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The formation environment and developmental models of argillaceous dolomite in the Xingouzui Formation, the Jianghan Basin
Marine and Petroleum Geology 67 (2015) 692e700
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Research paper
The formation environment and developmental models of argillaceous dolomite in the Xingouzui Formation, the Jianghan Basin Wenhao Li a, b, *, Shuangfang Lu a, Haitao Xue a, Pengfei Zhang b, Shiqiang Wu c a
Research Institute of Unconventional Petroleum and Renewable Energy, China University of Petroleum, Qingdao, 266580, China School of Geosciences, China University of Petroleum, Qingdao, 266580, China c Research Institute of Exploration and Development of Jianghan Oilfield, SINOPEC, Wuhan, 430000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 16 June 2015 Accepted 22 June 2015 Available online 30 June 2015
Shale oil resources are found in the argillaceous dolomite in the Xingouzui Formation of the Jianghan Basin, revealing a new field for petroleum exploration. Studying the environment in which argillaceous dolomite forms reveals details regarding the formation and distribution of shale oil in the basin. By combining a detailed analysis of the basic geological data from petroleum exploration with methods based on organic petrology and geochemistry, the formation environment and developmental models of argillaceous dolomite in the YajiaoeXingou Uplift and Chentuokou Sag are discussed. The results are as follows: the average TOC values in the argillaceous dolomite exceed 1.0% in the YajiaoeXingou Uplift and Chentuokou Sag, reaching better source rock standards. The type of organic matter of the argillaceous dolomite from the Chentuokou Sag is obviously better than that in the YajiaoeXingou Uplift. The maturity of organic matter in the source rocks is lower. Tricyclic terpanes, pregnane and homopregnane, which originated from algae, are detected in the argillaceous dolomite in the Chentuokou Sag, while these compounds are not found in YajiaoeXingou Uplift. In addition, the alginite and sporinite are respectively detected in the two areas described above. Therefore, the lake productivity is higher in the Chentuokou Sag relative to the YajiaoeXingou Uplift. The Pr/Ph, Fe2þ/Fe3þ and gammacerane/C30 hopane data reveal that argillaceous dolomite developed in an anoxic environment with better water column stratification. The anoxic environment is the major factor controlling the formation of argillaceous dolomite. The central belt of the Chentuokou Sag was where the lake productivity was higher and preservation conditions of organic matter were excellent, making it the most favorable zone for forming argillaceous dolomite that tends to generate oil. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Lake productivity Anoxic environment Argillaceous dolomite The Jianghan Basin
1. Introduction The successful development of a shale gas reservoir in North America has made shale gas the most important unconventional hydrocarbon resource, because it has great potential for replacing conventional hydrocarbon resources worldwide (Rogner, 1977; Hill et al., 2004; Perry and Lee, 2007). However, unconventional hydrocarbon exploration has focused on shale oil rather than shale gas in recent years in America, making the exploration of shale oil of great value. Shale, widely distributed in China, is the main field for shale oil exploration and development. Shale oil has recently been found in the dolomite mudstone and argillaceous dolomite in * Corresponding author. Research Institute of Unconventional Petroleum and Renewable Energy, China University of Petroleum, Qingdao, 266580, China. E-mail address: [email protected] (W. Li). http://dx.doi.org/10.1016/j.marpetgeo.2015.06.011 0264-8172/© 2015 Elsevier Ltd. All rights reserved.
the Jianghan, Junggar and Santanghu Basins (Liang et al., 2012; Luo et al., 2013; Zou et al., 2013a,b), revealing a new field for exploring and developing shale oil. After discussing the actual geological conditions in China, in combination with the formation conditions, occurrence state and accumulation mechanism of shale oil and gas, the most favorable zones have been predicted based on the research data, exploration and the development of shale oil and gas in North America (Zhang et al., 2003; Nie et al., 2009; Zou et al., 2010, 2013a,b; Zhao et al., 2011; Zhang, 2011; Lu et al., 2012). The amount of residual hydrocarbons in shale can be calculated easily based on the current method for quantitatively evaluating source rocks (Lu and Zhang, 2008). However, hydrocarbon contents of shale in diverse formations are significantly different because the mineral composition, organic matter abundance, organic matter types and expulsion efficiency of hydrocarbon are obviously affected by the environment during the formation of the shale.
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However, TOC values depend on the formation of source rocks, specifically, supply and preservation conditions of organic matter. Organic matter supply includes the lower hydrobiont organisms in lakes or oceans and terrigenous organic matter, but the former tends to generate crude oil. Preservation conditions of organic matter are related to dissolved oxygen content of water column. Organic matter can be consumed in oxic water column and conserved in an anoxic environment. Two hypotheses were proposed regarding whether high organic matter abundance of the sediments depended on the preservation conditions or production (Tyson, 1987; Tyson and Pearson, 1991; Calvert and Pedersen, 1992). Currently, most scholars believe that high TOC values of the source rocks can be attributed to several factors including lake or ocean productivity, preservation conditions of organic matter, sedimentation rate, etc.; the most important ones are productivity and anoxic environment (Dean et al., 1997; Boucsein and Stein, 2000; Stow et al., 2001; Chen et al., 2006; Zhang et al., 2007; Hao et al., 2011; Li et al., 2013). Hao et al. (2011) analyzed the lateral source rock heterogeneities to reveal the environmental and ecological changes and to construct depositional models for lacustrine source rocks under different tectonic and climatic conditions. Zou et al. (2012) proposed that the environment in which shale or carbonate rocks formed controlled the extent of the enrichment of shale oil and gas, while large-scale delta and lacustrine carbonate rocks enabled the formation and distribution of the reservoir. Li et al. (2014) revealed the main controlling factors and depositional models of the Miocene marine source rocks in the Qiongdongnan Basin, based on analysis of terrestrial plants supply, paleoproductivity and redox conditions. Yan et al. (2015) investigated the mechanism of organic matter accumulation in Early Silurian sediments using multiple geochemical proxies, including redox parameters, productivity indices and clastic influx indicator. Shale oil of lacustrine argillaceous dolomite in the Xingouzui Formation in the YajiaoeXingou Low Uplift has been successfully explored in the Jianghan Basin since 2012, marking a significant breakthrough. Concurrently, progress has been made in the Chentuokou and Mianyang Sags of the basin. However, the environment in which the argillaceous dolomite forms remains unclear, restricting the exploration of unconventional resources in the Jianghan Basin. Consequently, the environment surrounding the formation of argillaceous dolomite is discussed through the analysis of organic petrology and geochemistry while using the YajiaoeXingou Low Uplift and Chentuokou Sag as examples.
