Gas Sources, Migration, and Accumulation Systems: The Shallow Subsurface and Near-Seafloor Gas Hydrate Deposits

Round 1

Reviewer 1 Report

The authors have conducted a very extensive review of shallow hydrates and various hydrate systems that are not thermodynamically stable. As a matter of fact - no hydrates in sediments are in thermodynamic equilibrium. All experimental researchers know from 1940 that a system of 1 hydrate former and water can only be in equilibrium if ONE and ONLY ONE independent thermodynamic variable is defined. But in natural settings then 2 independent variables (both T and P) are defined and the hydrate system with surroundings cannot reach thermodynamic equilibrium. And it is even worse that that because mineral surfaces  (adsorption and impact on hydrate formation and dissociation) and other routes to hydrate formation and dissociation. Far too many to discuss here. And for mixtures of guest molecules the system is even worse. It is very easy to perform an analysis of how many degrees of freedom there is. For mixtures of hydrate formers it is also necessary to include selective adsorption of guest molecules on liquid water and a variety of different hydrates can exists (different compositions, different levels of stability).

In order to perform such an analysis simply count:

Number of independent thermodynamic variables in all coexisting phases (temperatures, pressures, all mole-fractions) 

minus

Conservation laws (all mole-fractions in each phase sum to 1)

Equilibrium (equal temperatures, equal pressures and equal chemical potentials)

It is better than using Gibbs phase rule but the result is the same if also selective adsorption on minerals and liquid water are included.

Why this very long introduction in a review report?

In a thermodynamic non-equilibrium system then there is no P T EQUILIBRIUM curve, But there T P hydrate stability limits (but it is no longer an equilibrium curve) along with many other hydrate stability limits.

Example 1: Hydrate will dissociate if exposed to pure water or groundwater containing low concentration of quest molecules. This stability limit is a well defined stability limit and a 3 D map with T P and mole-fraction guest in surrounding 

CH4 stability limits towards seawater concentration

 

This stability limit is one of the most frequent reasons for the dynamic balance that dictates hydrate saturations for offshore hydrates. Hydrates dissociate towards incoming seawater that contain little or practically no CH4 and new hydrate forms from upcoming gas below.

 

It is also responsible for the existence of hydrate mounds on seafloor. gas coming up and meeting seawater at seafloor forms hydrate if T and P and  T and favorable. Then hydrate dissociates towards surrounding seawater and create a dynamic that feeds ecosystems with CH4 from dissolving hydrate.

 

The manuscript is very incomplete in analysis of reasons for hydrate formation and dissociation in non-equilibrium systems

Author Response

Response to Reviewer 1 Comments

Thank you very much for your comments.

In our paper, we introduce the origins of hydrate deposits in the shallow subsurfaces and outcrops at the seafloor in terms of the gas sources, migration conduits to the seafloor, and how these deposits form at the seafloor. However, we have decided against covering the dynamics (dissociation and recrystallization) of the outcrops in the bottom waters explicitly in this paper.

First, we conducted extensive research on ~1000 references and reports to determine the regions where such hydrate deposits occur. Then, we described each kind of system, considering three insights: the gas source, migration conduit, and how they form on the seafloor. Moreover, we have proposed several prerequisites for the formation and stability of hydrate mounds and pingoes on the seafloor in Section 7.2. For example, “a prerequisite for the formation and stability of hydrate deposits in the shallow subsurface and at the seafloor is a sufficient supply of gas-rich fluids through the hydrate stability zone. The amount of gas migrating from the deep source zone may be far larger than that trapped in hydrates.”

However, the dynamic processes of hydrate outcrops on the seafloor, such as dissolution rates and mechanism, may be determined by various factors, as you rightly mentioned, including P–T conditions, crystal structures, oil coatings, sediment cover, and biofilms. Although being an interesting topic, only a handful of studies have attempted to quantify the dynamic processes of a natural hydrate outcropping (as illustrated in Section 7.2). During our research, we found only a few following studies focusing on the dynamic processes observed at a hydrate outcropping under in situ conditions, and they were performed at Bush Hill (GC 185) and Barkley Canyon.

