Turtles And Snakes: Evidence For Molecular Shape-Selective Migration Of Crude Oil Hydrocarbons In The Bakken Petroleum System

Aromatic and aliphatic hydrocarbons were analyzed in extracts from 105 rock core samples collected from 16 wells across the Bakken Petroleum System in the Williston Basin, North Dakota (USA). Crude oil hydrocarbons recovered from the Upper (UBS) and Lower (LBS) Bakken shale source rocks had consistently higher proportions of aromatic hydrocarbons compared to aliphatic hydrocarbons than were found in the adjacent Middle Bakken (MB) and Three Forks (TF) target drilling zones. The higher aromatic/aliphatic (Ar/Al) ratios in the UBS and LBS source zones as compared to the adjacent MB and TF producing zones and the near molecular-sized nanometer pore throats in the UBS and LBS rocks suggest that primary migration of the aromatic hydrocarbons out of the source shale pores was (and still is) inhibited by the rounded, flat, and rigid "turtle" shape of the aromatic hydrocarbons significantly more than by the linear and flexible "snake" shape of the aliphatic hydrocarbons. Additional evidence for inhibited primary migration of aromatics versus aliphatics based on molecular shape was shown in laboratory experiments by comparing their recovery rates from the "as-received" UBS, LBS, and MB rock core samples from three wells using supercritical CO2 and supercritical ethane under reservoir conditions of 34.5 MPa and 110 degrees C. CO2 and ethane gave similar aromatic hydrocarbon recovery rates with all the rock samples regardless of whether recoveries from the individual rocks were slow or fast. Since CO2 as a solvent can better disrupt polar interactions between aromatics and the shale organic matrix than ethane, the similarity in recovery rates comparing CO2 and ethane indicates that sorption of the aromatics to the source shale organic matrix was not the primary reason for their inhibited migration. However, the recovery of aromatics as compared to the same-sized aliphatics (same carbon number) was much more inhibited from the UBS and LBS samples than from the MB samples, results which provide further evidence that shape-selective filtration during primary migration out of the source shale nanopores is the dominant mechanism for lower Ar/Al ratios in the oil-producing MB and TF zones than in the source UBS and LBS zones.