Lower Paleozoic Source Rocks Research Articles

Genetic type and maturity of Lower Paleozoic marine hydrocarbon gases in the eastern Tarim Basin

Hydrocarbon compositions and their carbon isotopic data for 94 natural gas samples were combined with the results of thermal simulation experiments to complete a comprehensive study of maturity, genetic types and sources of Lower Paleozoic marine hydrocarbon gases from the eastern Tarim Basin. All of the Lower Paleozoic gases are relatively dry (dryness coefficient >0.9) and belong to the marine sapropelic type of cracked gases. These gases are characterized by low δ 13 C 1 and low Δ(δ 13 C 2−δ 13 C 1) values due to a lack of terrestrial source input. Thermal simulation experiments on Upper Proterozoic to Lower Paleozoic source rocks resulted in a δ 13 C 1–R o regression equation that was used to discriminate the maturities and sources of the natural gases. These natural gases are divided into four genetic types: (1) humic, coal-derived pyrolytic gas; (2) lacustrine sapropelic oil-cracking gas; (3) the marine sapropelic, highly mature oil-cracking gas; and (4) overmature oil-cracking gas. Types (1) and (2) were mainly generated from the Upper Triassic-Middle and Lower Jurassic; types (3) and (4) are mainly generated from the Lower Paleozoic. The data can be used to describe multiple sources and multiple generating stages of natural gas.

Geochemical characteristics of type IV kerogen from lower paleozoic source rocks in lower Yangtze area

There is still some hydrocarbon potential in organic matter from the Lower Paleozoic source rocks in the Lower Yangtze area. The results obtained by elemental analysis of kerogen from this area were plotted on a Van Krevelen diagram. Comparing them with those of Tissot, we can see that most values are within the boundary of the Type IV kerogen of Tissot's division. But based on the result of biomarker analysis, its source material should be of Type I. Therefore, the author further analyzed its geologic setting and thermal history. It was discovered that although the key factors controlling the thermal evolution are temperature and time, geologic setting is by not means an ignorable factor. The higher O/C ratio in the remnant kerogen from this area is mainly due to the strata being uplifted later, which caused the organic matter to be transformed into anthroxolite and shale bitumen.