Systematic fractures analysis using image logs and complementary methods in the Marun Oilfield, SW Iran

Abstract

Fractures are considered as one of the important structures in fractured reservoirs due to their effect on fluid currents and reservoir<br />parameters such as porosity and permeability. Fracture parameters can only be directly calculated with core and image logs. Cores have<br />serious limitations, so image logs are the best method. The aim of this study is the systematic fractures analysis of the Asmari<br />Formation in the Marun field as one of the giant oilfields in world. The main objectives of image logs were evaluating structural dip,<br />characterizing natural fractures and field structure heterogeneity, and finally correlating the results with complimentary methods such<br />as Velocity Deviation Log (VDL), Repeat Formation Test (RFT), mud lost data, and isodip map in the carbonate Asmari Formation.<br />Generally, electric and ultrasonic imaging tools record vast amounts of high-resolution data. This enables geoscientists to describe in<br />detail the structural fracture networks. The results indicate that the highest fracture density is in the zones 1, 20, and 30 of the Asmari<br />reservoir that show high correlation with VDL and mud lost data. Image logs also show a range of bedding dips from 20˚ in the<br />northern limb to 30˚ in the southern limb with strikes ranging from 10˚ to 270˚N. Regarding the general pattern of fractures, it is<br />evident that they are related to the folding and are classified mainly as longitudinal, transverse, and oblique. The longitudinal pattern is<br />dominant and often forms open fractures. They are characterized by N50W-S50E and mainly observed in the upper Asmari zones.<br />Moreover, to find the vertical relation of the layer and fractures, RFT data were used. The findings revealed the presence of a vertical<br />relation in the upper horizons of the reservoir, especially in the eastern section due to the high fracture density.