Quantitative interpretations and assessments of a fractured gas hydrate reservoir using three-dimensional seismic and LWD data in Kutei basin, East Kalimantan, offshore Indonesia

Abstract

Concentrated fractured gas hydrate reservoirs in fine-grained sediments have been considered to have limited potential for commercially-viable production of natural gas. They may also pose significant geohazard where they overly conventional oil and gas production targets. The genesis of such accumulations is not well known and the characterization of the reservoirs is difficult. Thus, integrate characterization using 3D seismic interpretation, well logs, and geotechnical analyses are vital. The present study is the first detailed description of a fractured gas hydrate reservoir in the Kutei basin, offshore Indonesia.In this study, vertical fluid flow features (such as mud volcanoes and chimney structures) and other indicators of gas hydrate occurrence (including Bottom Simulating Reflectors (BSRs) and high-resistivity zones) are interpreted over a region of 500 km2. Seismic amplitude anomalies provide evidence of gas migration that has reached the seafloor. An interpreted double BSR below a mud volcano is inferred to indicate thermogenic gas present in the shallow sections. Geological and geotechnical analyses show overpressure associated with gas migration across the base of gas hydrate stability zone (BGHSZ) results in the formation of hydraulic and pneumatic fracturing. Seismic and well logs data indicate that the vertical migration of ascending free gas results in large-size macro-fractures whereas free gas accumulation below less permeable hydrated sediments allows for the generation of local small micro-fractures. Consequently, the fractures play roles as conduits and reservoirs for gas hydrate accumulations.Hydrate saturation was estimated from seismic and well log data by using seismic inversion and Archie equation, respectively. Gas hydrate is inferred to uniformly occur above the base of hydrate stability zone and its average thickness is estimated to be approximately 30 m. The gas hydrate is estimated to occupy generally 5–10% by volume in micro-fracture reservoirs, with as high as 30–35% near the top of macro-fracture reservoirs.