Publications

Peciko field is one of giant gas field that have dominated the geological thinking of Mahakam area for years. Four different orders of stratigraphic stacking pattern are distinguished in the Peciko Main Zone. Deltaic parasequence, the smallest stratigraphic building block (5th order, is a prograding siliciclastic lens 200-300 km long, 50-100 km wide and 15-50 m thick. Delta front mouth bars, surrounded by prodelta shale, constitute the internal architecture of this lens. A coal layer is developed at the delta top. Many deltaic parasequences are vertically stacked in parasequence sets according to a regressive pattern followed by a transgressive pattern. Third order Genetic Sequences bounded by maximum flooding surfaces at their bottoms and their tops represent the next stratigraphic hierarchy. No erosional surfaces are observed during the deposition of Peciko Main Zone. The subsidence is higher than eustatic sea level falls. Parasequence sets are vertically stacked in a third order Genetic Sequence according to a Regressive-Transgressive pattern. The whole Peciko Main Zone represents a second order Genetic Sequence. It starts at the bottom with the MF 11 Ma marker and ends at the top with MF 7 Ma marker. In the seismic cross section it is genetically related with the second order depocenter in the basin resulting from basinward shelf break shift after MF 11 Ma. The delta front mouth bars represent the dominating reservoir in the Peciko Main Zone. The bar geometrical model derives from subsurface data of Peciko and Tunu fields, the middle Miocene outcrops of Samarinda anticlinorium and the sedimentary process observed in the modern Mahakam delta front. The size of individual mouth bars and their connectivity depend on the position of the deltaic cycle in this fourfold stratigraphic hierarchy. In a regressive pattern the sand quantity increases, bars can reach extensive dimensions and can easily communicate. In a transgressive pattern bars are smaller in size, entirely surrounded by shales, thus limiting the communication.