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2. Geological background The Jianghan Basin lies in the center of the Jianghan Plain, which is in the Hubei Province and spans 28,760 km2. The northern portion of the basin is adjacent to the QinlingeDabie Orogenic Belt, the southern part is near the JiangnaneXuefeng Uplift, and the western part borders the Huangling Anticline. The evolution of the basin developed through the Yanshan and Himalayan movements from Cretaceous to Quaternary. The basin experienced three stages including the early Yanshan extrusion, the CretaceousePaleogene rift depression and the NeogeneeQuaternary depression. The Jianghan Basin can be divided into 15 secondary tectonic units including 11 sags (Jiangling Sag, Qianjiang Sag, Chentuokou Sag, etc.) and four uplifts (YajiaoeXingou Uplift, etc.) (Fig. 1). The Jianghan Basin is a polycyclic sedimentary basin, and its cap rocks are Cretaceous and Cenozoic clastic rocks. The basement of the basin is composed of pre-Cretaceous marine carbonate, clastic and terrestrial coal-bearing clastic rocks. On top of these rocks, the following were deposited: the Yuyang Formation in Cretaceous, the Shashi, Xingouzui, Jingsha, Qianjiang and Jinghezhen Formations in Palaeogene, Neogene Guanghuasi Formation and Quaternary Pingyuan Formation. From Cretaceous to Quaternary, the periphery terrain of the basin was higher in the northwest and lower in the southeast; the provenance was primarily from northwest, exhibiting a maximum thickness of sedimentary strata above 10,000 m and characteristic cyclicity and rhythmicity. During the sedimentary period of the Xingouzui Formation, the paleoclimate was initially subtropical, becoming humid semiarid and wetter in the late period. The sedimentary environment included shallow to semi-deep water and contained well-developed dark mudstones as the major source rocks in the basin. During the sedimentary period of the lower Xingouzui Formation, provenance was mainly from north of the basin, which formed a large-scale delta progression toward the basin. The delta front was broadly extended. Another large-scale provenance was observed from the northeast. When the upper Xingouzui Formation was deposited, the provenance and depositional system were obviously different from those of the lower Xingouzui Formation. The provenance was obviously limited and was far from the depocenter. The deltaic deposits developed in some regions in the northwest of the basin. However, most areas of the basin were covered by the gypsum and mud deposits in shallow to brackish lakes. The Jianghan Basin contained structural, structuralelithological, lithologic and stratigraphic reservoirs before 2012. Three tons of
Fig. 1. The structural location of the Jianghan Basin.
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We chose 15 samples to analyze the relative or total content, as well as the constituents, of organic materials in the rocks. The measurements were performed on a fluorescent microscopeLABORLUX 12 POL microphotometer (MPV-3). The measurements were collected using a 50objective lens at a total amplification of 800. The temperature of the detection system is (23 ± 2) C. Twenty-six samples were chosen and prepared to analyze the trace and major elements using approximately 25 mg of a powdered and homogenized sample loaded in polytetrafluoroethylene (PTFE) vessels; these samples were heated to 190 C for 72 h in PTFE autoclaves with 1 mL of HF (40%) and 1 mL of HNO3 (65%). The acids were removed through evaporation at 200 C on hot plates, and the wet residues were dissolved and re-evaporated twice using 0.5 mL of HNO3 (65%). After the final HNO3 treatment, the wet residues were heated to 130 C for 12 h in PTFE autoclaves with 5 mL of HNO3 (32.5%). The wet residues were diluted to 30 mL with 2% HNO3. During the analyses, the acid digests were spiked with Rh and Re as internal standards. Chinese standard samples GB7107 (shale) and GB7114 (dolomite) were used to assess the accuracy of the analyses. What is more, the carbon and oxygen isotopes of the 26 samples are measured in State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum in Beijing. 4. Results and discussion 4.1. Analysis of hydrocarbon potential of argillaceous dolomite in the Xingouzui Formation
Fig. 2. The comprehensive stratigraphic column of the Xingouzui Formation.
shale oil resources have been obtained per day in the Xingouzui Formation since 2012, stimulating exploration for shale oil in the Jianghan Basin (Li et al., 2015; Shen et al., 2015). The discovery of shale oil in the argillaceous dolomite of the Xingouzui Formation provides new strata and new area of hydrocarbon exploration in the basin. The lithology of the Xingouizui Formation is mainly argillaceous dolomite. There also developed shales in the bottom of the Xingouizui Formation (Fig. 2). 3. Analytical samples and methods Forty-nine argillaceous dolomite samples in the Xingouzui Formation were collected in Wells Xin391 and Chen100, respectively, in the YajiaoeXingou Low Uplift and the Chentuokou Sag of the Jianghan Basin. The samples were ground in an agate mortar and pestle to for 200-mesh particles. Two-hundred grams were weighed from each sample and subsequently crushed to a grain size of 0.18 mm at 50 C in the laboratory. The powdered samples were analyzed for total organic carbon and Rock-Eval. Twelve typical samples were chosen for GCeMS analysis based on the results of the experiments above. The samples were crushed to 80 mesh and extracted in a Soxhlet apparatus with dichloromethane (CH2Cl2) for 24 h. The extracts were evaporated, deasphaltened with n-hexane, and separated into saturated, aromatic and polar fractions through column chromatography using silica gel and alumina (3:1) while eluting with n-hexane, dischloromethane:n-hexane (2:1, v/v) and chloroform:ethanol (1:1, v/v), respectively. The saturated and aromatic fractions were analyzed by gas chromatography-mass spectrometry (GCeMS). GCeMS analyses were performed on a Thermo-Finnigan TraceDSQ equipped with an HP-5MS (60 m 0.25 mm 0.25 mm) fused silica capillary column while operating in full scan mode.
Researchers proposed that source rocks with TOC values below 0.6% could become better source rock standards in the salt lake environments; the hydrocarbon conversion rate is rather high, and the hydrocarbon mass could be generated in a relatively lower maturity of organic matter in such environment (Wang et al., 1995; Sun et al., 1997; Zhang et al., 2000; Bao et al., 2006). Organic geochemical analysis of the argillaceous dolomite in the YajiaoeXingou Low Uplift and Chentuokou Sag reveals that the TOC values range from 0.20% to 4.59% and from 0.16% to 4.27% with averages of 1.32% and 1.04%, respectively, and the values of (S1 þ S2) range from 0.44 to 43.86 mg/g and 0.33e29.44 mg/g, averaging 6.72 and 5.83 mg/g, respectively in the above areas (Table 1). A positive correlation exists between the TOC and (S1 þ S2) values, and these two parameters exceed 0.6% and 2 mg/g in most of the samples, respectively (Fig. 3). Therefore, source rocks in the Xingouzui Formation have better hydrocarbon potential. Hydrogen Index (HI) values range from 63.32 to 915 mg/g and 76.92e895.48 mg/g respectively in the YajiaoeXingou Low Uplift and the Chentuokou Sag, respectively. The average HI values in the latter are much higher than that in the former, indicating that the type of organic matter in the source rocks from the Chentuokou Sag is superior. The average Tmax values of argillaceous dolomite in the areas above are 425.26 C and 433.88 C respectively (Table 1), indicating that source rocks in the two areas are in immature and mature stages, respectively. 4.2. The formation environment of argillaceous dolomite 4.2.1. Lake productivity Organic materials that primarily generate oil are detected in the argillaceous dolomite in the YajiaoeXingou Low Uplift and Chentuokou Sag. Sporinite (Fig. 4aec) and alginite (Fig. 4def) are found in the above areas, respectively. Sporinite belongs to exinite, which tends to generate oil. Sporinites, particularly microsporinite, can be circular, ovular and strip-shaped; the examples from Well Xin391 in the Xingou Low Uplift are yellow-kelly under the fluorescence
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Table 1 Distribution of some of the geochemical parameters of argillaceous dolomite in the Xingouzui Formation in the YajiaoeXingou Uplift and Chentuokou Sag of the Jianghan Basin. Structural belt
Depth, m
TOC, %
S1 þ S2, mg/g
IH, mg/g
Tmax, C
YajiaoeXingou low uplift
