ムバラス油田の炭酸塩岩の地質学的特徴

貯留岩の堆積相中のLimemudとSpariteの問題について

抄録

The sedimentary facies which has strong relation to productivity was studied. The relations between porosity and permeability in different sedimentary facies were plotted and these data show that limemud contents in the sediment control the permeability. Chalky limemud deposited under low energy wave condition has good porosity but it has very low permeability. These sediments were studied under the microscope and scanning electron microscope (SEM). These studies reveal that the pores in such chalk and chalky limemud are made up mainly of very fine intercrystalline pores which are able to observed under the SEM.
The differences between core porosity measured by helium gas and visual porosity observed under the microscope are compared with permeability obtained by core analysis. The result shows that the latter shows better relation than the former to permeability. Such relation was examined using recent limemud changing the mud content in oolitic sand. The result coincides very well with the relation obtained in core samples.
The diagenesis of the carbonate was studied especially on the cementation which controled the porosity change. The porosity change in the grainstone is much influenced by the cementation of sparite, but in chalk or chalky limemudstone the porosity does not decrease so much, because there is no pore space large enough for sparite precipitation.
The porosity changes versus depths show the two types of characters. The porous zone in the Thamama Zone II (Lower Cretaceous) has linear relation between poresity and depth. However, those in the deeper oil zones (Thamama IVA and Yamama VA, B) show the linear relation between present pcrosity and relative depth at the end of the Dammam age (Eocene). These phenomena show that the porosity of Zone II decreases down-flank with increase of depth. But the porosity in deeper zones decreased before Miocene movement.