Investigation of pore-type heterogeneity and its control on microscopic remaining oil distribution in deeply buried marine clastic reservoirs

Pore-type heterogeneity is a key factor controlling the distribution of microscopic remaining oil in clastic reservoirs. It is important to quantify the characteristics of pore-type heterogeneity, understand its formation mechanisms, and analyze its impact on remaining oil at microscopic scale. Here we addressed these issues by adopting a new characterization method of flow zone indicator (FZI) with conventional velocity data as input and pore aspect ratio (α) as intermediate parameter, and conducting CT scan imaging on reservoir samples with different pore types, as identified by FZI, at different levels of water injection. The results demonstrate that this method can effectively discern pore-type changes and characterize the variabilities of velocity and permeability. A value of FZI>2.1 is characteristic of lithic quartzarenite with large amount of residual primary intergranular pores, which developed mainly in a shoreface environment. Clay rim cementation prevails in this reservoir type, as it can survive compaction, inhibit quartz overgrowth, and thus preserve intergranular pores with large pore-throat radius and good connectivity. The displacement pressure of this reservoir type is low during water flooding and the water swept area is wide; as a result, the majority of clustered flows are displaced and the remaining oil is present small patches in the form of multiporous flow. Samples with FZI<1.5 represent litharenite with large amount of intragranular dissolved pores, which developed mainly in foreshore and offshore-transition zone facies. This reservoir type underwent intense compaction and dissolution, which resulted in small pore-throat radius, poor connectivity and high displacement pressure. The injected water flows along the preferential pathways with relatively small flow resistance in this reservoir type during oil displacement such that the water swept area is limited; therefore the remaining oil is present continuous characteristics in the form of clustered flow. A transition zone with 1.5 < FZI<2.1 exists between the upper two reservoir types; this contains mixed lithologies and pore types, and exhibits medium pore-throat radius, connectivity and displacement pressure. The remaining oil in this reservoir type is present in the form of clustered flow and a small amount of multiporous flow, displaying continuous characteristics or small patches. Moreover, the water displacement process mainly occurs in relatively large pore-throats, which make a major contribution to permeability in each reservoir type. The heterogeneity of these pore-throats, as indicated by fractal dimension, controls the final oil displacement efficiency. The results of this study can be broadly applied to the characterization of similar clastic reservoirs and for further research in multi-scale heterogeneity.