Oils and source rocks from Pauls Valley, Anadarko Basin, Oklahoma, U.S.A.

Abstract

Thirty oils from the Pauls Valley area of the Anadarko Basin, Oklahoma, have been analyzed for n-alkane, terpane and sterane distributions in an effort to classify genetically related oils. Ten rock samples from the area were also analyzed in an attempt to relate oil types to their possible source rocks. The Pauls Valley area, located near the southeast corner of the Anadarko Basin, has producing formations ranging in age from Middle Ordovician to Late Pennsylvanian. Trapping mechanisms responsible for the accumulation of hydrocarbons vary from purely structural traps, to fractured reservoirs, to up-dip truncations of reservoirs and up-dip pinchouts of sandstones. Results show that 85% of oils in the area probably have a common source, the Woodford Shale. A group of oils reservoired in the Viola Group exhibit distinct geochemical characteristics and are thought to have a different source. The differences between the oils from these two sources include: enhanced C 24 tetracyclic terpane concentrations relative to C 26 tricyclic terpanes; diminished C 28–30 tricyclic terpanes relative to T s and T m ; diminished C 30-steranes relative to their C 29 counterparts; predominance of C 35 over C 34 extended hopanes and an even/odd predominance of the normal alkane distribution. Extracts from the Viola Group generally exhibit the same qualities as the associated oils and the limestone is proposed as their probable source rock. The predominance of C 29-steranes for oil samples which appear to have a marine source (inferred by their n-alkane distribution, the presence of C 30-steranes and high levels of tricyclic terpanes) suggests that caution be used in the interpretation of terrigenous sources based on the high abundance of C 29-steranes. The Deese Group, the Springer Formation, the Sylvan Shale, and the Arbuckle Group are not likely sources for the oils analyzed from the Pauls Valley area. Differences in API gravity and color, previously used to classify oils in this region, are not source-related but are related to maturation and migration effects.