The PESGB are sponsoring this Aberdeen Evening Lecture

What controls reservoir quality in Middle East carbonates?

Rachel Wood, School of Geosciences, University of Edinburgh

Tuesday 20th May 2008, 6pm for 6.30pm presentation
 Copthorne Hotel, 122 Huntly Street, Aberdeen AB10 1SU

Pore space in sedimentary rocks is known to decrease regularly with increasing burial depth as a result of cementation but oil reservoirs are a notable exception to this pattern. When oil fills pores in sediments, water is excluded, and as water is the medium for diagenetic reactions cementation is thought to be halted. It is an accepted paradigm therefore that oil charge has been responsible for the retention of high porosity in many reservoirs.

The oil filling of reservoirs is a gradual process, in some instances taking millions of years to complete; pores at the reservoirs crest are filled first and as pores become filled with oil progressively down structure so does termination of cement growth until the oil/water contact is reached.  Below the oil/water contact cementation can continue as long as conditions are right for precipitation in the water-saturated pores and space exists into which cement can grow.     

High porosities and low cement volumes occur in the crest of a giant carbonate reservoir from Abu Dhabi. Crestal pores are largely uncemented but a few pores contain cm-sized calcite cement crystals. Pore surfaces in the flank and below the oil water contact are all encrusted with mm-sized calcite cement crystals. Oxygen isotope analysis of cements indicates the temperature at which the cement grew, which is also a proxy for the depth of growth. Oxygen isotope values for the last growth zone as revealed by cathodoluminescence (Fig. 1) in the crest, flank and below the oil/water contact of these calcite cements are all the same implying they were precipitated under the same conditions. The high current porosity in the reservoir crestal rocks is therefore due to the presence of only a few large cement crystals compared to the low porosity rocks in the flank; below the oil/water contact cement crystals are smaller but more numerous.  These observations contradict the hypothesis that the presence of oil stops or inhibits the growth of calcite cement, and is responsible for high reservoir porosity.

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