Diagenetic Controls on Carbonate Reservoir Quality of Jurassic Middle Marrat Formation in Burgan Field, Kuwait
Chakrabarti, Bhaskar 1; Al-Wadi, Meshal 1;
Abu Hebiel, Hanan 1; Al-Enezi, Abdulmohsen 1
(1)Kuwait
Oil Company, Kuwait, Kuwait.
Middle Marrat Formation in Kuwait was deposited in an inner ramp
setting. It was subsequently modified in near surface and burial diagenetic
environments. In present study an attempt has been made to establish
paragenetic sequence, effect of diagenesis on reservoir quality and delineate
reservoir facies.
480 ft conventional core and 125 thin sections were studied; SEM
and XRD analysis were carried out in 10 samples. Subsequently facies and
property modeling were carried out to delineate reservoir facies.
Five major facies were identified in Middle Marrat in Burgan viz.
Lime mudstone, Wackestone, Oncoidal wackestone, Dolomitic oncoidal wackestone
and Packstone. In Lime mudstone, wackestone and Oncoidal wackestone bladed,
isophaceous fibrous high Mg calcite cement formed in shallow marine diagenesis.
Subsequent exposure to meteoric diagenetic environment has resulted in leaching
of high Mg calcite, aragonite resulting in non-touching moldic pores, aragonite
neomorphism and equant calcite cementation. Abundance of kaolinite in XRD
analysis could be linked to frequent exposure and induction of meteoric water.
Burial diagenesis has resulted in compaction, extensive stylolitization,
anhydrite cementation of molds and over dolomitization causing drastic
reduction in porosity and very low permeability.
In Dolomitic oncoidal wackestone meteoric diagenesis caused
leaching and development of moldic porosity. Matrix replacive euheadral medium
grained dolomite crystals formed in near surface diagenesis. These dolomites
show interconnected intercrystalline matrix porosity (upto15%). In Packstone
meteoric diagenesis caused extensive leaching, isophaceous cement rims along
margins of ooids and partial equant calcite cementation. Excellent
interparticle and moldic porosity (upto 25%) and permeability (100-200 md)
developed in this facies.
Paragenetic sequence observed in Middle Marrat is marine bladed, isophaceous fibrous high Mg calcite cement, meteoric equant calcite cementation coarsening towards pore centre, matrix dolomitization, deep burial baroque dolomite and anhydrite cement. Different facies have reacted differently during diagenesis, which ultimately controls porosity. Porous facies have good permeability, thus making Packstone and Dolomitic wackestone intervals potential reservoirs. Facies and property maps delineate the area of good reservoir facies. The study helped in Middle Marrat reservoir characterization and delineation.
AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.