Sedimentary characteristics of intersalt fine-grained sedimentary rocks and their control on oil-bearing ability of shales in the Qianjiang Formation, Qianjiang Sag

doi:10.13745/j.esf.sf.2020.12.8

Abstract

Abstract:

The Jianghan Basin is a typical inland salt lake basin in China. The Qianjiang Sag, in the middle of the basin, developed intersalt petroliferous fine-grained sedimentary rocks, and it has seen a recent breakthrough in oil and gas exploration. In order to study its sedimentary characteristics and depositional control, eight kinds of lithofacies are identified in the sedimentary rocks using core and various tests: calcite-bearing argillaceous dolomite, dolomite, argillite or calcite mixed fine-grained sedimentary rocks, calcareous mudstone/mudstone, dolomitic mudstone, sulfate mixed fine-grained rock, and dolomitic/argillaceous glauberite. In addition, two lithofacies associations, types A and B, are identified. The type A association contains calcite-bearing argillaceous dolomite, dolomite or argillite mixed fine-grained sedimentary rocks, with small amounts of calcite mixed fine-grained sedimentary rocks, dolomitic mudstone, sulfate mixed fine-grained rock, and dolomitic/argillaceous glauberite. The type B association involves dolomitic mudstone, sulfate mixed fine-grained rock dolomitic/argillaceous glauberite, small amounts of calcareous mudstone/mudstone and argillite mixed fine-grained sedimentary rocks. The type A lithofacies association is rich in carbonate and organic matter; its cores are gray-black argillaceous rocks (argillite and calcite mixed fine-grained sedimentary rocks) and yellow-brown dolomitic rocks (calcite-bearing argillaceous dolomite and dolomite mixed fine-grained sedimentary rocks) frequently interchanging, with white sulfate rocks interspersed in the form of lenses or layers, indicating the environmental salinity at that time was relatively low. Laminates under the microscope mostly show continuous fine wave or aligned horizontally. The sedimentary characteristics show the type A association is formed in a reductive and low salinity deep water environment. The type B lithofacies association, on the other hand, is rich in sulfate and has low organic matter content; its cores are white high sulfate rocks with black argillaceous layers or yellowish-brown dolomitic layers. Microscope observation reveals rhombohedral glauberite, commonly with interspersed twin crystals, and obvious grained sequence change can be seen. The thin slices with high dolomite content are characterized by the absence of laminates under the microscope. The sedimentary characteristics show the formation environment of the type B lithofacies association is shallow water with weak reducibility and high salinity. Geochemical parameters, such as V/(V+Ni), (S₁+S₂)/TOC, Sr/Ba, and Ga/C₃₁H, combining with lithofacies characteristics, further support the above assessment and show the types A and B associations are formed in humid and dry climate, respectively. The intersalt deposition is divided into three stages, B, A, B, in a proposed sedimentary model: In the first stage, the climate is dry and the water is shallow, a lot of salts precipitate from the lake water to form mainly the type B lithofacies association. With increasing sulfate minerals, organic matter oxidation is facilitated by sulfate-reducing bacteria, which is not conducive to organic matter enrichment. In the second stage, increasing rainfall and large-scale fresh water injection cause the water salinity and depth to increase. At this time, the water has good stratification and strong reducibility to develop the type A lithofacies association, which improves the preservation conditions for organic matter. Therefore, the type B association, with low TOC content, is not the main lithofacies for oil and gas accumulation, as enriched sulfate minerals can dilute the organic matter, and the high salinity environment in the lithofacies can slow down the microbial activity. Conversely, the type A association, with high TOC content, is the main lithofacies for oil and gas enrichment. In its forming environment, frequent rainfall lowers the water salinity to create a good environment for the organic matter-producing planktonic microorganisms to grow; at the same time, pores between minerals beneficial to hydrocarbon storage develop in the dolomitic laminate, making it a high-quality source rock and favorable lithofacies for oil and gas exploration and development in the area.

Key words: fine-grained sedimentary rocks, sedimentary characteristics, lithofacies, depositional model, Qianjiang Sag

CLC Number:

P618.13

CHEN Chen, JIANG Zaixing, KONG Xiangxin, WU Shiqiang, CHEN Fengling, YANG Yepeng. Sedimentary characteristics of intersalt fine-grained sedimentary rocks and their control on oil-bearing ability of shales in the Qianjiang Formation, Qianjiang Sag[J]. Earth Science Frontiers, 2021, 28(5): 421-435.

Figures/Tables 12

Fig.1 Diagram showing the locations of the structural wells in the Qianjiang Sag

Fig.2 Composite stratigraphic profile compiled for the Qianjiang Sag. Modified after [24].

Fig.3 Mineral composition of the Eq34-10 rhythm layer of Well BYY2

Table 1 Quantitative classification of each lithofacies in the Eq34-10 rhythm layer of Well BYY2 in the Qianjiang Sag

大类	岩相	碳酸盐矿物含量/%		硫酸盐矿物含量/%	泥质碎屑含量/%
大类	岩相	方解石	白云石	硫酸盐矿物含量/%	泥质碎屑含量/%
碳酸盐岩	含灰泥质云岩	1020	>50	<10	>20
碳酸盐岩	泥质云岩	<10	>50	<10	>20
泥岩	灰质泥岩泥岩	>20 <10	<10 <10	<10 <10	>50 >50
钙芒硝岩	云质钙芒硝泥质钙芒硝岩	<10 <10	>20 <10	>50 >50	<10 >20
混合细粒岩	云质混合细粒岩	<50	<50(最高)	<50	<50
	泥质混合细粒岩	<50	<50	<50	<50(最高)
	灰质混合细粒岩	<50(最高)	<50	<50	<50
	硫酸盐质混合细粒岩	<50	<50	<50(最高)	<50

Fig.4 Composite diagram showing the lithofacies association in the Eq34-10 rhythm layer of Well BYY2 in the Qianjiang Sag

Fig.5 Microphotographic images revealing the sedimentary characteristics of type A lithofacies association in the Eq34-10 rhythm layer of Well BYY2 in the Qianjiang Sag

Table 2 Mineral composition and TOC content of each lithofacies in the Eq34-10 rhythm layer of Well BYY2 in the Qianjiang Sag

Fig.6 Microphotographic images illustrating the sedimentary characteristics of calcareous fine-grained sediment in the Eq34-10 rhythm layer of Well BYY2 in the Qianjiang Sag

Fig.7 Microphotographic images showing the sedimentary characteristics of type B lithofacies association in the Eq34-10 rhythm layer of Well BYY2 in Qianjiang Sag

Fig.8 Composite diagram characterizing the geochemical parameters of the Eq34-10 rhythm layer of Well BYY2 in the Qianjiang Sag

Fig.9 Correlation between TOC and mineral contents in the Eq34-10 rhythm layer of Well BYY2 in the Qianjiang Sag

Fig.10 Depositional model of the Qianjiang Sag, Jianghan Basin

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