Middle Miocene deep marine stratigraphic traps in the Transylvanian Basin, Romania

Deep marine middle Miocene stratigraphic traps filled with microbial origin methane gas have been drilled and produced in the Transylvanian Basin alongside the structural controlled ones for a long time. They were either targeted directly through exploration wells, especially by using 3D seismic data in more recent times or during recompletions of older productions wells. New exploration targets were identified by mapping and interpreting bright seismic reflection units that were not related to structural traps. In some prospects the stratigraphic traps are associated with deep marine channels and crevasse/frontal splays. As these targets are not structural traps, the lateral/updip trap seal becomes the most important risk factor. A key factor in assessing the containment risk is the amplitude shut-off related to and interpreted in the context of the local sedimentological conditions of the reservoir sequence. Stratigraphic traps were much easier found in production wells situated on or close to structural closures through recompletions where the reservoirs are part of the same sequence of deep marine channels and crevasse/frontal splays. After the production tests, it was discovered that gas filled sandstones are located beyond the deepest contour line of the strictly structural closure. In some wells this situation was due to synkinematic sedimentation that forced the reservoir channel(s) to deviate from a rising salt- cored anticline. However, in other wells drilled on structural traps, due to the pronounced lateral facies change, separate gas accumulations exist in isolated channel bedforms. Some exploration wells that drilled a bright amplitude fan shape, situated in a low relief concave up structure, were converted to production wells after the discovery. The gas volumes hosted by these accumulations are above the economical threshold, although they provide smaller resources compared to the already drilled large structural traps.