Abstract
The main focus of study is to characterize lower and upper cretaceous carbonate deposits with Low Resistivity Pay, in Persian Gulf. Four oil reservoirs in the Cretaceous including the Zubair, Buwaib, Shuaiba and Khatiyah Formations of Southern fields have been analyzed. Here is a look at that to determine main factors on decreasing resistivity in pay zone. In some intervals resistivity responses reach less than 6 to 1 ohm·m. Significant hydrocarbon accumulations are “hidden” in low resistivity Pay zone, (LRPZ). LRPZ reservoirs have been found in some formations in Persian Gulf. Causes of LRPZ reservoirs on the basis of experimental analysis include clay-coated grains, carbonate with interstitial dispersed clay. On the other side Smectite and Kaolinite of main clays types have high CEC and greater impact on lowering resistivity. Micritization and Pyritization of digenetic process have noticeable impact on LRPZ. It is mentioned that Lonoy method applied to address pore throat sizes which contain Inter crystalline porosity, Chalky Limestone, Mudstone micro porosity. Pore systems are classified at class 2 and 3 Lucia and pore size varies from 0.5 to 4 micron. NMR Core and Log results show different pore size distribution. NMR core and MRIL results explain that decreasing of resistivity in pay zone is related to texture and grain size variation not being existence of moved water. Irreducible water estimate for this reservoir was between 30% and 50%. T2 cut off estimates, for defining irreducible water saturation, 115 ms.
Highlights
Pore systems are classified at class 2 and 3 Lucia and pore size varies from 0.5 to 4 micron
Water saturation varies between 35% to 50% results extract based on 15 core samples selected for Nuclear magnetic resonance (NMR) analysis in oil brine and air brine condition to get information about pore size Clay bound water, Moved oil and water and residual water saturation
Four oil reservoirs in the Cretaceous from the Zubair, Buwaib, Shuaiba and Khatiyah formations of Southern fields have susceptible for LRP formations
Summary
B. Arbab et al 860 some of LRPZ examples. The examples are from Wells data of Balal, Salman and Reshadat fields from southern of Persian Gulf. Khatiyah (Cenomanian), Shuaiba (Aptian), Zubair and Buawib (Barreminan) formations have been selected for reservoir studying. The reservoir characterization of low resistivity pay has been challenging due to high reservoir heterogeneity [1]. The pay zone includes carbonate reservoirs that resistivity logs have a resistivity between 6 to 1 ohm-meters and even in some intervals reach less than 1 ohm·m. Due to influence of digenetic effects, determination of pore type size and pore distribution has significant impact. The type of clay has a main role on resistivity response. Smectite is main clay types of mentioned formations. Smectite and Montmorillonite have high CEC (cation exchange capacity) and greater impact on lowering resistivity [2]. For each reservoirs microfacies analysis, digenetic process, rock fabric and pore systems have been defined
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