1380.15 1381.55 1382.70 1385.20 1385.80 1386.80 1388.30 1389.30 1392.65 1393.30 1394.10 1426.30 1427.80 1428.25 1442.85 1445.40 1460.45 1460.95 1461.95 1462.45 1464.00 1464.55 1464.90 2106.40 2107.43 2108.20 2109.10 2113.40 2115.75 2116.25 2117.03 2117.58 2118.65 2150.90 2152.10 2153.60 2154.75 2155.90 2156.85 2158.65 2159.35 2188.85 2191.05 2191.90 2192.60 2193.75 2194.95 2195.85 2196.90
0.67 1.47 0.62 1.47 0.65 0.84 0.91 0.20 3.56 0.61 1.03 0.94 1.08 1.03 0.34 0.57 4.59 0.67 0.48 1.02 0.76 3.97 2.84 0.51 0.50 0.58 0.68 0.64 0.34 1.06 0.75 4.27 0.56 0.95 0.43 0.16 0.53 1.93 3.10 1.20 0.52 0.94 2.80 1.09 1.33 1.22 0.45 0.45 0.17
0.79 6.73 0.76 6.45 1.06 0.81 1.67 0.44 33.83 1.07 1.98 3.88 2.22 1.90 0.69 0.66 43.86 0.87 1.34 2.14 1.63 19.13 20.74 0.85 0.70 4.11 1.08 0.75 0.51 2.81 5.03 18.20 1.12 6.56 2.17 0.41 0.95 11.51 29.44 7.46 2.18 2.87 27.73 4.00 12.48 3.99 2.21 2.06 0.33
83.83 408.84 92.38 404.08 128.83 63.32 120.61 153.85 557.02 109.48 148.54 318.71 166.67 145.63 154.30 78.67 915.03 90.23 220.13 145.10 142.29 140.81 659.86 119.14 91.27 288.16 92.65 76.92 97.92 215.09 212.28 220.14 108.54 274.45 329.44 163.52 120.98 560.10 895.48 464.17 302.68 192.55 892.50 282.57 681.20 190.16 215.73 213.81 129.63
413 428 415 433 421 425 425 488 428 423 426 417 421 420 428 397 432 415 429 426 425 422 424 430 426 428 427 428 428 434 429 433 421 429 432 434 431 436 437 437 436 425 438 438 437 420 416 426 473
Chentuokou Sag
microscope. Alginite is a sapropelinite, which has excellent hydrocarbon potential for oil generation. Vermicular scattered and laminated alginite are detected in the argillaceous dolomite in Well Chen100 in the Chentuokou Sag; these samples are yellow under the fluorescence microscope. The organic material data indicate that the ancient lake productivity in the Chentuokou Sag is higher than that in the YajiaoeXingou Uplift during the sedimentary period of the Xiugouzui Formation. Biomarkers are often used to discuss the origin of organic matter, which indicates lake productivity. Biomarker characteristics of the steranes and terpanes of the source rocks in the YajiaoeXingou Low Uplift and Chentuokou Sag show that the gammacerane content is relatively high, and the aaa20RC27, aaa20RC28 and aaa20RC29 regular steranes are distributed in “V” or approximate “V” shapes (Fig. 5). The aaa20RC27/aaa20RC29 ratios range from 0.54 to 2.25, averaging 1.06 (Table 2). Notably, the tricyclic terpanes, pregnane and homopregnane that originated from algae are detected in the core samples from Well Chen100; these
Fig. 3. A cross plot showing the relationship between TOC and S1 þ S2 values of argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin.
compounds are nearly absent in Well Xin391 (Fig. 5). The above biomarker characteristics indicate that organic matter in the source rocks includes lower hydrobiont and higher terrestrial plants in the study area. In addition, the ancient lake productivity in the Chentuokou Sag is low to medium, while that in the YajiaoeXingou Uplift is lower. The argillaceous dolomite in the YajiaoeXingou Uplift contains high organic matter abundance despite low lake productivity because large amounts of terrigenous organic matter were delivered by the Hanshui River, which is in the northern portion of the basin. However, organic matter and clastic materials were not delivered to the Chentuokou Sag due to the silt from the YajiaoeXingou Low Uplift; in this case, lake productivity was not diluted by large amounts of detrital materials. Consequently, source rocks containing high levels of organic matter can be formed in the Chentuokou Sag. 4.2.2. Redox environment Organic matter can be accumulated in anoxic water (Demaison and Moore, 1980; Hunt, 1996; Cheng et al., 1996; Ten et al., 2004; Li et al., 2012). The following parameters are used to discuss redox conditions in this paper. The Pr/Ph ratio in the samples varies from 0.38 to 1.07, averaging 0.58 (Table 2); these data indicate an anoxic environment. The Fe2þ/Fe3þ ratios range from 0.79 to 5.22 (only one sample generated a ratio below 1), averaging 2.07 (Table 3); these data also indicate an anoxic environment. In addition, the salinity is also important during source rock formation. An anoxic environment can be formed in the bottom water due to high salt content at the sedimentewater interface (Li, 1993; Katz, 1995). Gammacerane is commonly used to reveal the sedimentary environment of organic matter, and high gammacerane contents are related to highly saline environments and strati et al., 1995; Zhang et al., 1999; Wang fied water (Sinninghe Damste et al., 2002; Feng et al., 2009). A relatively high content of gammacerane is present in the argillaceous dolomite in the study area (Fig. 5). The gammacerane index (gammacerane/C30 hopane) varies from 0.13 to 1.22, averaging 0.49 (Table 2); these values indicate stratified water with a relatively high salinity, which tends to form anoxic water. The range of salinity levels is wide, while the Pr/Ph ratio remains steady, indicating that the body of water was continuously anoxic during the sedimentary period of the argillaceous dolomite in the Xingouzui Formation, which is supported by the high Fe2þ/Fe3þ values (Table 2).
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Fig. 4. Organic micrograph of argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin. Note: a. Well Xin391 (1393.3 m) microsporinite, yelloweyellow green fluorescence; b. Well Xin391 (1464 m) sporinite, yelloweyellow green fluorescence; c. Well Chen100 (2117 m) microsporinite, yelloweyellow green fluorescence; d. Well Chen100 (2155.9 m) scattered alginite, yellow fluorescence; e. Well Chen100 (2156.9 m) laminar alginite, yellow fluorescence; f. Well Chen100 (2191.05 m) laminar alginite, yellow fluorescence. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4.3. Main controlling factors of formation environment of argillaceous dolomite The TOC values of the argillaceous dolomite ranged from 0.16% to 3.10%, averaging 0.95%, in Well Chen100 (Fig. 6). Three sections (lower, middle and upper sections) are created to illustrate the formation environment of the argillaceous dolomite. The TOC values of the argillaceous dolomite in the lower section are moderate, averaging approximately 1%. The middle section exhibits values from 0.16% to 3.10%, averaging 1.33%. The values from the upper section are much lower, ranging from 0.50% to 1.06% and averaging 0.66%. Phosphorus is usually used to determine marine or lake productivity (Tyrrell, 1999). The percentage of phosphorus in the argillaceous dolomite is relatively high in the lower and upper sections; this value is low in the middle section. Therefore, the ancient lake productivity was relatively high during the sedimentary period of the low and upper sections of the Xingouzui Formation; the productivity is lower during the sedimentary period of the middle section. The average Fe2þ/Fe3þ ratio of the argillaceous dolomite exceeds 1 in three sections (Table 3, Fig. 6), revealing excellent preservation conditions of organic matter. The Fe2þ/Fe3þ ratios of the samples from the middle section are higher than in other two sections; therefore, the middle section had better preservation conditions of organic matter due to the strong anoxic environment during the sedimentary period of
the middle section of the Xingouzui Formation. The Mg/Ca ratios of the samples in the lower section are high; these values are lower in the middle and upper sections (Table 3, Fig. 6). The d18O values do not obviously vary, while the d13C values clearly change as these ratios increase (Fig. 6); therefore, the climate changed from warm and humid to relatively dry and cold during the sedimentary periods from the lower to upper sections. Therefore, the anoxic environment is the major factor controlling the argillaceous dolomite, followed by lake productivity. During the sedimentary period of the middle section of the Xingouzui Formation, the lake productivity in the Chentuokou Sag is relatively lower, but argillaceous dolomite with abundant organic matter can still form in strong anoxic environments. The degree of water reduction in the source rocks in the lower section is almost the same as that in the upper section. However, a warm and humid climate remains favorable for alga propagation. Therefore, the lake productivity is relatively high, making organic matter abundance of argillaceous dolomite higher than in the upper section. 4.4. The developmental model of argillaceous dolomite The argillaceous dolomite developed in a shallow lake environment in the YajiaoeXingou Uplift. Due to the influence of the Hanshui River, large-scale delta deposits were observed in the
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Fig. 5. Mass chromatograms of the steranes and terpanes in the argillaceous dolomite of the Xingouzui Formation in the Jianghan Basin.
