Compositional variability in oils and formation waters from the Ayoluengo and Hontomيn fields (Burgos, Spain). Implications for assessing biodegradation and reservoir compartmentalization

The main scope of this work is the geochemical characterization of 12 crude oils and formation waters of the Ayoluengo oilfield (Basque-Cantabrian Basin, NW Spain) to delineate its reservoir compartmentalization. The Ayoluengo oil characteristics were also compared with data obtained from crude oil and formation water of the Hontomín-2 well located 25 km to the SE of Ayoluengo. The 13 Ayoluengo and Hontomín oil samples are very similar in composition, bulk properties and isotopic signature, indicating that these oils are genetically related. However, the Hontomín oil is more mature and it is slightly biodegraded, which suggests some differences in the reservoir environments. The molecular parameters of both oils confirm a marine origin related to a Liassic source rock, coincident with the occurrence of lateral facies changes along the Sedano sub-basin. On the other hand, the connate waters are analogous and both classified as chloride–calcium type. Given the complex nature of the sedimentology and structures of the reservoir, gas chromatographic fingerprints of the analyzed crude oils have been used to detect minor variations in composition that are indicative of reservoir compartmentalization.