Resolving key issues of mature Mumbai offshore fields using ocean bottom node seismic interpretation

Neelam–Heera fields are located in Mumbai offshore basin and are significant contributors to India's oil and gas production. These mature oil and gas fields are producing mainly from Eocene–Oligocene carbonate reservoirs. Key challenges of Neelam–Heera fields include seismic imaging issues in areas perturbed by shallow gas, discrepancy in time and depth structures caused by lateral and vertical velocity variations and delineation of karst/discontinuities in the Neelam–Bassein formation to improve well planning and avoid mud loss. The seismic interpretation study we conducted used recent ocean bottom node seismic data to provide new insights into these challenges and are discussed in this paper. An integrated study scaling from interpretive processing to high-resolution mapping/attribute analysis was conducted to solve the key challenges of the Neelam–Heera fields. A significant improvement is seen in the imaging of the Neelam–Heera area because of prestack depth migration processing of ocean bottom node data, which led to detailed mapping and better understanding of various formations. One of the key challenges was to understand the structural changes in areas masked by shallow gas clouds in the Heera high area. The PP data could not provide clear imaging of areas masked by shallow gas; however, the PS data is not affected by fluid presence and provided imaging below the gas cloud. The gas-bearing Bandra formation in the crestal part of the Heera field is mapped with the help of PS data. The Neelam–Heera area shows prominent trends related to a velocity anomaly at a shallow level (dissolution features), forming the valleys in the time cube that are not seen in the depth cube. The key formations are mapped carefully with the integration of a depth cube. The new depth maps provided insights in areas hindered by previous false structures and are key to future development planning. To gain insights on karstification and mud-loss issues in Neelam field (Bassein formation), various structural attributes were derived and correlated with well data. Most positive/negative curvature attributes at different azimuthal stacks alongside variance maps provided detailed high-resolution insights on faults/lineaments/fractures, and posting of well mud-loss information validated the disturbed-zone-related mud-loss areas. The availability of ocean bottom node data with high-quality processing and imaging and subsequent detailed seismic interpretation using PP and PS data provided new insights to solve the key challenges of Neelam–Heera fields. The key outcome of this study includes an improved understanding of areas perturbed by shallow gas and better depth maps for integration into the future development model in areas with discrepancies caused by significant lateral and vertical velocity variations. In addition, we were better able to delineate karst/discontinuities in the Neelam–Bassein formation, which improved well planning and helped avoid mud loss.