Table 2 Distribution of some of the biomarker parameters of argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin.
Table 3 Distribution of some of the major elements and parameters of argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin.
Well
Depth (m)
Pr/Ph
Ga/C30H
C27/C29
Well
Depth (m) MgO (%) CaO (%) P2O5 (%) “P” (%) Mg/Ca Fe2þ/Fe3þ
Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100
1386.80 1393.30 1394.10 1428.25 1442.85 1462.45 2113.40 2116.25 2117.03 2191.05 2191.90 2195.85
0.63 0.38 0.63 0.40 0.58 0.58 1.07 0.62 0.44 0.53 0.61 0.51
0.41 0.33 0.28 0.13 0.19 1.22 0.39 0.20 0.60 0.67 1.11 0.35
0.54 0.94 1.17 0.57 1.07 2.25 2.14 1.06 0.61 0.61 0.65 1.06
Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100
1380.15 1381.55 1385.80 1386.80 1393.30 1394.10 1428.25 1442.85 1445.40 1461.95 1462.45 1464.00 2106.40 2107.43 2108.20 2113.40 2116.25 2117.03 2118.65 2150.90 2153.60 2154.75 2155.90 2156.85 2188.85 2191.90
Note: Pr/Ph ¼ pristane/phytane; Ga/C30H ¼ gammacerane/C30 hopane; C27/ C29 ¼ C27/C29 steranes.
northern portion of the uplift, providing abundant terrigenous organic matter. However, lake productivity could be diluted by mass of detrital materials delivered by the river-delta system, as shown by the phosphorus content of the argillaceous dolomite in Well Xin391 (Table 3). The source rocks were developed in brackish to semi-saline environments with favorable preserving conditions of organic matter (Fig. 7) and contained abundant organic matter, which generates oil and gas.
3.18 7.37 7.56 3.35 4.16 4.15 2.18 3.53 3.35 13.95 3.21 3.87 1.79 5.15 13.20 4.34 4.29 13.04 8.62 18.94 12.57 4.80 5.90 7.71 5.17 4.45
2.72 12.41 10.82 2.80 5.29 2.10 0.57 8.72 3.49 22.62 2.83 4.13 6.81 6.20 27.82 1.10 3.94 22.53 11.53 28.87 25.09 9.96 10.23 12.66 2.96 2.19
0.13 0.13 0.22 0.10 0.12 0.17 0.09 0.16 0.10 0.12 0.13 0.16 0.13 0.15 0.11 0.30 0.15 0.15 0.19 0.07 0.15 0.10 0.15 0.14 0.32 0.16
0.05 0.06 0.10 0.04 0.05 0.07 0.04 0.07 0.04 0.05 0.06 0.07 0.05 0.06 0.05 0.13 0.07 0.07 0.08 0.03 0.06 0.04 0.06 0.06 0.14 0.07
0.99 0.50 0.59 1.01 0.66 1.67 3.22 0.34 0.81 0.52 0.96 0.79 0.22 0.70 0.40 3.33 0.92 0.49 0.63 0.55 0.42 0.41 0.49 0.51 1.47 1.71
2.24 3.22 3.17 1.08 1.96 1.67 1.31 1.80 1.47 2.83 2.04 2.35 1.56 1.10 5.22 1.09 1.56 2.16 1.10 2.94 3.32 3.14 1.37 2.17 1.24 0.79
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Fig. 6. The geochemical profile of Well Chen100 in the Xingouzui Formation in the Jianghan Basin.
Fig. 7. The developmental model of the argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin.
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Argillaceous dolomite was formed in shallow lake environment in the slope of the Chentuokou Sag, where the organic matter derived from both lower aquatic organisms and higher terrestrial plants was deposited. The lake productivity was lower, and the terrestrial organic matter originated primarily from the Jiangnan Uplift, which is to the south of the Jianghan Basin (Fig. 7). Although no exploratory wells are present, numerous source rocks containing abundant organic matter can form, and these rocks can generate both oil and gas. The source rocks in the central depression belt of the Chentuokou Sag developed in a semi-saline to hypersaline environment; these rocks have excellent hydrocarbon potential due to the better preservation conditions of organic matter. The argillaceous dolomite in this area formed in shallow to semi-deep lake environments; the organic matter originated primarily from lower aquatic organisms. Although the lake productivity was moderate in this area, as revealed by Well Chen100, organic matter could be preserved effectively. The source rocks developed in this area have better oil-generation potential. 5. Conclusions (1) High organic matter abundance of argillaceous dolomite in low mature to mature stage in Xingouzui Formation were formed both in the YajiaoeXingou Uplift and Chentuokou Sag of the Jianghan Basin. Organic matter type of argillaceous dolomite in the Chentuokou Sag is obviously better. (2) The ancient lake productivity in the Chentuokou Sag was higher than that in the YajiaoeXingou Uplift. The argillaceous dolomite formed in a continuous anoxic environment. (3) The anoxic environment is the major factor controlling the formation of argillaceous dolomite. Source rocks in the central depression belt of Chentuokou Sag have high organic matter abundance that tend to generate oil. Acknowledgments This study is financially supported by Key Program of National Natural Science Foundation (No: 41330313) and National Natural Science Foundation (No: 41172134) of China. References Bao, J.P., Ma, A.L., Li, X.Q., 2006. The Study of Immature and Low-mature Oil in Saltlake Basin. Geological Publishing House, Beijing (in Chinese). Boucsein, B., Stein, R., 2000. Particulate organic matter in surface sediments of the Laptev Sea (Arctic Ocean): application of maceral analysis as organic-carbonsource indicator. Mar. Geol. 162, 573e586. Calvert, S.E., Pedersen, T.F., 1992. Organic carbon accumulation and preservation in marine sediments: how important is anoxia? In: Whelan (Ed.), Productivity, Accumulation and Preservation of Organic Matter in Recent and Ancient Sediments. New York Columbia University Press, pp. 231e263. Chen, J.F., Zhang, S.C., Sun, S.L., Wu, Q.Y., 2006. Main factors influencing marine carbonate source rock formation. Acta Geol. Sin. 80 (3), 467e472 (in Chinese with English abstract). Cheng, K.M., Wang, Z.Y., Zhong, N.N., 1996. Theory and Practice of Hydrocarbon Generation. Petroleum Industry Press, Beijing (in Chinese). Dean, W.E., Gardner, J.V., Piper, D.Z., 1997. Inorganic geochemical indicators of glacial-interglacial changes in productivity and anoxia on the California continental margin. Geochim. Cosmochim. Acta 61 (21), 4507e4518. Demaison, G.J., Moore, G.T., 1980. Environments and oil source bed genesis. Org. Geochem. 2 (1), 9e31. Feng, Z.H., Huo, Q.L., Wang, X., Fang, W., Song, Z.G., 2009. Geochemical research on the Late Cretaceous strata of well SK1 in Songliao Basin. Earth Sci. Front. 16 (5), 181e191 (in Chinese with English abstract). Hao, F., Zhou, X.H., Zhu, Y.M., Yang, Y.Y., 2011. Lacustrine source rock deposition in response to co-evolution of environments and organisms controlled by tectonic subsidence and climate, Bohai Bay Basin, China. Org. Geochem. 42, 323e339. Hill, D.G., Lombardi, T.E., Martin, J.P., 2004. Fractured shale gas potential in New York. Northeast. Geol. Environ. Sci. 26 (1/2), 57e78. Hunt, J.M., 1996. Petroleum Geochemistry and Geology. Freeman, New York.
