Dealing with Foraminiferal Reservoir Complexity: A Dual-Contact Mechanism in DA Gas Field, Madura Strait, Indonesia

Abstract The development drilling campaign in DA Gas Field, Madura Strait PSC has been completed. Therefore, updating the reserve estimates is prioritized task as a guideline for production monitoring, allocation, and reservoir management. By integrating exploration data, it was found that each well has a different Gas-Water Contact (GWC). A comprehensive study should be conducted to determine the most suitable contact and generate a reliable OGIP estimate corresponding to the initial foraminiferal reservoir condition. The identification of hydrocarbon contact involved integrating direct formation pressure data from RFT, MDT, and DST to create the formation pressure profile. The results were combined with wireline log evaluations and total gas readings, while seismic Direct Hydrocarbon Indicator (DHI) interpretations confirmed the hydrocarbon distribution on a field-wide scale. It is revealed that variations in fluid contacts corresponded with the "non-flat" DHI responses. To investigate the non-flat GWC, facies analyses using core data and petrophysical properties were conducted. Fluid substitution modeling was also applied to quantify the effect of varying gas saturation on compressional wave velocity and bulk density. The integration of the analyses identified two distinct mechanisms influencing the non-uniform GWC in the DA Field. Facies classification indicated that two wells in the eastern area of the field have different GWC due to the reservoir geometry that formed as clinoform which affecting its quality. This condition leads to higher capillary pressure (Pc). The other two wells in the western area only encountered a paleo-GWC 35 feet below the current GWC. This paleo-GWC was confirmed through a residual gas mark from one exploration well and fluid saturation modeling. It is interpreted as the result of some gas that escapes through faults into shallower formations. This paleo-GWC is also defined as a Free Water Level (FWL), resulting in the interval between the current GWC and the paleo-GWC interpreted as a transition zone. Despite these variations, the mechanisms combine to form a single GWC system with no compartmentalization. This study introduces a novel methodology for defining a "non-flat" contact by integrating multiple theories, leading to a more accurate OGIP estimate prior to building the reservoir modeling and simulation.