SEISMIC PROCESSING AND IMAGING OF UPPER ASSAM SHELF BASIN

Abstract

Geophysical techniques are applied for delineating the subsurface structures, and exploration of conventional and unconventional energy resources. In the present study, we applied seismic reflection methods for imaging the major formations and their structural setting within the Upper Assam Shelf Basin and interpreted the formations with the available well log data. The Upper Assam Basin is a composite foreland basin which is located between the eastern Himalayan foot hills and the Assam - Arakan thrust belt with the number of faults striking NE-SW. The Assam-Arakan Sedimentary Basin is a shelf-slope-basinal system. The shelf part of the basin spreads over the Brahmaputra valley and the Dhansiri valley, and the area of study lies in between the Naga thrust and Margherita thrust. The shelf part rests on Precambrian granitic basement, whereas the basinal part lies on transitional to oceanic crust. With the growing energy demand and climatic concerns, there is a need to shift the demand from conventional to unconventional energy resources such as Coal Bed Methane (CBM). Coal Bed methane is a form of natural gas extracted from coal beds. Methane is adsorbed into the solid matrix of the coal with very little heavier hydrocarbons such as propane or butane, lacking natural gas condensate and hydrogen sulphide. Coalbed methane is distinct from typical sandstone or other conventional gas reservoir, as the methane is stored within the coal, and this property can be applied for identifying CBM. High amplitude on the seismic section, low Gamma Ray count (<30), high resistivity (100-200 ohmm), low density (<1.65 g/cc), high porosity (>0.45) and high slowness (>380 micros/m) are all combined indicator of CBM. In the present study, we applied seismic reflection methods for imaging the major formations of Upper Assam Shelf Basin. Eleven 2D profiles, three parallel (strike-line) and eight perpendicular (dip-line) to the strike, with a source interval of 40 m, receiver interval of 5 m, nominal fold of 60, sampling interval of 1 ms, and record length of 4 s were used. The study area had undergone thrusting and faulting and therefore pre-stack time migrated (PSTM) processing steps were used for processing and imaging. PSTM sections were further interpreted with the help of the well log data. Well-to-seismic tie was performed on the profiles with correlation coefficient of 0.5 between the synthetic seismogram and the seismic vi data near the well. During the well drilling, three formation tops were identified, namely Tipam, Barail, and Sub Girujan Clay at different depths in the Toklong well. Two formation tops, Coal Streak top and bottom were identified and used to calculate the throw in the study area. A throw of 684.459 (455.96) m (ms) was observed for Coal Streak formation top. A throw of 698.18 (518.158) m (ms) was observed for Coal Streak formation bottom. Based on this throw, throw for other formations were also estimated. Three thrusts, Thrust 1, Thrust 2, Margherita thrust, were identified in different seismic sections with a general dip direction of South-East (SE). In general, time relief map for Tipam, Barail, Coal Streak Top, and Coal Streak Bottom indicates decrease in time migrated on moving from North-West (NW) to South-East (SE). Observations of time relief map are in accordance with the geological setting of the study area. Thrusting with dip direction SE will result into the upward movement of the reflector in the SE side in comparison to the NW side. Structural interpretation of the area at various formation levels indicated mostly thrust play types for Coal Bed Methane exploration.