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Research paper
The formation environment and developmental models of argillaceous dolomite in the Xingouzui Formation, the Jianghan Basin Wenhao Li a, b, *, Shuangfang Lu a, Haitao Xue a, Pengfei Zhang b, Shiqiang Wu c a
Research Institute of Unconventional Petroleum and Renewable Energy, China University of Petroleum, Qingdao, 266580, China School of Geosciences, China University of Petroleum, Qingdao, 266580, China c Research Institute of Exploration and Development of Jianghan Oilfield, SINOPEC, Wuhan, 430000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 16 June 2015 Accepted 22 June 2015 Available online 30 June 2015
Shale oil resources are found in the argillaceous dolomite in the Xingouzui Formation of the Jianghan Basin, revealing a new field for petroleum exploration. Studying the environment in which argillaceous dolomite forms reveals details regarding the formation and distribution of shale oil in the basin. By combining a detailed analysis of the basic geological data from petroleum exploration with methods based on organic petrology and geochemistry, the formation environment and developmental models of argillaceous dolomite in the YajiaoeXingou Uplift and Chentuokou Sag are discussed. The results are as follows: the average TOC values in the argillaceous dolomite exceed 1.0% in the YajiaoeXingou Uplift and Chentuokou Sag, reaching better source rock standards. The type of organic matter of the argillaceous dolomite from the Chentuokou Sag is obviously better than that in the YajiaoeXingou Uplift. The maturity of organic matter in the source rocks is lower. Tricyclic terpanes, pregnane and homopregnane, which originated from algae, are detected in the argillaceous dolomite in the Chentuokou Sag, while these compounds are not found in YajiaoeXingou Uplift. In addition, the alginite and sporinite are respectively detected in the two areas described above. Therefore, the lake productivity is higher in the Chentuokou Sag relative to the YajiaoeXingou Uplift. The Pr/Ph, Fe2þ/Fe3þ and gammacerane/C30 hopane data reveal that argillaceous dolomite developed in an anoxic environment with better water column stratification. The anoxic environment is the major factor controlling the formation of argillaceous dolomite. The central belt of the Chentuokou Sag was where the lake productivity was higher and preservation conditions of organic matter were excellent, making it the most favorable zone for forming argillaceous dolomite that tends to generate oil. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Lake productivity Anoxic environment Argillaceous dolomite The Jianghan Basin
1. Introduction The successful development of a shale gas reservoir in North America has made shale gas the most important unconventional hydrocarbon resource, because it has great potential for replacing conventional hydrocarbon resources worldwide (Rogner, 1977; Hill et al., 2004; Perry and Lee, 2007). However, unconventional hydrocarbon exploration has focused on shale oil rather than shale gas in recent years in America, making the exploration of shale oil of great value. Shale, widely distributed in China, is the main field for shale oil exploration and development. Shale oil has recently been found in the dolomite mudstone and argillaceous dolomite in * Corresponding author. Research Institute of Unconventional Petroleum and Renewable Energy, China University of Petroleum, Qingdao, 266580, China. E-mail address: [email protected] (W. Li). http://dx.doi.org/10.1016/j.marpetgeo.2015.06.011 0264-8172/© 2015 Elsevier Ltd. All rights reserved.
the Jianghan, Junggar and Santanghu Basins (Liang et al., 2012; Luo et al., 2013; Zou et al., 2013a,b), revealing a new field for exploring and developing shale oil. After discussing the actual geological conditions in China, in combination with the formation conditions, occurrence state and accumulation mechanism of shale oil and gas, the most favorable zones have been predicted based on the research data, exploration and the development of shale oil and gas in North America (Zhang et al., 2003; Nie et al., 2009; Zou et al., 2010, 2013a,b; Zhao et al., 2011; Zhang, 2011; Lu et al., 2012). The amount of residual hydrocarbons in shale can be calculated easily based on the current method for quantitatively evaluating source rocks (Lu and Zhang, 2008). However, hydrocarbon contents of shale in diverse formations are significantly different because the mineral composition, organic matter abundance, organic matter types and expulsion efficiency of hydrocarbon are obviously affected by the environment during the formation of the shale.
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However, TOC values depend on the formation of source rocks, specifically, supply and preservation conditions of organic matter. Organic matter supply includes the lower hydrobiont organisms in lakes or oceans and terrigenous organic matter, but the former tends to generate crude oil. Preservation conditions of organic matter are related to dissolved oxygen content of water column. Organic matter can be consumed in oxic water column and conserved in an anoxic environment. Two hypotheses were proposed regarding whether high organic matter abundance of the sediments depended on the preservation conditions or production (Tyson, 1987; Tyson and Pearson, 1991; Calvert and Pedersen, 1992). Currently, most scholars believe that high TOC values of the source rocks can be attributed to several factors including lake or ocean productivity, preservation conditions of organic matter, sedimentation rate, etc.; the most important ones are productivity and anoxic environment (Dean et al., 1997; Boucsein and Stein, 2000; Stow et al., 2001; Chen et al., 2006; Zhang et al., 2007; Hao et al., 2011; Li et al., 2013). Hao et al. (2011) analyzed the lateral source rock heterogeneities to reveal the environmental and ecological changes and to construct depositional models for lacustrine source rocks under different tectonic and climatic conditions. Zou et al. (2012) proposed that the environment in which shale or carbonate rocks formed controlled the extent of the enrichment of shale oil and gas, while large-scale delta and lacustrine carbonate rocks enabled the formation and distribution of the reservoir. Li et al. (2014) revealed the main controlling factors and depositional models of the Miocene marine source rocks in the Qiongdongnan Basin, based on analysis of terrestrial plants supply, paleoproductivity and redox conditions. Yan et al. (2015) investigated the mechanism of organic matter accumulation in Early Silurian sediments using multiple geochemical proxies, including redox parameters, productivity indices and clastic influx indicator. Shale oil of lacustrine argillaceous dolomite in the Xingouzui Formation in the YajiaoeXingou Low Uplift has been successfully explored in the Jianghan Basin since 2012, marking a significant breakthrough. Concurrently, progress has been made in the Chentuokou and Mianyang Sags of the basin. However, the environment in which the argillaceous dolomite forms remains unclear, restricting the exploration of unconventional resources in the Jianghan Basin. Consequently, the environment surrounding the formation of argillaceous dolomite is discussed through the analysis of organic petrology and geochemistry while using the YajiaoeXingou Low Uplift and Chentuokou Sag as examples.
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2. Geological background The Jianghan Basin lies in the center of the Jianghan Plain, which is in the Hubei Province and spans 28,760 km2. The northern portion of the basin is adjacent to the QinlingeDabie Orogenic Belt, the southern part is near the JiangnaneXuefeng Uplift, and the western part borders the Huangling Anticline. The evolution of the basin developed through the Yanshan and Himalayan movements from Cretaceous to Quaternary. The basin experienced three stages including the early Yanshan extrusion, the CretaceousePaleogene rift depression and the NeogeneeQuaternary depression. The Jianghan Basin can be divided into 15 secondary tectonic units including 11 sags (Jiangling Sag, Qianjiang Sag, Chentuokou Sag, etc.) and four uplifts (YajiaoeXingou Uplift, etc.) (Fig. 1). The Jianghan Basin is a polycyclic sedimentary basin, and its cap rocks are Cretaceous and Cenozoic clastic rocks. The basement of the basin is composed of pre-Cretaceous marine carbonate, clastic and terrestrial coal-bearing clastic rocks. On top of these rocks, the following were deposited: the Yuyang Formation in Cretaceous, the Shashi, Xingouzui, Jingsha, Qianjiang and Jinghezhen Formations in Palaeogene, Neogene Guanghuasi Formation and Quaternary Pingyuan Formation. From Cretaceous to Quaternary, the periphery terrain of the basin was higher in the northwest and lower in the southeast; the provenance was primarily from northwest, exhibiting a maximum thickness of sedimentary strata above 10,000 m and characteristic cyclicity and rhythmicity. During the sedimentary period of the Xingouzui Formation, the paleoclimate was initially subtropical, becoming humid semiarid and wetter in the late period. The sedimentary environment included shallow to semi-deep water and contained well-developed dark mudstones as the major source rocks in the basin. During the sedimentary period of the lower Xingouzui Formation, provenance was mainly from north of the basin, which formed a large-scale delta progression toward the basin. The delta front was broadly extended. Another large-scale provenance was observed from the northeast. When the upper Xingouzui Formation was deposited, the provenance and depositional system were obviously different from those of the lower Xingouzui Formation. The provenance was obviously limited and was far from the depocenter. The deltaic deposits developed in some regions in the northwest of the basin. However, most areas of the basin were covered by the gypsum and mud deposits in shallow to brackish lakes. The Jianghan Basin contained structural, structuralelithological, lithologic and stratigraphic reservoirs before 2012. Three tons of
Fig. 1. The structural location of the Jianghan Basin.
