Using Large-Size Three-Dimensional Marine Electromagnetic Data for the Efficient Combined Investigation of Natural Hydrogen and Hydrocarbon Gas Reservoirs: A Geologically Consistent and Process-Oriented Approach with Implications for Carbon Footprint Reduction

Round 1

Reviewer 1 Report

Please see my comments as attached.

Comments for author File: Comments.pdf

Author Response

Reviewer#1

 Comments on the paper minerals-2355279 “Using large-size three-dimensional marine electromagnetic data for efficient combined investigation of hydrogen and hydrocarbon reservoirs: A geologically-consistent and process-oriented approach with implications for carbon footprint reduction“ by Max Meju and A. Shahir B.-Saleh

In this paper the authors presented and reviewed the framework for hydrocarbon and hydrogen reservoir investigation. This provides a guideline for exploring the basins, the prospects, the sweet spots, and the drilling sites. Especially, the authors used the CSEM and MT data in combination with seismic one to delineate the target via the constraints defined by the cross gradient. This will be of interest to oil & gas industry.

Reply: Thank you.

 

However, to make the paper be publishable, the following suggestions are for the authors to consider when revising the paper:

 

1. There exist slight repetitions in sections of Abstract and Introduction. For example, “ Easy to find oil and gas accumulations no longer exist”, “ finding the right basin (and block), selecting the right prospect, drilling the right well”, and many others have repeatedly occur in the text. It will great if the authors can rewrite them.

Reply: We have sufficiently re-worked the Abstract and Introduction as suggested to reduce the repetitions. However, we kept some of the essential phrases “Find the right basis, select the right prospect and drill the right well” since these are very important in the Play-based exploration concept practiced in the industry. They also help drive home a consistent message!

2. The insert in Fig.2 has not been clearly explained.

Reply: We agree and have re-drawn Figure 2 and explained all the computations.

3. There are typos: line 260 “in” should be “is”, line 492 “or and”.

Reply: Thank you. Typos on line 260(now 270) ‘in’ changed to ‘is’;  line 492 (now 503)  “or and” changed to “and/or”.

4. From my experience with EM exploration, MT is insensitive to horizontal resistive layers, however, from Fig8a, the inversion results from MT data reveal resistive targets (RP), please clarify.

Reply: MT actually maps horizontal resistive as well as conductive layers depending on their thickness and depth of occurrence. Hence, MT is used routinely to map groundwater aquifers and geothermal reservoirs on land.  All the marine MT results presented in this paper were draw from recent publications that showed the actual field data and verified the reconstructed 3D resistivities using available resistivity logs from several explorations wells in the various basins [7,17,]. In this paper, we only interpreted MT models verified at borehole sites and the regional play RP in Fig 8a is sufficiently thick and was verified with resistivity well logs in the related publications [7,17].

 

5. The discussion section is a bit bumpy. I suggest the authors put the contents in a separate section, but only address the concerns and future prospect in the Discussion section.

Reply: We agree and have now moved section 4.3 to section 2.3 where it fits quite well.

 

6. It seems that the data are unavailable as I checked the reference [7]. Thus, the inversion results provided in this paper cannot be verified by other methods and we have to trust everything presented. It will be great if the data can be released to public for scientific research when the exploration and drilling are finished.

Reply: The actual MT field data and the model verification using resistivity logs from several exploration wells are presented in [7, 17]. Some of the data were released to industrial collaborators [7] and academia (UBC-Vancouver’s CompuGeo) on a case-by-case basis.  The data were released to University of Science Malaysia- Penang for an ongoing PhD study [17].

 I’ve now added this statement to the caption of Figure 8: “The actual MT field data and the model verification using resistivity logs from several exploration wells are presented in [7, 17].”

 

In summary, I find that the paper has enough technical contents that warrantee

the publication of a paper. A moderate revision is recommended.

