Regional Globigerina Limestone Characterization by Fluid Replacement Model and Pre Stack Inversion for Predicting Future Exploration and Development in Northeast Java Basin Indonesia

Abstract Since 1980, Globigerina limestone reservoirs have been explored and successfully discovered biogenic gas. At least, 15 wells were drilled with 13 of them are contributing to energy sustainability in Madura and East Java communities of Indonesia. Gas accumulation of this unique reservoir is very well pronounced in seismic data, either found as seismic flat spot, seismic dim spot, and seismic bright spot. Each of this seismic response correlated to its reservoir characteristic, where porosity ranges up to 55% with permeability up to 500mD. The facies of Globigerina limestone foraminifera are varied from grain-stone to pack-wackestone. Even though the success ratio of this play is high, due to its marginal resource, a thorough reservoir characterization has to be established, or the field may end un-economic when the properties are far worse than expected. Regional AI - Vp/Vs cross plot is the initial tool to define the filled gas reservoir qualitatively among existing wells, roughly describe the reservoir quality through rock density and p-wave velocity. By using the Zoeppritz equations, the reflection coefficients are calculated and generated P-impedance, S-impedance, and density as outputs. Others elastic moduli such as bulk modulus and LMR were derived from these individual parameters for further discriminations among lithology, porosity, and fluid effect. The aim of this regional study is to deliver the workflow in dealing with such unique reservoir, with very limited deposition over the world. This workflow can be used as exploration or appraisal and development tools where size of the field is quite challenging yet still tempting to look and economically valuable for investment in Indonesia.