Quantitative Characterization of Bitumen in the Complex Carbonate of Sichuan Basin: A Case Study

Abstract

In Sichuan Basin, Southwest of China, the occurrence of bitumen has been reported in the deep-buried carbonate gas field, where the reservoir spaces are mainly dissolved pores and vugs. The bitumen occupies the pore volume and blocks the flow channel of fluid, adversely impacted on the reservoir performance. Hence, identification and quantification of bitumen are critical for the reservoir evaluation. However, lacking of contrast from conventional logs between bitumen and other fluids makes it difficult to differentiate them. Fortunately, with the invention of some hi-end logging tools, the identification of bitumen is becoming reliable, even the quantification is becoming possible. In this case, nuclear magnetic resonance (NMR) and neutron spectroscopy tools are used, originally to acquire the accurate porosity and mineralogy. The NMR response of bitumen is dominated by short transverse relaxation times (T2), providing an opportunity to identify bitumen by porosity deficit. With the application of 2D NMR, the saturation and fluid density could be determined, which are important inputs of bitumen quantification. High-energy pulse neutron tool measures the main element of bitumen, such as carbon (C) and sulfur (S) from collision of fast neutron, and calculates the grain density, providing another crucial parameter to calculate the pore volume occupied by bitumen. This case study demonstrates the bitumen volume derived from the combination of NMR, neutron spectroscopy and conventional logs. The presence of bitumen is validated by cores. Based on the bitumen analysis, engineer could optimize their completion and acid fracturing strategy to improve the reservoir performance; meanwhile geologist could further study the evolution progress of reservoir. This method applies to those conventional reservoirs that suffer the harm of bitumen, and is also suitable in the heavy oil evaluation.