“Lapham L L, Chanton J P, Chapman R, et al. Methane under-saturated fluids in deep-sea sediments: Implications for gas hydrate stability and rates of dissolution. Earth and Planetary Science Letters, 2010, 298(3-4): 275-285.

MacDonald I R, Guinasso N L, Sassen R, et al. Gas hydrate that breaches the sea floor on the continental slope of the Gulf of Mexico. Geology, 1994, 22: 699-702.

Solomon E A, Kastner M, Jannasch H, et al. Dynamic fluid flow and chemical fluxes associated with a seafloor gas hydrate deposit on the northern Gulf of Mexico slope. Earth and Planetary Science Letters, 2008, 270: 95-105.

Vardaro M F, Macdonald I R, Bender L C, et al. Dynamic processes observed at a gas hydrate outcropping on the continental slope of the Gulf of Mexico. Geo-Marine Letters, 2006, 26: 6-15.”

 

Only one study proposed the rate and mechanism of hydrate outcrop dissolution at the seafloor as follows:

“Lapham L L, Wilson R M, Macdonald I R, et al. Gas hydrate dissolution rates quantified with laboratory and seafloor experiments. Geochimica et Cosmochimica Acta, 2014, 125: 492-503.”

 

Due to the different properties of hydrate outcroppings on the seafloor (including with or without the oil coating) and inadequate in situ studies, we decided to exclude the description of dynamic processes (dissociation or recrystallization) of seafloor outcroppings from our paper, which would be explained at the end of Section 7.2 of our paper. We thank you for your advice again.

 

 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This is comprehensive review of naturally occurring gas hydrates in the Earth. Although there are many review papers on natural gas hydrates, it is highly recommended to publish this review paper to Energies. However, there are minor points to be corrected before its publication.

 

(1)    In Figure 1, it is shown the distribution of reported shallow gas hydrate deposits. On the other hand, to date, distribution of natural gas hydrate deposits in the sea floor and permafrost areas. It would be useful to show all of hydrate deposit regions in the Earth reported so far.

For instance,

T. Collet et al., J. Chem. Eng. Data 2015, 60, 319−329.

(2)    In Fig.21, two types of natural gas hydrate morphology are shown. On the other hand, core-scale hydrate morphology in grain sediments.  

For instance,

1. Hassanpouryouzband et al., Chem. Soc. Rev., 2020, 49, 5225-5309.

It is recommended to address features of shallow gas hydrates comparing with other type of gas hydrate sediments.

(3)    In the last decade, several drilling projects have been performed in several locations. However, in this review paper, there are few reference papers published in the last decade. It is recommended to add up-to-date reference papers.

Author Response

Response to Reviewer 2 Comments

Response 1:

We thank you very much for your kind suggestion and the reference. We must highlight that in this paper, we have merely focused on the shallow hydrates and outcrops on the seafloor. Thus, in Figure 1, we only display the areas where shallow hydrates and/or outcrops occur, which were selected after sifting through nearly a thousand references and reports.

We have carefully read the reference by Collet et al. (2015) that you have kindly provided; however, we found that Figures 1 and 4 in their paper showed the occurrences of all recovered and inferred hydrate deposits worldwide. Therefore, in Figure 1 of our paper, if we mark all hydrate deposit regions on Earth, the focus of this paper may not be highlighted. Nevertheless, we thank you for your valuable advice again and the good reference paper.

 

Response 2:

We thank you very much for your kind advice.

First, the two types of hydrate morphologies in Figure 21 of our paper refer to the two hydrate types at the Southern Hydrate Ridge and not the common types in global terms.