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We chose 15 samples to analyze the relative or total content, as well as the constituents, of organic materials in the rocks. The measurements were performed on a fluorescent microscopeLABORLUX 12 POL microphotometer (MPV-3). The measurements were collected using a 50objective lens at a total amplification of 800. The temperature of the detection system is (23 ± 2) C. Twenty-six samples were chosen and prepared to analyze the trace and major elements using approximately 25 mg of a powdered and homogenized sample loaded in polytetrafluoroethylene (PTFE) vessels; these samples were heated to 190 C for 72 h in PTFE autoclaves with 1 mL of HF (40%) and 1 mL of HNO3 (65%). The acids were removed through evaporation at 200 C on hot plates, and the wet residues were dissolved and re-evaporated twice using 0.5 mL of HNO3 (65%). After the final HNO3 treatment, the wet residues were heated to 130 C for 12 h in PTFE autoclaves with 5 mL of HNO3 (32.5%). The wet residues were diluted to 30 mL with 2% HNO3. During the analyses, the acid digests were spiked with Rh and Re as internal standards. Chinese standard samples GB7107 (shale) and GB7114 (dolomite) were used to assess the accuracy of the analyses. What is more, the carbon and oxygen isotopes of the 26 samples are measured in State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum in Beijing. 4. Results and discussion 4.1. Analysis of hydrocarbon potential of argillaceous dolomite in the Xingouzui Formation
Fig. 2. The comprehensive stratigraphic column of the Xingouzui Formation.
shale oil resources have been obtained per day in the Xingouzui Formation since 2012, stimulating exploration for shale oil in the Jianghan Basin (Li et al., 2015; Shen et al., 2015). The discovery of shale oil in the argillaceous dolomite of the Xingouzui Formation provides new strata and new area of hydrocarbon exploration in the basin. The lithology of the Xingouizui Formation is mainly argillaceous dolomite. There also developed shales in the bottom of the Xingouizui Formation (Fig. 2). 3. Analytical samples and methods Forty-nine argillaceous dolomite samples in the Xingouzui Formation were collected in Wells Xin391 and Chen100, respectively, in the YajiaoeXingou Low Uplift and the Chentuokou Sag of the Jianghan Basin. The samples were ground in an agate mortar and pestle to for 200-mesh particles. Two-hundred grams were weighed from each sample and subsequently crushed to a grain size of 0.18 mm at 50 C in the laboratory. The powdered samples were analyzed for total organic carbon and Rock-Eval. Twelve typical samples were chosen for GCeMS analysis based on the results of the experiments above. The samples were crushed to 80 mesh and extracted in a Soxhlet apparatus with dichloromethane (CH2Cl2) for 24 h. The extracts were evaporated, deasphaltened with n-hexane, and separated into saturated, aromatic and polar fractions through column chromatography using silica gel and alumina (3:1) while eluting with n-hexane, dischloromethane:n-hexane (2:1, v/v) and chloroform:ethanol (1:1, v/v), respectively. The saturated and aromatic fractions were analyzed by gas chromatography-mass spectrometry (GCeMS). GCeMS analyses were performed on a Thermo-Finnigan TraceDSQ equipped with an HP-5MS (60 m 0.25 mm 0.25 mm) fused silica capillary column while operating in full scan mode.
Researchers proposed that source rocks with TOC values below 0.6% could become better source rock standards in the salt lake environments; the hydrocarbon conversion rate is rather high, and the hydrocarbon mass could be generated in a relatively lower maturity of organic matter in such environment (Wang et al., 1995; Sun et al., 1997; Zhang et al., 2000; Bao et al., 2006). Organic geochemical analysis of the argillaceous dolomite in the YajiaoeXingou Low Uplift and Chentuokou Sag reveals that the TOC values range from 0.20% to 4.59% and from 0.16% to 4.27% with averages of 1.32% and 1.04%, respectively, and the values of (S1 þ S2) range from 0.44 to 43.86 mg/g and 0.33e29.44 mg/g, averaging 6.72 and 5.83 mg/g, respectively in the above areas (Table 1). A positive correlation exists between the TOC and (S1 þ S2) values, and these two parameters exceed 0.6% and 2 mg/g in most of the samples, respectively (Fig. 3). Therefore, source rocks in the Xingouzui Formation have better hydrocarbon potential. Hydrogen Index (HI) values range from 63.32 to 915 mg/g and 76.92e895.48 mg/g respectively in the YajiaoeXingou Low Uplift and the Chentuokou Sag, respectively. The average HI values in the latter are much higher than that in the former, indicating that the type of organic matter in the source rocks from the Chentuokou Sag is superior. The average Tmax values of argillaceous dolomite in the areas above are 425.26 C and 433.88 C respectively (Table 1), indicating that source rocks in the two areas are in immature and mature stages, respectively. 4.2. The formation environment of argillaceous dolomite 4.2.1. Lake productivity Organic materials that primarily generate oil are detected in the argillaceous dolomite in the YajiaoeXingou Low Uplift and Chentuokou Sag. Sporinite (Fig. 4aec) and alginite (Fig. 4def) are found in the above areas, respectively. Sporinite belongs to exinite, which tends to generate oil. Sporinites, particularly microsporinite, can be circular, ovular and strip-shaped; the examples from Well Xin391 in the Xingou Low Uplift are yellow-kelly under the fluorescence
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Table 1 Distribution of some of the geochemical parameters of argillaceous dolomite in the Xingouzui Formation in the YajiaoeXingou Uplift and Chentuokou Sag of the Jianghan Basin. Structural belt
Depth, m
TOC, %
S1 þ S2, mg/g
IH, mg/g
Tmax, C
YajiaoeXingou low uplift
1380.15 1381.55 1382.70 1385.20 1385.80 1386.80 1388.30 1389.30 1392.65 1393.30 1394.10 1426.30 1427.80 1428.25 1442.85 1445.40 1460.45 1460.95 1461.95 1462.45 1464.00 1464.55 1464.90 2106.40 2107.43 2108.20 2109.10 2113.40 2115.75 2116.25 2117.03 2117.58 2118.65 2150.90 2152.10 2153.60 2154.75 2155.90 2156.85 2158.65 2159.35 2188.85 2191.05 2191.90 2192.60 2193.75 2194.95 2195.85 2196.90
0.67 1.47 0.62 1.47 0.65 0.84 0.91 0.20 3.56 0.61 1.03 0.94 1.08 1.03 0.34 0.57 4.59 0.67 0.48 1.02 0.76 3.97 2.84 0.51 0.50 0.58 0.68 0.64 0.34 1.06 0.75 4.27 0.56 0.95 0.43 0.16 0.53 1.93 3.10 1.20 0.52 0.94 2.80 1.09 1.33 1.22 0.45 0.45 0.17
0.79 6.73 0.76 6.45 1.06 0.81 1.67 0.44 33.83 1.07 1.98 3.88 2.22 1.90 0.69 0.66 43.86 0.87 1.34 2.14 1.63 19.13 20.74 0.85 0.70 4.11 1.08 0.75 0.51 2.81 5.03 18.20 1.12 6.56 2.17 0.41 0.95 11.51 29.44 7.46 2.18 2.87 27.73 4.00 12.48 3.99 2.21 2.06 0.33
83.83 408.84 92.38 404.08 128.83 63.32 120.61 153.85 557.02 109.48 148.54 318.71 166.67 145.63 154.30 78.67 915.03 90.23 220.13 145.10 142.29 140.81 659.86 119.14 91.27 288.16 92.65 76.92 97.92 215.09 212.28 220.14 108.54 274.45 329.44 163.52 120.98 560.10 895.48 464.17 302.68 192.55 892.50 282.57 681.20 190.16 215.73 213.81 129.63
413 428 415 433 421 425 425 488 428 423 426 417 421 420 428 397 432 415 429 426 425 422 424 430 426 428 427 428 428 434 429 433 421 429 432 434 431 436 437 437 436 425 438 438 437 420 416 426 473
Chentuokou Sag
