Heat Flow and Hydrocarbon Generation in the Transylvanian Basin, Romania

Abstract

The Transylvanian basin in central Romania is a Neogene depression superimposed on the Cretaceous nappe system of the Carpathian Mountains. The basin contains the main gas reserves of Romania, and is one of the most important gas-producing areas of continental Europe; since 1902, gas has been produced from more than 60 fields. Surface heat flow in the Transylvanian basin as estimated in other studies ranges from 26 to 58 mW/m2, with a mean value of 38 mW/m2, relatively low compared to surrounding areas. The effect of sedimentation on heat flow and temperature in the Transylvanian basin was estimated with a numerical model that solved the heat equation in one dimension. Because both sediment thickness and heat flow vary widely throughout the Transylvanian basin, a wide range of model variables were used to bracket the range of possibilities. Three different burial histories were considered (thin, average, and thick), along with three different values of background heat flow (low, average, and high). Altogether, nine different model permutations were studied. Modeling results show that average heat flow in the Transylvanian basin was depressed approximately 16% during rapid Miocene sedimentation, whereas present-day heat flow remains depressed, on average, about 17% below equilibrium values. This result suggests that low heat flow in the Transylvanian basin is at least in part a lingering effect of Miocene sedimentation. We estimated source rock maturation and the timing of hydrocarbon generation by applying Lopatin's method. Potential source rocks in the Transylvanian basin are Oligocene-Miocene, Cretaceous, and Jurassic black shales. Results show that potential source rocks entered the oil window no earlier than approximately 13 Ma, at depths of between 4200 and 8800 m. Most simulations encompassing a realistic range of sediment thicknesses and background heat flows show that potential source rocks presently are in the oil window; however, no oil has ever been discovered or produced in the Transylvanian basin. Model simulations of the thermal history of the Transylvanian basin show that in most scenarios potential source rocks are too cold for significant gas generation. Only simulations with averag or high heat flow and average to high sediment thicknesses yield predictions of significant gas generation. There is a correlation between heat flow and gas occurrence in the Transylvanian basin, but gas fields are also likely to be found in areas with relatively thin sedimentary cover.