Second, through a comprehensive review of the shallow hydrate systems globally, we reached an important finding that “shallow hydrate systems are primarily distributed in sand-poor locales and are controlled by migration conduits…,” stated in “Section 7.1,” and the hydrate morphologies and the properties of the host sediments for each hydrate system have been listed in Table 1. They are different from the pore-fillings in sand reservoirs and the finely disseminated accumulations in the fine-grained and relatively undeformed sediments. We have added this point at the end of the first paragraph of Section 7.1.

 

Response 3:

We thank you very much for your kind suggestion.

To the best of our knowledge, we have checked all the references related to the hydrate deposits in the shallow subsurfaces, outcrops, and hydrate pingoes via various document databases. Thus, we believe that our paper involves nearly all such shallow hydrate systems reported so far. Some results of the drilling projects over the recent decade are not cited herein as they might be unrelated to the shallow hydrate systems.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The same problem as earlier. Inflow of water can dissociate hydrate in 2 ways:

1) If the temperature of the water coming in is inside P T hydrate formation temperature then the hydrate will still dissociate if the chemical potential of guest molecules are lower (more negative) in the liquid water than in the hydrate then the hydrate dissociates. Incoming water from seafloor is normally at almost infinite dilution in CH4 and chemical potential of CH4 in the incoming water is lower that chemical potential in the hydrate it meets. 

2) Temperature of incoming water is higher than hydrate stability in P T projection and hydrate will dissociate BOTH due to T and chemical potential difference (point 1)

Author Response

We thank you very much for your constructive suggestions. Following your comments, a number of improvements have been made primarily to Sections 7.2 and 7.3, including “ways of exposed hydrate outcrop dissolution at bottom water,” “crucial prerequisites for their existence,” and “other factors that support the hydrate steady-state.”

The main points are as follows:

“Hydrate outcrops or chunks are observed close to or at the seafloor level, where the P-T conditions are within the stability field of hydrate formation (Table 1). Nevertheless, hydrates will still dissociate as the inflow of seawater from the seafloor is normally at almost infinite dilution in methane since the chemical potential of methane in the incoming water is much lower than in hydrates. Thus, surrounded by seawater that is under-saturated with respect to enclosed methane, hydrate outcrops will accordingly dissociate and create a dynamic that feeds the cold-seep ecosystems. Moreover, if the temperature of incoming seawater is beyond the limit known as the “hydrate stability zone,” without a doubt, hydrate will dissociate due both to temperature and chemical potential differences.

Therefore, in addition to favorable P–T conditions, one crucial prerequisite for the existence of hydrate deposits at the shallow subsurface and seafloor levels is a sufficient supply of gas-rich fluids through the gas-hydrate-stability zone (GHSZ). Such deposits are limited to regions of relatively high flux, where gas flow is focused through sulfate-reduction domains.

Furthermore, heavier hydrocarbon concentrations and deeper water depths (Table 1) support the existence of multiple stability fields and resultantly mitigate the inhibition of steady-state hydrate conditions caused by stressors such as fluctuations in water temperature (e.g., GOM, Solomon et al., 2008) and salinity. Some biological phenomena (e.g., microbial mats and their excretions) (Fig.4; Vardaro et al., 2006) and physical processes (e.g., sediment cover and oil coating) (Figs.10A and 23; Vardaro et al., 2006) may also help stabilize exposed hydrate lobes in methane-undersaturated bottom waters.

However, likely due to technical limitations of in situ time-series observations and deployment difficulties, our research team has found only a few studies that focus on the dynamic processes observed for a hydrate outcropping under in situ conditions at both the Bush Hill location (GC 185) (Solomon et al., 2008; Vardaro et al., 2006) and Barkley Canyon (Lapham et al., 2010). Results consistently showed that there were few or no significant changes in the shapes or sizes of sediment/oil-covered hydrate mounds.

As such, due to the different properties of hydrate outcroppings on the seafloor (e.g., with or without oil-coating) and inadequate in situ studies, our team decided to exclude descriptions regarding the rates and mechanisms of dynamic processes (dissociation or recrystallization) of seafloor outcroppings in this paper.”

We thank you again for your kind patience and guidance.

 

Author Response File: Author Response.docx