“…1). The Lucaogou Formation in the Jimusar Sag was mainly composed of fine-grained sediments with various mineral compositions, including siliciclastic, carbonate, and volcaniclastic rocks (Jiang et al 2015;Xi et al 2015;Qiu et al 2016).…”
Section: Geological Settingmentioning
confidence: 99%
“…This relationship indicated that sufficient nutrient elements promoted algal blooms, which enabled a high primary productivity. The high content of the nutrient materials may also be related to volcanic activity within the basin or its peripheral (Jiang et al 2015;Zhang et al 2009). Vertically, the change in P and Cu content was similar to that of the TOC.…”
Section: Influence Of Paleoproductivitymentioning
confidence: 99%
“…However, understanding of the Lucaogou Formation has been very limited, as has any exploration efforts. While significant studies have been conducted to identify the key lithological features, reservoir properties, source rock, and the hydrocarbon generation potential (Kuang et al 2013;Wang et al 2014;Jiang et al 2015;Shao et al 2015;Xi et al 2015;Gao et al 2016;Zha et al 2017;Yang et al 2017), few studies have focused on the paleoenvironment, the paleoproductivity, or the other factors controlling the accumulation of organic matter. In this work, detailed core observations were made, samples were collected from the common lithologies observed in sedimentary cycles present in the formation, the organic-rich rocks were characterized, and the factors controlling the deposition in brackish, lacustrine environments with a high frequency of water level fluctuation were identified.…”
The Lucaogou Formation in the Jimusar Sag of the eastern Junggar Basin is an important sedimentary stratum accumulating huge amounts of lacustrine tight oil in China, where organic-rich rocks are commonly observed. Focusing on the Lucaogou Formation, a precise analysis of the inorganic and organic petrology and the inorganic geochemistry characteristics was conducted. The paleoclimate and paleoenvironment during sedimentation of the Lucaogou Formation were established, and the key factors that were controlling the accumulation of organic matter during this time were identified. The results of this study suggest that during the sedimentation of the Lucaogou Formation, the paleoclimate periodically changed from a humid environment to an arid environment. As a result, the salinity of the water and the redox environment fluctuated. During the sedimentation period, the lake showed sufficient nutrient supplies and a high primary productivity. The interval studies in the Lucaogou Formation were divided into five sedimentary cycles, where the first, second, and fifth sedimentary cycles consisted of cyclical paleoclimate fluctuations varied from a humid environment to an arid environment and shifted back to a humid environment with levels of salinity from low to high and decreased again. The third and fourth cycles have cyclical fluctuations from a humid to an arid environment and corresponding salinity variation between low and high levels.During the period when organic-rich rocks in the Lucaogou Formation deposited in the Jimusar Sag, the paleoclimate and the water body were suitable for lower aquatic organisms to flourish. As a result, its paleoproductivity was high, especially during the early period of each cycle. A quiet deep water body is likely to form an anoxic environment at the bottom and is also good for accumulation and preservation of organisms. Fine-grained sediments were accumulated at a low deposition rate, with a low dilution of organic matter. Therefore, high paleoproductivity provided a sufficient volume of organisms in the studied area in a quiet deep water body with an anoxic environment and these were the key factors controlling formation of organic-rich rocks.
“…1). The Lucaogou Formation in the Jimusar Sag was mainly composed of fine-grained sediments with various mineral compositions, including siliciclastic, carbonate, and volcaniclastic rocks (Jiang et al 2015;Xi et al 2015;Qiu et al 2016).…”
Section: Geological Settingmentioning
confidence: 99%
“…This relationship indicated that sufficient nutrient elements promoted algal blooms, which enabled a high primary productivity. The high content of the nutrient materials may also be related to volcanic activity within the basin or its peripheral (Jiang et al 2015;Zhang et al 2009). Vertically, the change in P and Cu content was similar to that of the TOC.…”
Section: Influence Of Paleoproductivitymentioning
confidence: 99%
“…However, understanding of the Lucaogou Formation has been very limited, as has any exploration efforts. While significant studies have been conducted to identify the key lithological features, reservoir properties, source rock, and the hydrocarbon generation potential (Kuang et al 2013;Wang et al 2014;Jiang et al 2015;Shao et al 2015;Xi et al 2015;Gao et al 2016;Zha et al 2017;Yang et al 2017), few studies have focused on the paleoenvironment, the paleoproductivity, or the other factors controlling the accumulation of organic matter. In this work, detailed core observations were made, samples were collected from the common lithologies observed in sedimentary cycles present in the formation, the organic-rich rocks were characterized, and the factors controlling the deposition in brackish, lacustrine environments with a high frequency of water level fluctuation were identified.…”
