Abstract: Oil genesis, migration and accumulation around and in the Nagylengyel paleokarst (SW-Hungary)

KORPAS, Z., and I., VETO

Since its discovery in 1951, the giant Nagylengyel field has produced 21 Mt oil from an estimated oil in place of approximately 50 Mt. The gas-free, high-viscosity, sulphur-rich but non-biodegraded oil is pooled in karstified Mesozoic carbonates at 90-120 degreesC temperatures. The field is build-up by 16 hydrodynamically independent reservoirs consisting of Upper Cretaceous limestones and Upper Triassic dolomites. The primary production has been replaced by recovery with CO₂ since 1979.

The 300-800 m thick, organically rich Kossen Marl + Rezi Dolomite, deposited in a restricted basin inside the late Triassic carbonate platform and characterized by highly reactive, sulphur-rich Type II kerogen are considered as source rock. Alternating periods of subsidence and emergence up to the late Eocene resulted in (i) a burial of a maximum of 1400 m of the source rock and (ii) karstification of the carbonates under- and overlying it. The late Eocene characterized by a subduction-related volcanism and likely high heat flow was the first period of oil generation. The mid-Miocene opening of the clastic back-arc Pannonian Basin resulted in a further burial of 1500-2000 m leading to a massive generation, migration, and accumulation of oil which are possibly continuing up to present.

Based on the diagenetic model of paleo-karstification three independent reservoir levels of some ten meters thickness and with 1-2% fracture porosity can be delineated in the 2000 m thick Upper Triassic Main Dolomite. In the 200-300 m thick Upper Cretaceous Ugodi Limestone four interconnected reservoir levels of several meters to 20 m in thickness and with 1-5% cave-type porosity having been developed.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria