Measurement and Modeling of Viscous Oil and Tar Mat Formation with a Single, Low-maturity Charge

Tar mats are common features in carbonate and sandstone petroleum reservoirs in many basins throughout the world and greatly reduce fluid flow through affected rock. They are comprised in part by a solid or very viscous carbonaceous organic phase deposited in the rock pore spaces and in fractures and frequently found at the bottom of the reservoirs at the oil–water contact (OWC). Tar mats can be thin or quite thick (e.g., 10 m thick) and can be patchy or laterally continuous across the entire OWC of a reservoir. Tar mats and possible associated viscous oil have a huge impact on pressure support and aquifer sweep during oil production. In spite of their importance, two key properties of tar mats have not been explained in the literature: (1) the mechanism of their formation and (2) why tar mats are at the base of the reservoir. In this article, these two questions are resolved; the predominant mechanisms of formation of a tar mat and viscous oil column is clarified for a large reservoir representative of a simple class of reservoirs; those with a single low maturity charge, and with no alteration processes such as biodegradation. Here, extensive chemical analysis of the tar and viscous oil is reviewed constraining possible explanations for the origin of the tar mat. The formation of the tar mat and viscous oil is demonstrated numerically using reservoir flow simulation (Eclipse) using 2D model simulations. First-principles fluid mechanics considerations support simulation models. The nanocolloidal characterization of asphaltenes in oil codified by the Yen–Mullins model is key to predicting viscous oil and tar mat distributions at the 100 km length scale. Boycott convection is responsible for transport of asphaltene gravity currents across reservoir length scales. In addition, the role of Boycott convection in inhibiting tar mat formation clarifies why tar mats are generally form only after the reservoir charge is complete, and thus found at or near the OWC. With the understanding and modeling of this “simple” process of viscous oil and tar mat formation, more complex processes such as those involving multiple, incompatible charges are now readily accessible to reservoir simulation and for forecasting production especially with water injection.