Geological and Geophysical Evaluation of Sawan Area, Lower Indus Basin, Pakistan

Abstract

Sawan gas field located in Lower Indus Basın, Pakistan, was selected for geophysical and geological investigations to check the productivity. Digital interpretation along with seismic attribute analysis was conducted for this purpose. Five horizons namely, Chiltan Formation, Lower Goru Formation (Top B, Top C, and Top D sands) and Sur Main Limestone were marked. It was discovered that the "C" sand, deposited at the shelf margin during regression, is resolvable into seismic events called progrades. Detailed mapping of these prograding clinoforms (TWT, amplitude and isopach) could not be carried out in this field but some analysis were made to understand its genetic packages and their distribution within the basin. Contour maps were generated both in time for all the horizons but depth map for Lower Goru Top C interval was generated. Negative flower structures were interpreted conforming to transtensional tectonic regime in the area. C sand interval is acting as main reservoir for this field. At the level of productive sands, no structural closure recognized in the study area indicating subtle trap. Seismic attributes including cosine of phase, trace envelope, instantaneous frequency and phase along with minimum and rms amplitude maps were tested and their effectiveness verified. These attributes gave unique and concluding results as it revealed prograding sequences within Lower Goru interval. These were identified on the attribute section through on-lapping and down-lapping reflector configurations. Faults (structural) and reflection geometries (stratigraphic) were seen on selected attributes particularly on cosine of phase and instantaneous amplitude. It was derived that some of the attributes did not give positive result on the data provided which probably may have not been optimized during seismic processing. In addition, a stratigraphic framework was established for identifying reservoir intervals and to predict its lateral extent within the field which helped in seismic reservoir characterization.