Taking the Tishrine oilfield in Syria as an example

this paper discusses a characterization of a special type of carbonate reservoir-chalk reservoir.The Upper Cretaceous-Eocene reservoirs in this oilfield are rich in micro-pores and fractures

and typically have high porosity and ultra-low permeability.Lab analysis shows that their core plug porosity can be as high as 35%

permeability is less than 1×10

-3

μm

2

for most samples

formation resistivity is rather low(0.1-5 Ω·m)

and the irreducible water saturation can be as high as 70-95%.The study suggests that the Upper Cretaceous-Eocene are chalk or chalkoid(chalk-like)reservoirs deposited in shelf and off-shelf environments

which contain a large number of tiny coccolithophores of a few microns in diameter and well developed intergranular micropores.Their pore throat radius is in the range from 0.5 to 1 μm.Coupled with high salinity(3-7 times higher than that of sea water)

these reservoirs have very low formation resistivity.Factors such as sedimentary environment

shale content

calcite cementation

recrystallization and compaction are considered as the key forces controlling the development of the reservoirs.As far as the study area is concerned

such conditions as no exposure to atmospheric water

highstand deposition and no clastic input were favorable for forming high-quality reservoirs.The late fracturing event played a very important role in the forming of present permeability of these reservoirs.The fractures are mostly associated with late strike-slip faulting or tectonic inversion and also control the geometry and heterogeneity of the reservoirs.The challenges we are going to facing in the study of this special reservoir include the prediction of fracture distribution

identification of oil-bearing intervals

and determination of reserves estimating parameters