AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Dolomitization and Enhancement of Rock Properties During Burial by Hypersaline Brines. Arab C Member of the Arab Formation (Jurassic) Dolostone Reservoirs (Kingdom of Saudi Arabia)
(1) Carbonate Reservoirs, shell International Exploration and Production B.V., Rijswijk (ZH),, Netherlands.
(2) Shell International Exploration and Production, Den Haag, Netherlands.
(3) SRAK, South Rub Al-Khali Company Limited, Al-Khobar, Saudi Arabia.
(4) Expertise and Deployment, shell International Exploration and Production B.V., Rijswijk (ZH),, Netherlands.
The Jurassic Arab C Member of the Arab Formation in the Rub’ Al Khali region (Saudi Arabia) is made of carbonates and evaporites arranged in salinity increase parasequences, which formed in a large restricted shelf with sporadic inflow of open sea-waters. Complete parasequences are made, from bottom to top, by dolostone-limestone-anhydrite. The carbonate part of the parasequences was deposited in more open marine conditions in an overall salinity-stressed environment. The dolostones (i) correspond to originally coarser grain sizes (grainstones and rudstones/floatstones), (ii) contain evidence of more open marine conditions (e.g. sponge spicules) than limestones and anhydrite beds and (iii) is localized immediately above flooding surfaces (i.e. maximum freshening of the system). The anhydrite beds evidence maximum restriction from the open sea and precipitation in salina to sabkha environments. The limestones are mostly non-dolomitized, which limits the application of depositional to early diagenetic dolomitization models (e.g. dolomite precipitation in early stages of restriction and increase of brine salinity; reflux dolomitization). Detailed paragenesis, coupled to isotopic composition and fluid inclusion analyses support that dolomitization occurred at burial stages from (i) hypersaline brines, residual from gypsum/anhydrite precipitation, and retained in the Arab C formation, together with (ii) increased alkalinity and PCO2 related to maturation of organic matter. Dolomitization was mostly constrained in the lower part of the parasequences with higher preserved porosities and original organic content. Locally sourced dolomitizing fluids in an almost closed system, were essential for the improvement of the properties of the dolostone reservoirs in Arab C. This case study opens the possibility for similar dolomitization scenarios in restricted shelf settings with carbonate-evaporite transitions.