Reply: Thank you. We have sufficiently re-worked the Abstract and Introduction and corrected all the typos that the 3 reviewers found.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Editor and authors,

 

Following is my comment on the manuscript "Using large-size three-dimensional marine electromagnetic data for efficient combined investigation of hydrogen and hydrocarbon reservoirs: A geologically consistent and process-oriented approach with implications for carbon footprint reduction" by Meju and Shahir. Authours proposed CSEM-MT workflow to efficiently investigate hydrocarbon and hydrogen, emphasizing the reduction of carbon footprint. Authors performed case studies to demonstrate that the 3D CSEM-MT approach (structure guided inversion) can facilitate the interpretation of the resistivity information regarding conductivity structures. Authors also used the approach in geothermal energy investigations and show the usage of the approach in humanity's Net-Zero energy transition. It is good and well-written. I recommend the publication of the manuscript in Minerals after minor revision.

My minor suggestions are as follows:

1, Introduction, it is a lit bit confusing for me. I suggest the authors re-organize the introduction part to begin with the introduction of the main problem followed by the present statues as well as what is done by this study'

2, Figure 2, please improve the quality of the figure;

3， Equation 1, please define each symbol after the equation, what are f and \tau meaning?

4, line 329, what do you mean "In the second method" ?

5, 4D time-larpe electromagnetic imaging: how do the authors judge where the model structure difference originates? How to make sure the difference originates from the change of the underneath conductivity structure or the noise or the inversion uncertainty?

6, Do authors consider the bathymetry in course of CSEM-MT inversion?

Dear Editor and authors,

 

Following is my comment on the manuscript "Using large-size three-dimensional marine electromagnetic data for efficient combined investigation of hydrogen and hydrocarbon reservoirs: A geologically consistent and process-oriented approach with implications for carbon footprint reduction" by Meju and Shahir. Authours proposed CSEM-MT workflow to efficiently investigate hydrocarbon and hydrogen, emphasizing the reduction of carbon footprint. Authors performed case studies to demonstrate that the 3D CSEM-MT approach (structure guided inversion) can facilitate the interpretation of the resistivity information regarding conductivity structures. Authors also used the approach in geothermal energy investigations and show the usage of the approach in humanity's Net-Zero energy transition. It is good and well-written. I recommend the publication of the manuscript in Minerals after minor revision.

My minor suggestions are as follows:

1, Introduction, it is a lit bit confusing for me. I suggest the authors re-organize the introduction part to begin with the introduction of the main problem followed by the present statues as well as what is done by this study'

2, Figure 2, please improve the quality of the figure;

3， Equation 1, please define each symbol after the equation, what are f and \tau meaning?

4, line 329, what do you mean "In the second method" ?

5, 4D time-larpe electromagnetic imaging: how do the authors judge where the model structure difference originates? How to make sure the difference originates from the change of the underneath conductivity structure or the noise or the inversion uncertainty?

6, Do authors consider the bathymetry in course of CSEM-MT inversion?

Author Response

Reviewer#2

Following is my comment on the manuscript "Using large-size three-dimensional marine electromagnetic data for efficient combined investigation of hydrogen and hydrocarbon reservoirs: A geologically consistent and process-oriented approach with implications for carbon footprint reduction" by Meju and Shahir. Authours proposed CSEM-MT workflow to efficiently investigate hydrocarbon and hydrogen, emphasizing the reduction of carbon footprint. Authors performed case studies to demonstrate that the 3D CSEM-MT approach (structure guided inversion) can facilitate the interpretation of the resistivity information regarding conductivity structures. Authors also used the approach in geothermal energy investigations and show the usage of the approach in humanity's Net-Zero energy transition. It is good and well-written. I recommend the publication of the manuscript in Minerals after minor revision.

Reply: Thank you.

My minor suggestions are as follows:

1, Introduction, it is a lit bit confusing for me. I suggest the authors re-organize the introduction part to begin with the introduction of the main problem followed by the present statues as well as what is done by this study'

Reply: We agree and have now sufficiently re-worked the Abstract and Introduction as suggested. The abstract is now well-focused. Thank you.

2, Figure 2, please improve the quality of the figure;

Reply: We agree and have re-drawn Figure 2 and explained all the computations.

3， Equation 1, please define each symbol after the equation, what are f and \tau meaning?

Reply: We have now improved the font-size and defined the variables used in all the equations. Thank you.

4, line 329, what do you mean "In the second method" ?

Reply: We stated that two methods are of interest in this paper. These were described in paragraphs starting as: “In the first method…..”  and  “In the second method…”.

We’ve now re-formatted this section to make it clearer what the 2 methods are.

