Shallow Gas: From Hazard to Prospect, Prediction of Reservoir Porosity Distribution Based on Log and Seismic Data Analysis

Abstract

The accumulation of shallow gas in the Mada Field, North East Java Basin, is widespread and constitutes a significant volume, occurring in sandstone of the Early Pleistocene Lidah Formation, as confirmed by well log data and well tests. Initially, this shallow gas was considered a drilling hazard for deeper main reservoirs in the Early Pliocene Mundu–Selorejo Formations. The increasing gas demand and opportunities for new field development have shifted this shallow gas into a potential resource. Therefore, the purpose of this study is to characterize the shallow gas reservoir and identify the sweetspot area to support future development. A model-based seismic inversion technique is used to generate a P-impedance volume, which is subsequently transformed into total porosity. Seismic attributes were utilized to delineate reservoir boundaries, as shallow gas accumulations are typically indicated by amplitude anomalies, including bright spots and polarity reversals. The porosity, seismic attribute, and structure maps were integrated to identify prospective gas-bearing reservoir areas. Petrophysical data show that the target zone of Lidah Formation sandstone contains gamma ray readings of 10-80 API, porosity values of more than 0.33, and P-impedance values of 8,500-13,500 (ft/s)*(gr/cc). Study results indicate that the crossplot reveals a strong inverse correlation between P-impedance and porosity (R = 0.871). Low impedance corresponds with the high total porosity that indicates porous sandstone with high saturated gas. Based on the integrated analysis, a prospective gas reservoir area was identified to the south and southeast of the Mada-1 well, situated at the structural crest and characterized by the highest porosity values. This study provides a comprehensive technique for evaluating shallow gas accumulation and offers practical implications for optimizing well placement and development strategies to unlock remaining hydrocarbon potential in mature basins such as the North East Java Basin.