Abstract

The main gas reservoir in the Yadana field, offshore Myanmar, occurs in Early Miocene shallow marine limestones. Losses of drilling mud were recorded in the same intervals as karstic features observed in cores: subaerial exposure surfaces, breccia and large solution cavities filled with drilling mud. Thin section analysis confirms multiple stages of karstic dissolution and infilling. The stable carbon and oxygen composition of most bulk limestones are typical of normal marine carbonate, however neomorphosed corals in some intervals have isotopic compositions characteristics of fresh water diagenesis. Karst features are also recorded on wireline logs and correspond to intervals cemented during emergence. They appear dense on sonic log and correspond to strong amplitude events on seismic. Seismic amplitude maps reveal an anisotropic distribution of karst related events superimposed on an atoll like deposition model.

Dense intervals with porosities of approximately 5% are scattered through the reservoir which has an average porosity of approximately 25%. The karst related processes enhanced vertical permeability in the dense intervals, allowing good fluid flow between the better reservoir units.