Identification and quantification of Gas hydrates in Makran offshore area, Pakistan

Abstract

Gas hydrates are crystalline solid compounds formed under specific pressure and temperature conditions from water and gas molecules. These naturally occurring substances are of significant interest due to their potential as an immense energy resource and their influence on geological and geophysical processes. Pakistan has a high potential for gas hydrates, particularly in the country's offshore areas. In 1998, Pakistan and Germany collaborated on a large expedition and discovered that the gas hydrates submerged beneath the Makran coast having approximately 29% saturation of methane gas with 7.5% free gas also present. Furthermore, the bottom simulating reflector (BSR) in the Arabian Sea can be found at depths ranging from 500 to 800 meters below the seafloor. This dissertation delves into the methodologies and techniques employed for the identification and quantification of gas hydrates in Makran offshore area, Pakistan, through the integration of advanced seismic data processing, seismic attributes and pre-stack simultaneous inversion. Seismic data processing plays a pivotal role in unravelling subsurface characteristics, aiding in the detection of gas hydrate deposits. By utilizing advanced processing algorithms, the seismic data can be enhanced to reveal subtle features indicative of gas hydrate presence. Specialized processing techniques employed in this research including de-signature processing by inversion of direct arrivals, de-noising, Pre-stack time migration and spatially continuous velocity analysis (SCVA) played an excellent role in the identification and resolution enhancement of the BSR, which is regarded as the first order indicative parameter for the presence of gas hydrates. After the successful application of our advanced data processing methodology, the vertical and horizontal resolution of the BSR is optimum that enhanced its interpretability throughout the study area. Seismic attributes further enrich the analysis by providing a comprehensive understanding of the subsurface. Attributes like amplitude variations and frequency attributes highlight distinct geological features associated with gas hydrates. These attributes aid in the identification of hydrate-bearing sediments and assist in distinguishing them from other geological formations. Moreover, the integration of multi-attribute analysis refines the interpretation process, reducing uncertainty and improving the reliability of the results. All the attributes employed in this research study illuminate the BSR as distinctive feature, in the time window of 3.3-3.9ms, although the illumination strength is different throughout the seismic profile i.e. Strong illumination (CMP 13500-14100, 14400-14900, 15000-15600 and 15800-16200) and relatively weak illumination (CMP 13300-13499, 14101-14399, 14901-14999, 15601-16199 and 16201-16500) that may further be correlated with the accumulation of gas hydrates above and Free Gas below it. Pre-stack simultaneous inversion techniques facilitate the integration of various data types, enabling the estimation of key reservoir parameters, including hydrate saturation and concentration. This inversion process leverages the complementary nature of seismic data, enhancing the accuracy of the quantification process. The inversion results clearly mark BSR as the boundary between high and low impedances above and below respectively and we can clearly see the low impedance thin layer imbedded in the high impedance strata. The typical values of the inverted P-impedance vary between 3800 m/s*g/cc to 4300 m/s*g/cc for the BSR and the inverted S-impedance at the zone of interest vary between 900 m/s*g/cc and 1200 m/s*g/cc. The amalgamation of these methodologies forms a robust workflow for the identification and quantification of gas hydrates. The synergy between seismic data processing, seismic attributes, and pre-stack simultaneous inversion empowers geoscientists and researchers to characterize subsurface hydrate reservoirs with enhanced precision. As energy exploration ventures into more complex and challenging environments, this integrated approach holds the potential to unlock valuable insights into gas hydrate occurrences, paving the way for informed decision-making and sustainable resource exploitation.