microscope. Alginite is a sapropelinite, which has excellent hydrocarbon potential for oil generation. Vermicular scattered and laminated alginite are detected in the argillaceous dolomite in Well Chen100 in the Chentuokou Sag; these samples are yellow under the fluorescence microscope. The organic material data indicate that the ancient lake productivity in the Chentuokou Sag is higher than that in the YajiaoeXingou Uplift during the sedimentary period of the Xiugouzui Formation. Biomarkers are often used to discuss the origin of organic matter, which indicates lake productivity. Biomarker characteristics of the steranes and terpanes of the source rocks in the YajiaoeXingou Low Uplift and Chentuokou Sag show that the gammacerane content is relatively high, and the aaa20RC27, aaa20RC28 and aaa20RC29 regular steranes are distributed in “V” or approximate “V” shapes (Fig. 5). The aaa20RC27/aaa20RC29 ratios range from 0.54 to 2.25, averaging 1.06 (Table 2). Notably, the tricyclic terpanes, pregnane and homopregnane that originated from algae are detected in the core samples from Well Chen100; these
Fig. 3. A cross plot showing the relationship between TOC and S1 þ S2 values of argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin.
compounds are nearly absent in Well Xin391 (Fig. 5). The above biomarker characteristics indicate that organic matter in the source rocks includes lower hydrobiont and higher terrestrial plants in the study area. In addition, the ancient lake productivity in the Chentuokou Sag is low to medium, while that in the YajiaoeXingou Uplift is lower. The argillaceous dolomite in the YajiaoeXingou Uplift contains high organic matter abundance despite low lake productivity because large amounts of terrigenous organic matter were delivered by the Hanshui River, which is in the northern portion of the basin. However, organic matter and clastic materials were not delivered to the Chentuokou Sag due to the silt from the YajiaoeXingou Low Uplift; in this case, lake productivity was not diluted by large amounts of detrital materials. Consequently, source rocks containing high levels of organic matter can be formed in the Chentuokou Sag. 4.2.2. Redox environment Organic matter can be accumulated in anoxic water (Demaison and Moore, 1980; Hunt, 1996; Cheng et al., 1996; Ten et al., 2004; Li et al., 2012). The following parameters are used to discuss redox conditions in this paper. The Pr/Ph ratio in the samples varies from 0.38 to 1.07, averaging 0.58 (Table 2); these data indicate an anoxic environment. The Fe2þ/Fe3þ ratios range from 0.79 to 5.22 (only one sample generated a ratio below 1), averaging 2.07 (Table 3); these data also indicate an anoxic environment. In addition, the salinity is also important during source rock formation. An anoxic environment can be formed in the bottom water due to high salt content at the sedimentewater interface (Li, 1993; Katz, 1995). Gammacerane is commonly used to reveal the sedimentary environment of organic matter, and high gammacerane contents are related to highly saline environments and strati et al., 1995; Zhang et al., 1999; Wang fied water (Sinninghe Damste et al., 2002; Feng et al., 2009). A relatively high content of gammacerane is present in the argillaceous dolomite in the study area (Fig. 5). The gammacerane index (gammacerane/C30 hopane) varies from 0.13 to 1.22, averaging 0.49 (Table 2); these values indicate stratified water with a relatively high salinity, which tends to form anoxic water. The range of salinity levels is wide, while the Pr/Ph ratio remains steady, indicating that the body of water was continuously anoxic during the sedimentary period of the argillaceous dolomite in the Xingouzui Formation, which is supported by the high Fe2þ/Fe3þ values (Table 2).
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Fig. 4. Organic micrograph of argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin. Note: a. Well Xin391 (1393.3 m) microsporinite, yelloweyellow green fluorescence; b. Well Xin391 (1464 m) sporinite, yelloweyellow green fluorescence; c. Well Chen100 (2117 m) microsporinite, yelloweyellow green fluorescence; d. Well Chen100 (2155.9 m) scattered alginite, yellow fluorescence; e. Well Chen100 (2156.9 m) laminar alginite, yellow fluorescence; f. Well Chen100 (2191.05 m) laminar alginite, yellow fluorescence. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4.3. Main controlling factors of formation environment of argillaceous dolomite The TOC values of the argillaceous dolomite ranged from 0.16% to 3.10%, averaging 0.95%, in Well Chen100 (Fig. 6). Three sections (lower, middle and upper sections) are created to illustrate the formation environment of the argillaceous dolomite. The TOC values of the argillaceous dolomite in the lower section are moderate, averaging approximately 1%. The middle section exhibits values from 0.16% to 3.10%, averaging 1.33%. The values from the upper section are much lower, ranging from 0.50% to 1.06% and averaging 0.66%. Phosphorus is usually used to determine marine or lake productivity (Tyrrell, 1999). The percentage of phosphorus in the argillaceous dolomite is relatively high in the lower and upper sections; this value is low in the middle section. Therefore, the ancient lake productivity was relatively high during the sedimentary period of the low and upper sections of the Xingouzui Formation; the productivity is lower during the sedimentary period of the middle section. The average Fe2þ/Fe3þ ratio of the argillaceous dolomite exceeds 1 in three sections (Table 3, Fig. 6), revealing excellent preservation conditions of organic matter. The Fe2þ/Fe3þ ratios of the samples from the middle section are higher than in other two sections; therefore, the middle section had better preservation conditions of organic matter due to the strong anoxic environment during the sedimentary period of
the middle section of the Xingouzui Formation. The Mg/Ca ratios of the samples in the lower section are high; these values are lower in the middle and upper sections (Table 3, Fig. 6). The d18O values do not obviously vary, while the d13C values clearly change as these ratios increase (Fig. 6); therefore, the climate changed from warm and humid to relatively dry and cold during the sedimentary periods from the lower to upper sections. Therefore, the anoxic environment is the major factor controlling the argillaceous dolomite, followed by lake productivity. During the sedimentary period of the middle section of the Xingouzui Formation, the lake productivity in the Chentuokou Sag is relatively lower, but argillaceous dolomite with abundant organic matter can still form in strong anoxic environments. The degree of water reduction in the source rocks in the lower section is almost the same as that in the upper section. However, a warm and humid climate remains favorable for alga propagation. Therefore, the lake productivity is relatively high, making organic matter abundance of argillaceous dolomite higher than in the upper section. 4.4. The developmental model of argillaceous dolomite The argillaceous dolomite developed in a shallow lake environment in the YajiaoeXingou Uplift. Due to the influence of the Hanshui River, large-scale delta deposits were observed in the
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Fig. 5. Mass chromatograms of the steranes and terpanes in the argillaceous dolomite of the Xingouzui Formation in the Jianghan Basin.
