Predicting the occurrence of bulk bitumen in uncored wells: a core-log calibration method, Uwainat Reservoir, Dukhan field, Qatar

Most Jurassic reservoirs in the Middle East contain bulk bitumen. Core data from the Middle Jurassic Uwainat reservoir in the Dukhan field of Qatar show that bulk bitumen predominantly occurs in grainstones, and to a lesser degree, in bioclastic packstone and wackestone intervals. Bitumen fills interparticle and intraparticle porosities, hairline fractures, and stylolites. The thickness of the bitumen varies, generally in response to the distribution of the porous-permeable lithologies. For this study a learning dataset was constructed, from which numerical estimations were made to isolate petrophysical characteristics of bitumen. Then qualitative estimates were made to predict the occurrence of bitumen. Data from cores and thin sections were integrated with porosity and resistivity logs of twenty cored wells to develop a technique for determining the presence of bulk bitumen in the uncored wells. As this methodology is largely qualitative, a binary flag system was employed to indicate bitumen. The technique involves cross-plotting of bulk density versus thermal neutron porosity, across zones where bulk bitumen has been identified in cores. A transform of the form y=abxxc relates the neutron porosity to the bulk density. This transform is then applied to the neutron data to arrive at a calculated bulk density, theoretically predicting a bulk density response to bitumen. When the difference between the calculated and the actual bulk density is within ± 0.03 gm/cc, a positive flag is generated (suggesting that bulk bitumen is present). Using cut-offs based on calculated bulk density values and the microresistivity log further refines this flag. This method is useful in detecting bulk bitumen in uncored wells, when bulk bitumen forms in intervals more than 2 feet thick. The zones identified as having bulk bitumen in cores were correctly identified by the log data. This technique has about 80 percent reliability when the rock interval containing bulk bitumen is more than 10 feet thick. The approach however, has limitations where bitumen occurs in low porosity rocks, as it becomes difficult to differentiate resistivity increases due to reduction in porosity from those due to bitumen filling the pore spaces. The agreement between core and log requires an accurate depth matching between core and logs.