| 26 Jun 2025
Geological factors and fracture distribution in deep and ultra-deep sandstones in Kuqa Depression, Tarim Basin, China
Abstract. Deep and ultra-deep sandstone reservoirs hold great potential for hydrocarbon resources, yet complex geological challenges hinder the successful exploitation of oil and gas. Fractures in deep and ultra-deep sandstones are prevalent and significantly enhance rock permeability, and critically impact fluid flow and hydrocarbon productivity. Relationships between geological factors and fracture distribution in deep sandstone reservoirs, despite its significance, have remained poorly understood. This study utilizes core, thin section, acoustic emission tests and geophysical well logs to elucidate the interplay between geological elements and fracture occurrences in tight sandstones of the Kuqa Depression, which is a tectonically active foreland basin. The controls of sedimentation, sandbody distribution and earth stress on fracture distribution are analyzed. The research then unravels the effects of lithology units, earth stress fields, and broader tectonic context on fracture distribution patterns. Geological factors, including sedimentary factors (lithology, sandbody thickness and sandbody distribution), earth stress, and tectonic structure are integrated to comprehensively evaluate the fracture distributions in Kuqa Depression. The different lithologies are identified, and fractures in different lithologies are characterized. High-angle fractures and vertical fractures are mainly fracture types in Bozi-Dabei area. The fracture density increases as the sandbody thickness increases. The presence of thinner sandstones in conjunction with thin mud layers facilitates the formation of fractures. Paleostress affects the generation of natural fractures, and high fracture density is associated with high paleostress magnitudes. In situ stress affects the subsequent modification of natural fractures, and high in situ stress results in low fracture aperture. Structure factors including the position at folds and the proximity to faults are crucial for the fracture distribution. Fractures are more abundant in the hinge areas of anticlines compared to the limb areas, and fracture density above the neutral planes is notably higher. In addition, fracture density is higher in the formation adjacent to the fault due to the effect of the regional stress field. This study helps unravel the geological controlling factors and distribution of fractures by integrating geological and geophysical data, and has implications for hydrocarbon resource exploration in deep and ultra-deep sandstones.
Fractures in ultra deep sandstones are of considerable scientific and practical interest.
The Introduction needs to be modified to more clearly state the claims of the work. This element should come near the end of the Introduction. See comment under line 70.
The citations to the literature cover important references, but the precision of the citing is lacking in some instances. The authors should consider working to tailor the citations more specifically to the points being made. And in many cases adding ‘e.g.’ would be useful, to indicate that these are just some examples from the literature, because in some instances they are neither the earliest examples nor the most recent reviews.
The title says deep and ultradeep, but how deep is not sufficiently emphasized. I don’t think you mention actual depths at all in the Introduction. For the international reader this will be an interesting aspect of this paper and it needs to be underlined. Tell the reader what constitutes deep and ultradeep. And make it clear what depths your samples are from. For example figure 8 is a spectacular illustration of image log fracture intensity variations with rock type but it just has a simple, cryptic label and there is no emphasis that the observations are from 6778 to 6801 meters.
I’m not necessarily recommending you cite this work, but Laubach et al. 2023 might be a good guide for how to add emphasis on the deep observations you have to offer. This is part of what makes your study interesting so make this aspect more obvious to the reader. (Laubach, S.E., Zeng, L., Hooker, J.N., Wang, Q., Zhang, R.H., Wang., J., Ren, B., 2023. Deep and ultra-deep basin brittle deformation with focus on China. Journal of Structural Geology 175, 104938)
Some of the figure captions are too short and cryptic. Figure 8, for example. Explain what is in the figures and draw attention to key points. These need to be more than mere labels. (On the figures, the rose diagrams should show and n = for how many readings and say if these are equal area plots).
There are a few sections in the Results that should be moved to the Discussion. I’ve marked some of these below, but the entire text should be checked. Much of the section around line 256 on in situ stress belongs in the Discussion (where a more nuanced appreciation is needed). See comments below.
Although reasonably clearly written, the text could use additional polishing for concision.
