Criteria of identification of lithofacies objects as potential hydrocarbon traps in the Main Dolomite (Ca2) strata at the toe-of-slope of carbonate platforms and microplatforms in the central-western Poland

Abstract

In terms of oil reserves of Poland, the Lubiatów oilfield in the Międzychód area is the second largest after BMB oilfield. The discovery of this structural-lithological trap in a toe-of-slope position in relation to the Main Dolomite (Ca2) carbonate platform opened a new hydrocarbon opportunities in this unit. One of the conditions for trap occurrence in the "Lubiatów-type" deposits of the toe-of-slope zone in relation to Ca2 carbonate platforms and microplatforms is the presence of appropriate reservoir facies. These facies are characterized by occurrence of thick packets of porous Main Dolomite rocks (Ca2 lithosomes) composed mostly of sublittoral, redeposited carbonate sands (packstones, floatstones, sporadic rudstones), deposited below wave-base and resulting from activity of traction and suspension currents and gravity flows. The only method to detect such facies is via high resolution seismic images, preferably from three-dimensional (3D) surveys. Unfortunately, intensive exploration studies carried out in this zone between 2005-2008 did not bring measurable effects, that is discovery of new hydrocarbon deposits. Several boreholes drilled in Ca2 lithosomes found water-bearing horizons only. In several other dry holes, none of the facies assumptions made prior to drilling were confirmed. Comparison of seismic images of area of those dry holes made it posssible to find a geometric criterion for seismic detection of Ca2 lithosomes located at the toe-of-slope of carbonate platforms and microplatforms. This geometric criterion, along with the seismological one, should constitute an important interpretational premise in identifying or excluding areas with potential hydrocarbon traps in the Main Dolomite rocks in the area under study