Publications

One of the most profilic reservoirs in the North East Java Basin, Indonesia is the sandstone of Ngrayong Formation (Ardhana, 1993). This reservoir have supported hydrocarbon in this basin since the 19th century with total hydrocarbon exploited from this sandstone are 150 MMBO, 75 % of basin production (Ardhana, 1993). Hydrocarbon explorations in the southern and northern part of offshore Madura are also target the Ngrayong Formation as the main reservoir. In order to understand the characteristic of Ngrayong in those subsurface area, detail measured section were conducted in the Madura Island. Ngrayong Formation outcropped in Sampang and Surrounding, Madura Island is characterized by mixed siliciclastic – carbonate that gradually change from siliciclastic dominant in the lower part to the calcareous rock dominant in the upper part. The objective of this research is to know the potential of mixed siliciclastic and carbonate rocks of Ngrayong Formation as the reservoir in the offshore Madura area. This research elaborate surface data collected from Sampang and surrounding area, Madura Island which consists of geological map, geological cross-section, detailed measured section, lithofacies study and petrographic study of surface sample. For laboratory analysis, this study optimized 11 selected rock samples representing the entire area. Observation under microscope using thin section quadrant analytical calculation integrated with Mount (1985) methods was applied for rocks classification. Comparison of siliciclastic and carbonate component, sand and mud distribution as well as allochems and micrite portion were the main consideration for the description. Lithology of the mixed siliciclastic – carbonate are micritic mudrock, sandy micrite, muddy micrite, allochemic sandstone, allochem sandy limestone with porosity of the sample vary from 0.64% - 14.3%. The porosity is mainly created by precipitation process during diagenesis with dominantly vugy porosity. Refer to the detailed measured section and lithofacies study, the distribution of mixed siliciclastic – carbonate is predicted to develop until subsurface of offshore Madura. Keywords: Mixed Siliciclastic – Carbonate, Ngrayong Formation, Porosity, Reservoir