Identification and Mapping of Gas Reservoir Bitumen

Abstract

Reservoirs that have undergone a gas charge after being charge with oil will have oil trapped within the reservoir. The interest of this study is to identify ways to trace that heavy oil, referred to as bitumen, in gas reservoirs from openhole logs along with identifying the controlling parameters on the trapping of this bitumen. The bitumen could be pore-saturating or grain coating. This paper looked into the petrophysical logging responses from 3 wells with confirmed bitumen at surface as well as thre other wells with matching signatures, for the purposes of mapping. The main logs considered were Resistivity and density-neutron, with formation testing and elemental spectroscopy being used as additional data sources. Thin sections from the cores, were also used to further identify more controlling parameters that might exist. Regardless of the nature of the bitumen's existence within the pore-space it appears that the main controlling parameters are structural. The investigation of petrophysical signatures lead to minor structural changes within the silt being able to provide a trap for the bitumen charge to accumulate. Use of silt-sand-shale analysis allowed to map the presence of the sub-structures under which the bitumen resides further leading to the fact that bitumen presence in the reservoir is structurly controlled. This does explain why bitumen exists in some areas of the reservoirs and not in others. Moreover, this also explains the generally relative low thickness of bitumen. Mapping the layers and ditrubuting them into 3d space will allow for the prediction of where the bitumen occurs as long as a controlling depth, above which bitumen exists, can be identified. Alternatively, flatting the structure to conditions prior to the development of the current seal can be done to better identify where the bitumen could've been trapped. Bitumen in deep reservoirs will impact fluid composition and can have impacts on drilling and drilling strategy. Therefore, identifying where the bitumen occurs is of high value to the development of these reservoirs. Further work remains to be done for identification in carbonate reservoirs and potentially further study of more bitumen saturated samples might lead to a better understanding of the phenomenon.