Why does petroleum in carbonate reservoirs tend to occur at higher temperatures than in sandstones?

ABSTRACT Previous studies of the temperature distribution of conventionally recoverable oil and gas volumes have shown that most (74%–85%) of the Earth’s estimated ultimate recovery (EUR) occurs at reservoir temperatures of 60°C to 120°C ± 2°C, termed the “golden zone” for petroleum exploration. Both sandstone and carbonate reservoirs display similar overall conformance to this general pattern, with gradual decrease toward lower temperatures and precipitous dropoff >122°C. The present study compares these two main reservoir lithologies in greater detail in terms of their temperature distributions and volumetric dominance. The title of this paper stems from the observation of a significant difference between sandstone and carbonate reservoirs in their global temperature distributions of EUR. Using a global database with the largest 814 sandstone and 332 carbonate reservoirs (believed to be approximately representative of presently known conventionally recoverable global petroleum volumes), it is found that the average temperature of EUR occurrence is much lower for sandstone reservoirs (75°C) than for carbonate reservoirs (91°C). Closer inspection reveals that this difference is present in the 869 oil reservoirs in our database, but that no overall difference is apparent between the 204 sandstone and 73 carbonate reservoirs in which gas dominates over oil. Furthermore, it is mainly the 23 largest oil reservoirs (>4 billion barrels of oil equivalent) that account for most of the overall sandstone–carbonate difference: 14 sandstones versus 3 carbonates at <80°C and 2 sandstones versus 10 carbonates at >100°C. Five possible reasons for the above difference are considered. Expulsion of oil at higher temperatures in carbonate source rocks could have this effect, but no evidence is known that supports this hypothesis. Calcareous shales (presumably more frequent as seals to carbonate reservoirs) may undergo illitization at higher temperatures than do noncalcareous shales, but this does not explain why the difference in the temperature ranges of maximum EUR occurrences are observed for oil but not for gas reservoirs. Greater protection of porosity from high-temperature cementation in carbonate oil reservoirs is supported by the observation that this phenomenon is present in at least 7 of the 10 largest carbonate reservoirs at >100°C, but this explanation seems inconsistent with the relative paucity of similarly abundant large carbonate reservoirs also at lower temperatures. There may also exist some systematic differences, as yet unknown, in the overall petroleum systems between the larger sandstone and carbonate reservoirs. Finally, it is possible that the observed difference is simply coincidental (not related to any causal process). Whatever the cause, the empirical data are real and are worthy of further investigation.