Comments keyed to lines in the text
39 Check for typo
47 ‘makes it’
70 Near the end of the Introduction you need to state the claims of your paper. You have a list of what you did and topics you cover but not statement of claims. You really need these to draw in the reader. Consider adding a paragraph that starts with the phrase ‘here we show that…’ and then say what you show: your claims. Some of the extant text can be readily altered for this purpose. For example, in line 69 instead of saying “The results will provide new insights into geological…” say “The results show that [and then fill in what the insight is]”
111 What is the depth range of the samples?
143 From the images it looks as though most of the fractures are opening-mode fractures rather than faults. Why not use this terminology?
149-150 There is a problem with this sentence, since open fractures by definition can’t be filled with calcite. Maybe you mean that some opening-mode fractures are open or are only partly filled with mineral deposits like calcite, and some are filled with calcite.
150 (figure) why not mention sample depths in the figure caption?
173 (figure) The open, irregular pore space in these fractures is not necessarily due to dissolution. This kind of texture can arise from incomplete infill of fractures (see Lander and Laubach 2015, GSA Bulletin). In any case, in your description you should describe what you see in terms that do not imply a mechanism, otherwise you have a circular argument (And you also have to explain how you get dissolution in these siliciclastic rocks). If you use descriptive terms in the Results you can use arguments in the Discussion to make the case for how you think those textures arose. In other words, the Discussion is where you say ‘we interpret the irregular, partly open fractures to be the result of dissolution because …’
175 ‘Formation’ needs to be capitalized for formal units.
187 ‘often’ is a time term; ‘commonly’ is better
198-199 This line about fluid flow belongs in the Discussion
200-202 These acoustic effects can be interfered with by rugose borehole. How smooth is your wellbore?
226-230 Some of this material is fine, but it is out of place. Here in the Results describe what you found. More the text about what you expect given the fractured layer thickness to the Discussion.
228-230 This seems to contradict what you said in the Abstract, where I read “Fracture density increases as sandbody thickness increases” which would indeed be a surprising result. Check.
242 (figure 8) this is a nice, interesting illustration.
245-253 This material belongs in the Discussion.
256 (section) this interpretation of the effects of stress belongs in the Discussion. In the Results you could provide evidence of what the state of stress is in these sandstones. It is an interpretation that stress contrasts might have the effects that you describe but the rocks here are in all around compression and the difference between SHmax and Shmin is probably small (in the Results you can say what this is). But just because this effect might be important does not mean that it is important, so saying “Therefore, in situ stress has a significant influence on the fracture aperture and porosity” is not warranted unless you have observations that you have not presented on changes with stress in in situ aperture or pore space. In many deeply buried sandstones natural fracture aperture are quite sensitive to in situ stress changes going back to experiments on core by Warpinski in the late 1980s; in many moderately to deeply buries sandstones open fractures exists are a wide range of angles (including at right angles) to SHmax (Laubach et al., 2004, EPSL) a circumstance that can be explained by the precipitation on cement in the stressed host rock (Olson et al., 2007) or to partial mineral bridges like some of the ones visible in your images (Laubach et al., 2004). Olson et al. show that if host rock diagenesis is happening during fracture, the expected stiffening effect of host rock cements means that an unacceptably high stress would be needed to close the fractures. See figure 15 in Laubach et al. (2019) to see what the magnitude of modulus increase likely is in a sandstone like the ones you describe. There are several papers on diagenesis of sandstone in basins near yours that show that such cement accumulations were likely happening rapidly in these deeply buried and hot settings.
My suggestion: here focus on what you can observe about the stress state in your rocks; in the Discussion, where most of this text belongs, mention these alternative interpretations.
Laubach, S.E., Olson, J.E., and Gale, J.F.W., 2004, Are open fractures necessarily aligned with maximum horizontal stress? Earth & Planetary Science Letters, 222/1, 191-195.
Olson, J. E., Laubach, S. E., and Lander, R. L., 2007, Combining diagenesis and mechanics to quantify fracture aperture distributions and fracture pattern permeability: In Lonergan, L., Jolley, R.J., Sanderson, D.J. , Rawnsley, K., eds., Fractured Reservoirs, Geological Society of London Special Publication 270, 97-112.
Laubach, S.E., Lander, R.H., Criscenti, L.J., et al., 2019. The role of chemistry in fracture pattern development and opportunities to advance interpretations of geological materials. Reviews of Geophysics, 57 (3), 1065-1111. doi:10.1029/2019RG000671
275 type amone>among
296 ‘three wells’ (small number rule)