Shallow Gas Hydrate Accumulations at a Nigerian Deepwater Pockmark—Quantities and Dynamics

Abstract

The evolution of submarine pockmarks is often related to the ascent of fluid from the subsurface. For pockmarks located within the gas hydrate stability zone, methane oversaturation can result in the formation of gas hydrates in the sediment. An~600 m-wide sea floor depression in deep waters offshore Nigeria, Pockmark A, was investigated for distributions and quantities of shallow gas hydrates, origins of hydrocarbons, and time elapsed since the last major fluid ascent event. For the first time, pressure coring of shallow sediments and drilling of more than 50 m-long cores with the sea floor drill rig MARUM-MeBo70 were conducted in this pockmark. Unusually, high hydrate saturations of up to 51% of pore volume in the uppermost 2.5 m of sediment in the pockmark center substantiate that deepwater pockmarks are a relevant methane reservoir. Molecular and stable C and H isotopic compositions suggest that thermogenic hydrocarbons and secondary microbial methane resulting from petroleum biodegradation are injected into shallower sediments and mixed with primary microbial hydrocarbons. Two independent pore water chloride and sulfate modeling approaches suggest that a major methane migration event occurred during the past one to three centuries. A rough sea floor topography within the pockmark most likely results from combined sediment removal through ascending gas bubbles, hydrate clogging and deflection of migration pathways, gas pressure build-up, and hydrate sea floor detachment. This study shows for the first time the chronological interrelationship between gas migration events, hydrate formation, and sea floor shaping in a deep sea pockmark.

A Scheduling Model in Capturing Methane Gas from Methane Clathrates Deposits

“…The data used for this research was obtained from the studies of many scholars on the existing Pockmarks described to have an approximately 600 m wide seafloor depression in deep waters situated in the Niger Delta zone offshore Nigeria. According to [5], pockmarks are subcircular or circular, or elliptical sea floor depressions known from shallow to deep-water areas worldwide. They can equally be regarded as craters (circular pits or depressions) in the seafloor caused by erupting gas or liquid.…”

“…Accumulated gas volumes, gas-sediment ratios (at ambient pressure), methane concentrations (average porosity 83 %), and calculated fractions of hydrates in sediment cores recovered by pressure coring (GHsattr)[5] …”

Recent and episodic activity of decoupled mud/fluid discharge at Sartori mud volcano in the Calabrian Arc, Mediterranean Sea

“…Further evidence of MV activity comes from temperature and pore water concentration anomalies (Bohrmann et al, 2003;Castellini et al, 2006;Mastalerz et al, 2007;Feseker et al, 2009;Pape et al, 2010;Feseker et al, 2014;Pape et al, 2014), which are, in contrast to gas bubble escape in the water column, also identifiable after the eruptive phase of a MV. When pore water compositions show a significant depth gradient, any transient deviation from stationary conditions may be used to estimate the timing of sedimentdisturbing events (Niewöhner et al, 1998;Hensen et al, 2007;Strasser et al, 2013;Pape et al, 2020b).…”

Integration of Deep Learning and Information Theory for Designing Monitoring Networks in Heterogeneous Aquifer Systems

“…Groundwater, as an active component of the global hydrological cycle, is extensively distributed within pore spaces and fractures of crustal lithospheres (Aeschbach‐Hertig & Gleeson, 2012; J. Han et al., 2017; H. Dai et al., 2019; De Graaf et al., 2019; Debnath et al., 2019; Gleeson et al., 2020; Ferreira et al., 2021). An accurate representation of groundwater distribution within subsurface environment is therefore crucial for addressing the problems that emerge at geoscience‐, energy‐, and environmental‐related contents, including land subsidence (Miller & Shirzaei, 2015; H. Dai, Ye, et al., 2017; Ha et al., 2020; Zhu et al., 2020), contaminant transports (Z. Dai et al., 2009; Ghanbarian et al., 2015; Cao et al., 2021; X. Zhang, Ma, Yin, et al., 2021), nuclear waste disposal (Viswanathan et al., 1998; Juhlin & Stephens, 2006; Wolfsberg et al., 2017; X. Zhang, Ma, Dai, et al., 2021), water inrush in tunnel and mine excavations (Dong et al., 2020; Ma et al., 2017; Qin et al., 2019), methane migration (Barth‐Naftilan et al., 2018; M. R. Soltanian et al., 2018; Pape et al., 2020; Ershadnia, Wallace, Hosseini, et al., 2021; Stanish et al., 2021; Wen et al., 2021), geological carbon storage (Castelletto et al., 2013; Z. Dai et al., 2014; Ershadnia, Wallace, Hajirezaie, et al., 2021; Fu et al., 2017; Njiekak et al., 2013), and groundwater and surface water interactions (Brunner et al., 2017; Jia et al., 2021; Wallace & Soltanian, 2021a, 2021b; Xin et al., 2022).…”

“…Dai et al, 2009;Ghanbarian et al, 2015;Cao et al, 2021;X. Zhang, Ma, Yin, et al, 2021), nuclear waste disposal (Viswanathan et al, 1998;Juhlin & Stephens, 2006;Wolfsberg et al, 2017;, water inrush in tunnel and mine excavations (Dong et al, 2020;Ma et al, 2017;Qin et al, 2019), methane migration (Barth-Naftilan et al, 2018; M. R. Soltanian et al, 2018;Pape et al, 2020;Stanish et al, 2021;Wen et al, 2021), geological carbon storage (Castelletto et al, 2013;Z. Dai et al, 2014;Ershadnia, Wallace, Hajirezaie, et al, 2021;Fu et al, 2017;Njiekak et al, 2013), and groundwater and surface water interactions (Brunner et al, 2017;Jia et al, 2021;Wallace & Soltanian, 2021aXin et al, 2022).…”