5, 4D time-larpe electromagnetic imaging: how do the authors judge where the model structure difference originates? How to make sure the difference originates from the change of the underneath conductivity structure or the noise or the inversion uncertainty?

Reply: In our adopted anisotropic CSEM-MT inversion approach, the cross-gradient constraint enforces structural similarity between vertical and horizontal resistivities and between base and monitor surveys where justified by the observed data. The model’s structural changes are accepted as valid if there is structural consistency assuming that the reservoir (i.e., subsurface container) structure is not changing significantly with time and that only the injected fluid is responsible for the observed changes. This assumption may not entirely justified if there were to be sudden ground motion leading to structural collapse (subsidence) soon after fluid injection but the cross-gradient constraint will track the resulting new structure and enforce structural consistency. This constraint is a good tool to differentiate between realistic model changes and random noise or inversion artifact. 

6, Do authors consider the bathymetry in course of CSEM-MT inversion?

Reply: Yes. Bathymetry is crucial for correct simulation of 3D CSEM responses. It is also important for accurate anomaly placement and tectonic interpretation of joint CSEM-MT inversion results. We combined bathymetry measurements from CSEM-MT acquisition company (EMGS) with those from GEBCO and our company’s 3D seismic/Gravity/magnetic acquisitions and use the re-gridded composite product in our inversion. This is described in [7,11,17]. We now amended the CSEM-MT grid design statement in section 3.1.2 to read:

“The model grid size was 312 x 199 x 145 cells in x, y and z directions (excluding air layers) with cell sizes of 250 x 250 x 25 m and incorporates bathymetry.”

 

Rev#2 Further comments on annotated PDF version of the paper

Reply: (1) Textual changes: We replaced ‘bane’ with challenge as suggested. (1) Figure 2 redrawn. (3) Figure 5. CH4 log re-drawn in red colour for more clarity. (4) Figure 7. Bathymetry colour-scale now has numerical values. (5) Figure 9. Dots (MT stations) and red box (sweet-spot with best potential) are now described in the legend.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper extensively discusses the concept of integrated marine EM (CSEM+MT) data analysis/interpretation for hydrogen and hydrocarbon exploration. Authors formulate the main aspects of the proposed approach  and provide some case-study data showing its efficacy.

My little comments are as following:

1) In my view, Abstract looks more like introduction, so I'd suggest making it more brief and highlighting the main points of the paper;

2) It would be helpfull to discuss the resistivity properties of the hydrogen-saturated rock;

3) As the proposed approach for EM data inversion involves anisotropic models, showing the images of vertical resistivity along with horizontal resistivity is worth showing at least in some case-study figures.

Overall, I support the publication of this paper, and hope the authors will implement the discussed strategy into every-day industrial practice.

Author Response

Reviewer#3

The paper extensively discusses the concept of integrated marine EM (CSEM+MT) data analysis/interpretation for hydrogen and hydrocarbon exploration. Authors formulate the main aspects of the proposed approach and provide some case-study data showing its efficacy.

Reply: Thank you.

My little comments are as following:

1) In my view, Abstract looks more like introduction, so I'd suggest making it more brief and highlighting the main points of the paper;

Reply: We agree. We have re-written the Abstract and Introduction which are now well differentiated as suggested.

2) It would be helpfull to discuss the resistivity properties of the hydrogen-saturated rock;

Reply: We agree. Unfortunately, there are no publicly available resistivity well logs for the hydrogen discoveries. Only the hydrogen concentration (ppm) are shown in the publications drawn upon in this paper (Figure 5).

3) As the proposed approach for EM data inversion involves anisotropic models, showing the images of vertical resistivity along with horizontal resistivity is worth showing at least in some case-study figures.

Reply: We completely agree and thank you for pointing this out. Although they were shown in [7, 11], they needed to be shown here too for consistency. We’ve added anisotropy plot as new Figure 11b (offshore Borneo) and new Figure 14 (Gulf of Mexico).

Overall, I support the publication of this paper, and hope the authors will implement the discussed strategy into every-day industrial practice.

Reply: Thank you. The strategy has already been implemented in our company’s operations and received significant management support to acquire large-size CSEM-MT data in various proof-of-concept studies in 2018-2020.

Author Response File: Author Response.pdf