A Geochemical Probe of Formation Waters and Associated Crude Oils in a Source Shale Bed Toward an Understanding of the Role of Water-Mineral Interactions During Hydrocarbon Generation

Abstract

Crude oils and formation water samples in Devonian-Mississippian Woodford Shale produced from oil production fields in Payne County, north-central Oklahoma were analyzed for their rare earth element chracteristics and K/Rb weight ratios. The total rare-earth element contents of the crude oils were found varied between 0.162 and 0.445 ppb, the range of which is somewhat greater than that of formation waters with values between 0.09 and 0.31 ppb. The two fluids evidently had different rare-earth element distribution patterns when normalized to the rare earth element contents of the PAAS (Post-Archean Australian Shale). The crude oil samples were found to have varied PAAS-normalized REE distribution patterns, with some showing positive Eu and Ce anomalies and others having just slight negative or no Eu anomaly and Ce negative anomalies, but they all were found with middle rare-earth element enrichments. The formation water samples had been found with nearly uniform PAAS-normalized rare-earth element distribution patterns, typically characterized by Ce negative anomalies, Eu negative anomalies, middle rare-earth element enrichments, and trends with heavy rare-earth element (from Er to Lu) enrichments. As expected, the crude oils had very low K and Rb contents, with K values of 0.1 ppm or less and Rb concentrations of 0.146 ppb or less, whereas the formation waters had much higher concentrations of K with a range from 276 to 655 ppm and Rb with a range from 610 to 1280 ppb. Even though the crude oils had a relatively very low K and Rb concentrations, their K/Rb weight ratios, ranging from 660 to 1730, were much higher than that of the formations waters found to be with K/Rb weight ratios between 435 and 600. A noteworthy feature in the analyses of K and Rb contents of the formation waters is that when the concentrations of the formation waters are compared to that of normal sea waters or evaporated sea waters, the formation water K concentrations appeared depleted while Rb concentrations appeared enriched. The aforesaid geochemical features of crude oils and formation waters must be considered in the construction of a model for the origin of oil in source shale beds. It may be suggested from the data that oil generation most likely precedes mineral-water interactions in the vicinity of the oil generation sites. Then, the question arises about how much later the mineral-water interactions occurs.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017