Gas hydrate and free gas volumes in marine sediments: Example from the Niger Delta front

Bottom simulating reflectors (BSRs) detected on reflection seismic records from various deepwater locations world-wide, are known to occur as a response to the formation of gas hydrates above the reflector, and accumulation of small amounts of free gas below the reflector. Although estimates of enormous potential energy resources, and dramatic climate change scenarios have been discussed on account of the BSRs, our conclusions with respect to the commercially recoverable amounts of energy from gas hydrates and free gas associated with BSRs on the Niger Delta front, are disappointingly negative. We base our estimates on recent results from BSR-penetrating scientific drilling, a review of natural gas hydrate observations, and on theoretical considerations of gas hydrate formation and host sediment conditions. The main conclusions are as follows: 1. Natural gas hydrates and BSRs most probably occur as a direct consequence of focused and diffusive vertical fluid escape. 2. The mean maximum amount of gas hydrates residing in sediments above the BSR is most probably only 3% by volume. 3. The mean maximum amount of free gas charge in the sediments below even well developed (strong) BSRs is only 5% by volume. 4. Currently there is no commercial potential for recovering and exploiting 'trapped' energy sources in the form of BSR-associated gas hydrates and free gas on the Niger Delta front, mainly because they occur as dispersed, rather than concentrated deposits. (C) 1997 Elsevier Science Ltd.