam flooding may be an EOR option. The size of the potential major capital project required a detailed reservoir characterization.
The stratigraphy is dominated by highly dolomitized, meter-scale depositional cycles comprised of basal peloid dolopackstones (subtidal) that are capped by peloid dolowackestones and dolomudstones (intertidal). Bedded/nodular evaporites and algal bindstones are present. The meter-scale cycles are arranged as a series of sequences based on stacking patterns of facies and evidence of subaerial exposure. The sequences are bundled into two larger-scale composite sequences subdivided into transgressive and highstand sequence sets. The transgressive sequence sets contain sequences that are thicker and dominated by subtidal facies. The highstand sequence sets contain thinner sequences with a higher proportion of intertidal facies in the cycles. A field-wide pressure discontinuity is located near the base of the lower highstand sequence set. A vertical barrier to steam is observed somewhat above the pressure discontinuity.
The reservoir has complex diagenetic history: (1) early dolomitization and dissolution of depositional CaCO3; (2) dissolution of dolomite cores and precipitation of anhydrite cement; (3) late-stage diagenesis including anhydrite dissolution and calcite cementation. A primary control on porosity and permeability distribution was the permeability of pre-dolomitization facies which affected degree of dolomitization as well as the pathways for hydrocarbon and formation water migration within the reservoir. Detailed biostratigraphic analysis has allowed accurate dating. The style of sedimentation, in terms of lithofacies and stacked lithofacies successions, and the style of diagenesis, are consistent with rising temperatures during the Late Paleocene.
Three main fracture sets were indentified; Set 1 (NE-SW) parallel to the NE SHmax direction, Set 2 (E-W), and Set 3 (NW-SE) which is parallel to the axes of the anticlinal structures in the Wafra field. The fracture density is low with higher values in the LSP area averaging 0.0035 fractures/foot. A non-unique fracture model suggests the fractures may be organized in small clusters. Our interpretation indicates that this is a reservoir with fractures, but not a fractured reservoir.