Abstract: Influence of Composition and Temperature on Hydrocarbon Migration Through Morrow Fluvial Reservoirs, Las Animas Arch, Colorado
Dudley W. Bolyard
Precipitation of wax in pores may impair permeability and prohibit the flow of oil. Crude oil composition and temperature are the most important controlling factors. Oils are chemically complex, may contain up to 45 wax compounds, and may vary significantly even in the same pool.
High-wax oils are common in the Morrow of eastern Colorado. Narrow fluvial sandstones provide migration paths toward the Las Animas Arch from adjacent basins. Temperatures range from less than 110° F. on the top of the arch to 180° F. at a structural position only 1,400 feet lower. A range of 30° F. has been observed in individual pools. Wax has precipitated in the 120-140° F. range, creating relative permeability barriers which cut across the sandstones.
Wax barriers are impermeable to oil, but may be permeable to gas and water. They account for certain dry holes with high porosity, permeability and oil saturation (and low water saturation) in both core and electrical log analysis. They explain why some oil wells with impaired permeability are adjacent to structurally lower gas wells with good permeability. A network of wax barriers around the Las Animas Arch accounts for approximately 300 feet of variation in the structural position of a line separating oil from gas fields.
Since the low temperature bands may be short and discontinuous, wax barriers are more effective in narrow fluvial reservoirs than in blanket reservoirs.
AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada