scholarly journals Organic Matter Accumulation Mechanism in the Lower Cambrian Strata from Well Luntan 1 in the Tarim Basin, NW China

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-13
Author(s):  
Qian Deng ◽  
Haizu Zhang ◽  
Haozhe Wang ◽  
Zhiwei Wei ◽  
Bin Cheng ◽  
...  
Keyword(s):  
Trace Elements ◽  
Organic Matter ◽  
Tarim Basin ◽  
Enrichment Factors ◽  
Major And Trace Elements ◽  
Source Rocks ◽  
Drilling Depth ◽  
Nw China ◽  
High Productivity ◽  
Organic Matter Accumulation

A large amount of light crude oils have been found within 4000 to 7000 m deep strata of Cambrian, Ordovician, and Silurian reservoirs in the Tarim Basin, NW China. To enhance the understanding of parental materials of deep oils, a set of high-quality source rocks from the Yuertusi Formation in well Luntan 1 (maximum drilling depth of 8882 m) was studied in terms of their sedimentary condition and mechanism of organic matter enrichment. Total organic carbon (TOC) content, carbon isotope of kerogen (δ13Cker), and major and trace elements of the rocks from the Sinian Qigebulake, Cambrian Yuertusi, and Xiaoerbulake Formations in well Luntan 1 were analysed. The results showed that the δ13Cker value of the Yuertusi Formation barely changed with an average of -31.19‰. High TOC contents accompanied by enrichments of the bioessential trace elements, such as cadmium, chromium, copper, nickel, and zinc, occurred in the lower part of the Yuertusi Formation. Excess barium (Baxs) and phosphorus concentrations revealed high primary productivity during the deposition of the Yuertusi Formation. Moreover, variations in the enrichment factors of molybdenum, uranium, and vanadium and molybdenum-uranium covariation pattern indicated suboxic-anoxic conditions in the Qigebulake Formation, anoxic-euxinic conditions in the Yuertusi Formation, and suboxic-oxic conditions in the Xiaoerbulake Formation. The TOC contents were significantly correlated with the paleoproductivity and paleoredox parameters, indicating that high productivity and reducing conditions jointly controlled the organic matter accumulation and preservation in well Luntan 1.

scholarly journals The influences of sedimentary environments on organic matter enrichment in fine-grained rocks of the Paleogene Shahejie formation in Nanpu Sag, Huanghua Depression, Bohai Bay Basin

2021 ◽  
pp. 014459872110310
Author(s):  
Min Li ◽  
Xiongqi Pang ◽  
Guoyong Liu ◽  
Di Chen ◽  
Lingjian Meng ◽  
...  
Keyword(s):  
Organic Matter ◽  
Source Rocks ◽  
Bohai Bay ◽  
Bohai Bay Basin ◽  
Humid Climate ◽  
High Productivity ◽  
Fine Grained ◽  
Huanghua Depression ◽  
Shahejie Formation ◽  
Nanpu Sag

The fine-grained rocks in the Paleogene Shahejie Formation in Nanpu Sag, Huanghua Depression, Bohai Bay Basin, are extremely important source rocks. These Paleogene rocks are mainly subdivided into organic-rich black shale and gray mudstone. The average total organic carbon contents of the shale and mudstone are 11.5 wt.% and 8.4 wt.%, respectively. The average hydrocarbon (HC)-generating potentials (which is equal to the sum of free hydrocarbons (S1) and potential hydrocarbons (S2)) of the shale and mudstone are 39.3 mg HC/g rock and 28.5 mg HC/g rock, respectively, with mean vitrinite reflectance values of 0.82% and 0.81%, respectively. The higher abundance of organic matter in the shale than in the mudstone is due mainly to paleoenvironmental differences. The chemical index of alteration values and Na/Al ratios reveal a warm and humid climate during shale deposition and a cold and dry climate during mudstone deposition. The biologically derived Ba and Ba/Al ratios indicate high productivity in both the shale and mudstone, with relatively low productivity in the shale. The shale formed in fresh to brackish water, whereas the mudstone was deposited in fresh water, with the former having a higher salinity. Compared with the shale, the mudstone underwent higher detrital input, exhibiting higher Si/Al and Ti/Al ratios. Shale deposition was more dysoxic than mudstone deposition. The organic matter enrichment of the shale sediments was controlled mainly by reducing conditions followed by moderate-to-high productivity, which was promoted by a warm and humid climate and salinity stratification. The organic matter enrichment of the mudstone was less than that of the shale and was controlled by relatively oxic conditions.

scholarly journals Paleoenvironment of the Lower–Middle Cambrian Evaporite Series in the Tarim Basin and Its Impact on the Organic Matter Enrichment of Shallow Water Source Rocks

Minerals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 659
Author(s):  
Mingyang Wei ◽  
Zhidong Bao ◽  
Axel Munnecke ◽  
Wei Liu ◽  
G. William M. Harrison ◽  
...  
Keyword(s):  
Organic Matter ◽  
Shallow Water ◽  
Tarim Basin ◽  
Water Environment ◽  
Water Source ◽  
Major Elements ◽  
Source Rocks ◽  
Middle Cambrian ◽  
Sedimentary Environments ◽  
The Impact

Just as in deep-water sedimentary environments, productive source rocks can be developed in an evaporitic platform, where claystones are interbedded with evaporites and carbonates. However, the impact of the paleoenvironment on the organic matter enrichment of shallow water source rocks in an evaporite series has not been well explored. In this study, two wells in the central uplift of the Tarim Basin were systematically sampled and analyzed for a basic geochemical study, including major elements, trace elements, and total organic carbon (TOC), to understand the relationship between TOC and the paleoenvironmental parameters, such as paleosalinity, redox, paleoclimate, paleo-seawater depth, and paleoproductivity. The results show that the Lower–Middle Cambrian mainly developed in a fluctuating salinity, weak anoxic to anoxic, continuous dry and hot, and proper shallow water environment. The interfingering section of evaporites, carbonates, and claystones of the Awatag Fm. have higher paleoproductivity and higher enrichment of organic matter. Paleosalinity, redox, paleoclimate, paleo-seawater depth, and paleoproductivity jointly control the organic matter enrichment of shallow water source rocks in the evaporite series. The degree of enrichment of organic matter in shallow water source rocks first increases and then decreases with the increase in paleosalinity. All the samples with high content of organic matter come from the shallower environment of the Awatag Fm.

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