Predicting the presence of sealing or baffling shale layers in a reservoir is important as they are detrimental to the steam assisted gravity drainage (SAGD) recovery process widely used in the Athabasca oil sands. Baffles and barriers divert or halt steam chamber growth and thus slow oil production, possibly reducing bitumen recovery. However, predicting whether a shale layer is baffle or barrier is difficult. They are hard to correlate between appraisal wells as they are highly eroded in the fluvial depositional environment of the McMurray formation.
Several authors have observed that a barrier or baffle in a reservoir can be detected by discontinuities in the vertical grading of bitumen viscosity or composition that is caused by intense biodegradation. This paper presents a case study that tests bitumen geochemistry as a tool for predicting the presence of baffling or sealing shale at two different Athabasca oil sands fields.
Geochemical analysis of cuttings and core from multiple wells at two fields (Field 1 and 2) were analyzed to measure spatial variations in geochemical compositional at each field and to determine what parameters were the most robust in predicting barriers and baffles. Geochemical predictions of barriers or baffles were compared to barriers and baffles to SAGD steam chamber growth using field production and other geological data.
Results indicate that geochemistry is valuable for aiding in the characterization of barriers and baffles. For the wells tested in this study, geochemically predicted baffles and barriers were also baffles and barriers for SAGD steam chamber growth. Geochemistry is also successful in detecting baffles at thin shale layers that are difficult to see on gamma logs and in wells with significant lost core where shale layers cannot be visually examined.
In this study, Field 1 bitumen was more degraded than Field 2, and showed significant lateral heterogeneity in bitumen composition trends. Field 2 on the other hand had very little lateral variability in geochemical trends. Alkylnaphthalenes, alkylphenanthrenes, and steranes were found to be effective indicators of baffles and barriers at both fields, despite the greater heterogeneity at Field 1. Geochemical analysis from drill cuttings and from core stored at ambient conditions for 10 years provided good quality data for predicting baffles and barriers. Geochemistry provides a tool to help improve the interpretation of lateral shale continuity, and has future implications for SAGD design, such as determining pay thickness, optimal well location, and as a baseline in production allocation studies.