Table 2 Distribution of some of the biomarker parameters of argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin.
Table 3 Distribution of some of the major elements and parameters of argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin.
Well
Depth (m)
Pr/Ph
Ga/C30H
C27/C29
Well
Depth (m) MgO (%) CaO (%) P2O5 (%) “P” (%) Mg/Ca Fe2þ/Fe3þ
Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100
1386.80 1393.30 1394.10 1428.25 1442.85 1462.45 2113.40 2116.25 2117.03 2191.05 2191.90 2195.85
0.63 0.38 0.63 0.40 0.58 0.58 1.07 0.62 0.44 0.53 0.61 0.51
0.41 0.33 0.28 0.13 0.19 1.22 0.39 0.20 0.60 0.67 1.11 0.35
0.54 0.94 1.17 0.57 1.07 2.25 2.14 1.06 0.61 0.61 0.65 1.06
Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Xin391 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100 Chen100
1380.15 1381.55 1385.80 1386.80 1393.30 1394.10 1428.25 1442.85 1445.40 1461.95 1462.45 1464.00 2106.40 2107.43 2108.20 2113.40 2116.25 2117.03 2118.65 2150.90 2153.60 2154.75 2155.90 2156.85 2188.85 2191.90
Note: Pr/Ph ¼ pristane/phytane; Ga/C30H ¼ gammacerane/C30 hopane; C27/ C29 ¼ C27/C29 steranes.
northern portion of the uplift, providing abundant terrigenous organic matter. However, lake productivity could be diluted by mass of detrital materials delivered by the river-delta system, as shown by the phosphorus content of the argillaceous dolomite in Well Xin391 (Table 3). The source rocks were developed in brackish to semi-saline environments with favorable preserving conditions of organic matter (Fig. 7) and contained abundant organic matter, which generates oil and gas.
3.18 7.37 7.56 3.35 4.16 4.15 2.18 3.53 3.35 13.95 3.21 3.87 1.79 5.15 13.20 4.34 4.29 13.04 8.62 18.94 12.57 4.80 5.90 7.71 5.17 4.45
2.72 12.41 10.82 2.80 5.29 2.10 0.57 8.72 3.49 22.62 2.83 4.13 6.81 6.20 27.82 1.10 3.94 22.53 11.53 28.87 25.09 9.96 10.23 12.66 2.96 2.19
0.13 0.13 0.22 0.10 0.12 0.17 0.09 0.16 0.10 0.12 0.13 0.16 0.13 0.15 0.11 0.30 0.15 0.15 0.19 0.07 0.15 0.10 0.15 0.14 0.32 0.16
0.05 0.06 0.10 0.04 0.05 0.07 0.04 0.07 0.04 0.05 0.06 0.07 0.05 0.06 0.05 0.13 0.07 0.07 0.08 0.03 0.06 0.04 0.06 0.06 0.14 0.07
0.99 0.50 0.59 1.01 0.66 1.67 3.22 0.34 0.81 0.52 0.96 0.79 0.22 0.70 0.40 3.33 0.92 0.49 0.63 0.55 0.42 0.41 0.49 0.51 1.47 1.71
2.24 3.22 3.17 1.08 1.96 1.67 1.31 1.80 1.47 2.83 2.04 2.35 1.56 1.10 5.22 1.09 1.56 2.16 1.10 2.94 3.32 3.14 1.37 2.17 1.24 0.79
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Fig. 6. The geochemical profile of Well Chen100 in the Xingouzui Formation in the Jianghan Basin.
Fig. 7. The developmental model of the argillaceous dolomite in the Xingouzui Formation in the Jianghan Basin.
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Argillaceous dolomite was formed in shallow lake environment in the slope of the Chentuokou Sag, where the organic matter derived from both lower aquatic organisms and higher terrestrial plants was deposited. The lake productivity was lower, and the terrestrial organic matter originated primarily from the Jiangnan Uplift, which is to the south of the Jianghan Basin (Fig. 7). Although no exploratory wells are present, numerous source rocks containing abundant organic matter can form, and these rocks can generate both oil and gas. The source rocks in the central depression belt of the Chentuokou Sag developed in a semi-saline to hypersaline environment; these rocks have excellent hydrocarbon potential due to the better preservation conditions of organic matter. The argillaceous dolomite in this area formed in shallow to semi-deep lake environments; the organic matter originated primarily from lower aquatic organisms. Although the lake productivity was moderate in this area, as revealed by Well Chen100, organic matter could be preserved effectively. The source rocks developed in this area have better oil-generation potential. 5. Conclusions (1) High organic matter abundance of argillaceous dolomite in low mature to mature stage in Xingouzui Formation were formed both in the YajiaoeXingou Uplift and Chentuokou Sag of the Jianghan Basin. Organic matter type of argillaceous dolomite in the Chentuokou Sag is obviously better. (2) The ancient lake productivity in the Chentuokou Sag was higher than that in the YajiaoeXingou Uplift. The argillaceous dolomite formed in a continuous anoxic environment. (3) The anoxic environment is the major factor controlling the formation of argillaceous dolomite. Source rocks in the central depression belt of Chentuokou Sag have high organic matter abundance that tend to generate oil. Acknowledgments This study is financially supported by Key Program of National Natural Science Foundation (No: 41330313) and National Natural Science Foundation (No: 41172134) of China. References Bao, J.P., Ma, A.L., Li, X.Q., 2006. The Study of Immature and Low-mature Oil in Saltlake Basin. Geological Publishing House, Beijing (in Chinese). Boucsein, B., Stein, R., 2000. Particulate organic matter in surface sediments of the Laptev Sea (Arctic Ocean): application of maceral analysis as organic-carbonsource indicator. Mar. Geol. 162, 573e586. Calvert, S.E., Pedersen, T.F., 1992. Organic carbon accumulation and preservation in marine sediments: how important is anoxia? In: Whelan (Ed.), Productivity, Accumulation and Preservation of Organic Matter in Recent and Ancient Sediments. New York Columbia University Press, pp. 231e263. Chen, J.F., Zhang, S.C., Sun, S.L., Wu, Q.Y., 2006. Main factors influencing marine carbonate source rock formation. Acta Geol. Sin. 80 (3), 467e472 (in Chinese with English abstract). Cheng, K.M., Wang, Z.Y., Zhong, N.N., 1996. Theory and Practice of Hydrocarbon Generation. Petroleum Industry Press, Beijing (in Chinese). Dean, W.E., Gardner, J.V., Piper, D.Z., 1997. Inorganic geochemical indicators of glacial-interglacial changes in productivity and anoxia on the California continental margin. Geochim. Cosmochim. Acta 61 (21), 4507e4518. Demaison, G.J., Moore, G.T., 1980. Environments and oil source bed genesis. Org. Geochem. 2 (1), 9e31. Feng, Z.H., Huo, Q.L., Wang, X., Fang, W., Song, Z.G., 2009. Geochemical research on the Late Cretaceous strata of well SK1 in Songliao Basin. Earth Sci. Front. 16 (5), 181e191 (in Chinese with English abstract). Hao, F., Zhou, X.H., Zhu, Y.M., Yang, Y.Y., 2011. Lacustrine source rock deposition in response to co-evolution of environments and organisms controlled by tectonic subsidence and climate, Bohai Bay Basin, China. Org. Geochem. 42, 323e339. Hill, D.G., Lombardi, T.E., Martin, J.P., 2004. Fractured shale gas potential in New York. Northeast. Geol. Environ. Sci. 26 (1/2), 57e78. Hunt, J.M., 1996. Petroleum Geochemistry and Geology. Freeman, New York.
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