The Lucaogou Formation in the Jimusar Sag of the eastern Junggar Basin is an important sedimentary stratum accumulating huge amounts of lacustrine tight oil in China, where organic-rich rocks are commonly observed. Focusing on the Lucaogou Formation, a precise analysis of the inorganic and organic petrology and the inorganic geochemistry characteristics was conducted. The paleoclimate and paleoenvironment during sedimentation of the Lucaogou Formation were established, and the key factors that were controlling the accumulation of organic matter during this time were identified. The results of this study suggest that during the sedimentation of the Lucaogou Formation, the paleoclimate periodically changed from a humid environment to an arid environment. As a result, the salinity of the water and the redox environment fluctuated. During the sedimentation period, the lake showed sufficient nutrient supplies and a high primary productivity. The interval studies in the Lucaogou Formation were divided into five sedimentary cycles, where the first, second, and fifth sedimentary cycles consisted of cyclical paleoclimate fluctuations varied from a humid environment to an arid environment and shifted back to a humid environment with levels of salinity from low to high and decreased again. The third and fourth cycles have cyclical fluctuations from a humid to an arid environment and corresponding salinity variation between low and high levels.During the period when organic-rich rocks in the Lucaogou Formation deposited in the Jimusar Sag, the paleoclimate and the water body were suitable for lower aquatic organisms to flourish. As a result, its paleoproductivity was high, especially during the early period of each cycle. A quiet deep water body is likely to form an anoxic environment at the bottom and is also good for accumulation and preservation of organisms. Fine-grained sediments were accumulated at a low deposition rate, with a low dilution of organic matter. Therefore, high paleoproductivity provided a sufficient volume of organisms in the studied area in a quiet deep water body with an anoxic environment and these were the key factors controlling formation of organic-rich rocks.
“…The tectonic activity in the inside of the depression is relatively weak (Zhao et al 1994 ; Zou et al 2014 ), and body exploration site is gentle with 3°–5° structural dip. Recently, commercial tight oil flow has been obtained from many oil wells, and the sag has been the focus of tight oil exploration and development in China (Wang et al 2015 ; Jiang et al 2015 ; Cao et al 2016b , 2017 ). Fig.…”
Section: Geological Settingmentioning
confidence: 99%
“…LCG is the main target layer for tight oil exploration, which is mainly deposited in lacustrine environment (Kuang et al 2013a , b ; Zhang et al 2003 ; Peng et al 2011 ). LCG can be further subdivided into the lower Lucaogou Member (P 2 l 1 ) and the upper Lucaogou Member (P 2 l 2 ) (Jiang et al 2015 ). Tight oil is mainly distributed in the sweet spots of P 2 l 1 and P 2 l 2 , which is shown in Fig.…”
With the constant consumption of conventional oil and gas resources, unconventional oil and gas resources with great resource potential such as tight oil have gradually been valued and become the new exploration area. Jimsar Sag is the key tight oil exploration and development block in Junggar Basin of Northwestern China. Based on the data sets of geology, oil production test, logging, rock thin section, and geochemistry of Permian Lucaogou Formation (LCG), we studied the geochemical characteristics of hydrocarbon source rocks and their relation to the tight oil accumulation. Organic matter abundance of source rocks is high, the types of organic matter are mainly type I and type II, and the organic matter maturation is in the low mature stage to mature stage. Results of oil source correlation showed that the crude oil of sweet spots was mainly derived from the source rocks in the interior of the sweet spots. The LCG tight oil is mainly distributed in the plane where the source rocks have great thickness and the TOC is higher than 3.5%. It shows that the source rocks have obvious controlling on the occurrence and accumulation of tight oil.
The eastern Junggar Basin, controlled by continental extension (rift), was deposited by lacustrine dominated sediments during middle Permian Lucaogou period. An unusual porphyritic-like texture was observed in Lucaogou/Pingdiquan dark fine-grained organic-rich sediments in two sub-tectonic units in the basin. The “phenocrysts” are composed of two types of mineral assemblages. The first is a coarse euhedral calcite assemblage in the Jimusar Sag, and the second consists of dolomite, analcime, and pyrite in the Shishugou Sag. The lithological and mineralogical features indicate a hydrothermal origin for these phenocryst-like minerals. The chondrite-normalized rare earth element patterns show flat or positive Ce anomalies and negative Eu anomalies, which reflect a suboxic to anoxic, off-axis site from the center of the fault system, where the temperature of the hydrothermal fluid might be less than 250 °C. The high ratios of BaN/LaN at 1.6–65.5, strongly positive Sr anomalies at Sr/Sr* = 5.54–39.9, and relatively low 87Sr/86Sr isotopes at 0.705002–0.705776 in the coarse calcite suggest an origin of mixed sources of lake water, underlying biogenetic sediments, and deep magmatic water. However, the low 87Sr/ 86Sr ratios of 0.705321–0.705968 in the dolomite and δ34SV-CDT of 10.8‰–12.3‰ in the pyrite indicate that water–underlying-rock interaction and the abiotic thermochemical sulfate reduction of lake water or organic matter might have participated together resulting in the precipitation of the dolomite–analcime–pyrite assemblages in the Shishugou Sag.
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