Sequence stratigraphy and coal accumulation model of the Taiyuan Formation in the Tashan Mine, Datong Basin, China

Abstract

Datong Basin is a platform uplift formed in the Cenozoic era and has a significant coal-bearing geological unit in China. Tashan Mine is located in the middle-eastern part of the Datong Basin, the coal resources of the Taiyuan Formation are abundant, and the research value is enormous. Of the coal resources, coal seams 3–5 and 8 are the main. The lack of research in sequence stratigraphy, coal-forming environment, coal-accumulating model limits the layout efficiency of working face and the exploration and development efficiency of coal measures associated resources. This study used sedimentary and sequence stratigraphy and calculated and measured field drilling data to investigate the sequence stratigraphic characteristics, paleogeographic pattern, and coal accumulation model of the Taiyuan Formation. The sequence boundary and system tract boundary were identified, and the Taiyuan Formation was divided into four third-order sequences. The paleogeographic pattern was reconstructed with the third-order sequence as a unit by applying the single-factor analysis and multifactor comprehensive mapping. Additionally, the distribution characteristics of the main coal seams were clarified, and the corresponding coal accumulation model was reconstructed. The results exhibit that the sedimentary facies transformed from carbonate platform–tidal flat–lagoon–shallow water delta facies to shallow water delta facies from SQ1 to SQ2, coal seam 8 formed in interdistributary bay microfacies and peat swamp microfacies in SQ1. Transgression further expanded in SQ3, and the sedimentary facies were still dominated by the shallow water delta facies. Coal seams 3–5 are formed in distributary channel microfacies, which consist of interdistributary bay microfacies and peat swamp microfacies. The paleogeographic pattern was still dominated by shallow water delta in SQ4, in which interdistributary depressions and peat swamp microfacies widely developed. The thickness of coal seams 3–5 is in the range of 2.40–25.90 m, in which the northwestern study area is characterized by moderate water depth and sufficient fine sediment, and the widely developed distributary bay and peat swamp deposited a thick coal seam. The thickness of coal seam 8 is in the range of 0.5–10.5 m, and the thickness is stable (mostly thicker than 6 m) controlled by the widely developed lagoon, mud flat, and peat swamp microfacies and reduction water environment.

A model for superimposed coalbed methane, shale gas and tight sandstone reservoirs, Taiyuan Formation, Yushe-Wuxiang Block, eastern Qinshui Basin

“…The bottom boundary of the Taiyuan Formation is sandstone K1 (also named Jinci sandstone), and its continuity is not very good in the study area, as it is absent in certain areas and mudstone stratum deposits. The top boundary of the Taiyuan Formation is sandstone K7 (also named Beichagou sandstone), and its continuity is also poor in the study area and it is not a good signature stratum) 13 , 37 , 38 .

Figure 1 The tectonic location and structural characteristics of the Yushe-Wuxiang Block, modified from Ref.

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“…Additionally, signature strata K2 (also named Maoergou limestone), K3 (also named Xiedao limestone), K4 (also named Dongdayao limestone), K5 (also named Fucheng limestone with poor stability, and usually deposit marine mudstone caused by facies changing), and K6 limestone (also named Shangou limestone) are also developed from old to young in the Taiyuan Formation 38 , among which the former three have excellent stability, while the continuity of the latter two is poor 13 , 37 . Coal seam 15 in the Lower Taiyuan Formation is the main coal seam and a stable signature stratum with large thickness and fairly good continuity.…”

Identifying the coal seams and their sedimentary environments in the Nui Beo mine, Quang Ninh province using K-means and regression methods

“…Các lớp trầm tích cũng như vỉa than phát triển tương đối liên tục trong quá trình thành tạo cho nên chúng có mối liên hệ không gian gần gũi nhau. Các lớp đá hoặc vỉa than gần gũi nhau về không gian sẽ có chiều dày, góc dốc, số lớp kẹp cũng như đặc điểm, tính chất tương tự nhau, chính các yếu tố này là cơ sở cho việc đồng danh, liên kết vỉa, tập vỉa than [1][2][3] . Sau khi thành tạo, quá trình hoạt động kiến tạo về sau sẽ làm thay đổi thế nằm, tạo nếp uốn, dịch chuyển các vỉa than gây ra sự gián đoạn, phức tạp hóa trong quá trình nối vỉa.…”

“…Hầu hết các nghiên cứu trước đây về đồng danh, liên kết vỉa than ít chú ý đến môi trường thành tạo trầm tích. Tuy nhiên, trong cùng thời kỳ thành tạo, trong cùng một bồn trũng, than có thể hình thành trong môi trường đầm lầy (than dày, thế nằm thoải, dạng thấu kính, dạng ổ, chất lượng than tốt), môi trường bãi triều bùn (vỉa than có chiều dày vừa phải, thế nằm thoải, có dạng vỉa, chất lượng than tốt), môi trường bãi bồi ven sông, ven hồ (vỉa than có chiều dày thay đổi, thế nằm hơi dốc, có dạng hình nêm vát, dạng vành khăn, chất lượng than trung bình và kém) và nhiều môi trường khác [1][2][3][4][5][6] . Như vậy, trong cùng một thời kỳ, cùng một vỉa than nhưng chiều dày vỉa lại thay đổi, hình dạng thay đổi, và thậm chí chất lượng than cũng thay đổi, điều này sẽ dẫn đến phá vỡ tính đồng nhất tương đối gây ra sự phức tạp trong quá trình liên kết, nối vỉa.…”