This paper presents a detailed study on how to detect the presence of light-oil in a low-resistivity and interbedded sandstone-shale environment. Based on the basic log response (high GR readings, 45-55 API, low resistivity values, 3-5 ohm.m, low-to-moderate porosity), the studied interval was considered a non-oil bearing zone. The interval was re-evaluated by adding some advanced technologies and the results showed the zone to be oil bearing. Fluid sampling has confirmed that the zone is oil bearing zone.
The advanced logging technologies were magnetic resonance, mineralogy and high-resolution borehole electrical imaging. The mineralogy tool was logged to identify and quantify the formation minerals and also the clay typing. The magnetic resonance tool was used to identify the different fluids inside the pores and also to measure the fluids saturations. The high-resolution borehole electrical imaging was used to analyze the sedimentary facies and the formation structures. At last a sample of the formation fluid was successfully taken after flushing out the filtrate from the formation for about 10 hours and pumping out about 40 liters.
The NMR technology played an important role in detecting and quantifying the oil over the interbedded sandstoneshale zone. The fluid typing analysis of magnetic resonance measurements identified up to 50% oil saturation with permeability index ranging from 10 to 20 mD. As the tool can only read between 2 and 4.2 inches inside the formation, the saturation analysis is affected by the mud filtrate. This means that the actual oil saturation might be much higher than 50%. The mineralogy tool showed 25-35% volume fraction of clay, mainly illite, kaolinite, smectite, and the remaining volume is quartz. These fractions indicate that the interval is shaley. Because of its low vertical resolution, it was not possible to see any interbeds with the mineralogy tool. However, the high resolution borehole electrical imaging showed clearly the formation sedimentary structures which are interbedded sandstoneshale. The fluid sampling has confirmed that the low resistivity pay in the interbedded sandstone-shale interval is an oil bearing reservoir.
This type of reservoirs is usually underestimated when evaluated with only the standard logging technologies, because of the effect of the shale laminations on the measured formation resistivity. So, in such environment, the use of different technologies is required to first understand the nature of the environment and also to overcome the impact of the laminations in identifying the presence of oil and also quantify it.
The combination of the NMR, mineralogy, high resolution imaging and fluid sampling technologies has proven its successfulness over interbedded sandstone-shale reservoirs.