Paleoenvironment and Organic Matter Enrichment of the Middle Ordovician Marine Carbonates in the Ordos Basin of China: Evidence from Element Geochemistry

Abstract

The Middle Ordovician Majiagou marine carbonates in the Ordos Basin, China, are important natural gas exploration targets and potential gas source rocks. However, the hydrocarbon generation potential of marine carbonates is difficult to evaluate due to their old ages, high thermal maturities, and low total organic carbon (TOC) contents. The enrichments of organic matter deposited in the sedimentary period are of significance to the subsequent hydrocarbon generation process. Integrated petrology and geochemistry studies, including mineral and maceral contents, major and trace element contents, TOC, and organic carbon isotope signatures, were carried out on the Majiagou Formation. The detrital input, paleoproductivity, and preservation conditions of organic matter, which are essential control factors for the enrichment of organic matter, were discussed in detail. The results showed that sea level changes regulated the conditions for the enrichment of organic matter in the Majiagou sedimentary period. The third member of the Majiagou Formation (O2m3) resulted in high paleoproductivity (high Ba/Al, Cu/Al, and TOC values), terrigenous input (high Ti/Tiavergae shale and TCP/TOC values), and an anoxic (V/(V + Ni) > 0.6) and salty environment (high anhydrite contents) for the preservation of organic matter. Conversely, there is little organic matter input in the fourth member of the Majiagou Formation (O2m4) under the transgressive environment. Gypsum not only plays a potentially important role in the blooming of special organisms (halophilic archaea) but is also beneficial for the preservation of organic matter.

Paleoenvironmental Reconstruction and Organic Matter Accumulation of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin, South China

“…Copper (Cu) is an important nutrient for the biota as it can be incorporated into organic complexes and form essential metalloenzymes . The commonly observed correlation between Cu abundance and the intensity of plankton productivity in modern and ancient environments made it possible to assess the paleoproductivity. − Therefore, it can be used to indicate primary productivity through being normalized to Al for the removal of the influence of terrestrial materials. , The Cu/Al ratios mainly exceed 10 × 10 –4 in the LQS and are between 2 × 10 –4 and 10 × 10 –4 in the MQS–UQS, suggesting a higher primary productivity in the LQS (Figure B,D). In the MQS, the Cu/Al ratios in the north (5–9 × 10 –4 ) are relatively higher than in the south (2–6 × 10 –4 ), indicating that the northern basin had a higher productivity.…”

Paleoenvironment, Provenance, and Hydrocarbon Potential of Lower Permian Coal-Bearing Source Rocks in the Southern North China Basin: A Case Study of the Pingdingshan Coalfield

“…The Permian source rocks in the southern North China Basin mainly developed in a coal-forming environment with abundant terrestrial organic matter. − The high abundance of organic matter provided a necessary foundation for gas generation in source rocks . However, the accumulation of organic matter always depends on the paleoenvironment and paleoclimate in the sedimentary period . The anoxic water environment and stable provenance are favorable for the preservation of organic matter. , Compared with the marine shale, the organic matter in transitional source rocks is possibly derived from both marine and terrestrial inputs. , This difference in source inputs also indirectly leads to the uniqueness of transitional gas generation, accumulation, and storage.…”

“…19 However, the accumulation of organic matter always depends on the paleoenvironment and paleoclimate in the sedimentary period. 20 The anoxic water environment and stable provenance are favorable for the preservation of organic matter. 12,21 Compared with the marine shale, the organic matter in transitional source rocks is possibly derived from both marine and terrestrial inputs.…”

Characteristics of the Tectonic Background and Sedimentary Paleoenvironment of Coal-Bearing Strata from the Yan’an Formation in the Ordos Basin: Evidence from the Geochemistry of Major and Trace Elements

“…The Ordos Basin is the second largest sedimentary basin in China, and the Paleozoic stratum area is about 25 × 10 4 km 2 , which is located above the North China Craton. , It is a multicycle superimposed basin with multiple energy sources, such as oil, natural gas, coal, and uranium. , The oil and gas resources in the basin are mainly distributed in the Triassic Yanchang Formation and clastic rocks of the Fuxian and Yan’an Formation of Lower and Middle Jurassic. , Early Jurassic, the basin experienced Fuxian Formation deposition, and the depression continued to develop and form stable subsidence. During the deposition of the Yan’an Formation in the early Middle Jurassic, wide distribution and large thickness of multilayer coal seams were formed .…”

“…1,2 It is a multicycle superimposed basin with multiple energy sources, such as oil, natural gas, coal, and uranium. 3,4 The oil and gas resources in the basin are mainly distributed in the Triassic Yanchang Formation and clastic rocks of the Fuxian and Yan'an Formation of Lower and Middle Jurassic. 5,6 Early Jurassic, the basin experienced Fuxian Formation deposition, and the depression continued to develop and form stable subsidence.…”