Heterogeneous distribution and potential significance of solid bitumen in paleo-oil reservoirs: Evidence from oil cracking experiments and geological observations

The solid bitumen (SB) residing in reservoirs after oil cracking can reduce reservoir quality. To build a quantitative standard for the SB content for paleo-oil reservoir identification and to evaluate the heterogeneous distribution of SB and its effect on reservoir quality, we conducted a cracking experiment on eight dolostone samples injected with oil and analyzed the SB content (vol%) and porosity of 706 reservoir samples from the Sichuan Basin that had undergone oil cracking. The SB contents of the experimental samples (0.95-1.48 %) injected with normal oil were higher than those of the experimental samples (0.22-0.49 %) injected with light oil. Furthermore, for the experimental samples injected with the same type of oil, the SB contents of the samples with higher paleoporosities (PPs) were also higher than those of the samples with lower PPs. For the reservoir samples from the Sichuan Basin, the SB content exhibits a generally positive correlation with PP. When the PP was <2.5 %, the SB content was relatively low (<1.0 %); and when the PP was >2.5 %, the SB content increased significantly and was generally > 1.0 %. The data obtained from the oil cracking experiments and reservoir samples from the Sichuan Basin suggests that SB contents of 1.0 % and 0.4 % can be used as the lower limits for normal and light oil reservoir identification, respectively. The oil cracking experiments verified that SB preferentially resided in the smaller pores and throats, and the SB saturations of the samples with lower PPs were larger than those of the samples with higher PPs. For the reservoir samples from the Sichuan Basin, a paleo-oil reservoir with a PP > 8.0 % can be an effective gas reservoir, but some of the paleo-oil reservoirs with PPs of 2.5 %-8.0 % are ineffective gas reservoirs due to SB filling. As a result, the heterogeneous distribution of SB should be carefully considered in gas reservoir evaluation.

Automated summary

Through cracking experiments and analysis of reservoir samples, it was found that bitumen content is positively correlated with paleoporosity and can be used as a standard for identifying paleo-oil reservoirs. Samples with lower porosity are more susceptible to bitumen filling, therefore the heterogeneous distribution should be carefully considered in gas reservoir evaluation.