Assessment of the basin-centered gas accumulation in the Makó Trough

Abstract

The worldwide interest in unconventional hydrocarbon accumulations has increased continuously over the last
decade, largely due to a growing demand for natural gas, and equally important, because of the availability of a growing
range of advanced oilfield technologies. For these reasons the thermally mature, shaly sediments of the deep Makó Trough
of the Pannonian Basin have become a new target for exploration companies looking for unconventional natural gas
resources. So far the exploration efforts from TXM, MOL and ExxonMobil have been unsuccessful; no economic flow rate
has yet been tested from any of the wells. This study focuses primarily on the hydrocarbon potential of the effective source
rocks in the Makó Trough using state-of-the-art 3D basin and petroleum system modelling technology. The thermal and
maturity history, and the timing of the hydrocarbon generation from the effective source rocks were investigated. Thus the
generated volumes of hydrocarbons could be estimated, and then compared with the volumes needed to fill the assumed
pore space of unconventional reservoirs.
In order to reduce the uncertainty related to source rock parameters like the total organic carbon content (TOC),
hydrogen index (HI), and source rock thickness, a number of simulation runs were carried out in addition to the reference
model. Furthermore,, using the Monte Carlo sampling method the variations for each input parameter were determined;
these variations were based on triangular distribution in order to facilitate the calculation of the generated volume of
hydrocarbons for different probability confidences, for three drainage areas.
The estimated mean volume of gas generated (primarily from the Endrőd Formation) was calculated as 490–650
billion Sm3 (the are volumes always given at atmospheric pressure). This is much less than the necessary volume of at least
14 000 billion Sm3 needed to fill all the pore-spaces in the Szolnok Formation sand layers alone, and thus not enough to
justify the existence of a basin-centred gas accumulation. Therefore the Makó Trough is unlikely to contain a large basincentred, tight gas-sand accumulation. The Endrőd Marl is only a medium quality, mixed oil- and gas-prone source rock
with average immature TOC values of 0.75% in most wells, and only in the Hód–I and Makó–7 wells does this value reach
1.5%. These values are far below the average TOC values of the proven shale-gas systems of the USA and Canada. The very
high pressures and temperatures, and the high clay content represent additional negative factors for the Endrőd Marl as a
potential shale-gas accumulation.