Borehole Image Log Interpretation in Fractured Basement of Contai Area, Bengal Basin, India for Hydrocarbon Exploration

Abstract

 Fractured basement reservoirs are not being explored fully despite of their proven commercial success globally. Basement reservoirs consist of rocks such as granite, granodiorite, gneiss, basalt etc. All basement is not fractured. Fractures finding in basement rock through conventional petrophysical tools is not possible. Borehole image logging tools such as Formation micro resistivity image (FMI) and Circumferential borehole acoustic image log (CBIL) are most important tools in petroleum industry to find the fractures, dip magnitude and its direction of fractures, in-situ horizontal stress direction (SH), bedding plane and lithological boundaries etc. Fractures in conductive nature can be identified by both micro resistivity and acoustic image tools. Resolution of resistive image tool for identifying conductive fractures is much better than acoustic tool. But the demerit of resistivity image tool is that it cannot detect whether conductive fractures are either open or close. Acoustic tool can identify open or close fracture through amplitude and travel time. For open fracture, both amplitude and travel time of acoustic image tool will show low amplitude sinusoidal nature. Amplitude of acoustic tool has been reduced in open fracture due to mud entry in open fracture as a result dissipation of acoustic energy against open fractures occur and so amplitude is reduced. For closed fracture there is no mud entry within fractures, so amplitude of acoustic tool would be strong but travel time will not show any sinusoidal because of no dissipation of energy in travel time. When fractures are mineralized by minerals, they are called closed fractures and certainly those close fractures are devoid of hydrocarbon. Both FMI & CBIL tools were lowered in well R of ‘Oil & Natural Gas Corporation Ltd’ for finding fractures in basement of Contai block, Bengal basin, India. Well R was drilled up to the depth of 3950 m. Basement top is at a depth of 3606m and bottom depth was 3950m. Huge number of fractures in the depth interval 3606m – 3950m in basement of Bengal basin were identified by both tools of FMI and CBIL. Most of them are closed due to mineralization by conductive minerals. We may conclude that basement of area of Contai, Bengal basin, India is having fractures which are closed and therefore, this basement is devoid of hydrocarbon.