Ping’an Peng

Overpressure retardation of organic-matter maturation: a kinetic model and its application

Abstract A kinetic model, T–P–Ro, has been developed to address the issue that overpressure increases the activation energies during organic-matter maturation and that the retardation of organic-matter maturation by overpressure is due to the fact that organic-matter is hindered from progressive molecular ordering. Under overpressured conditions, the mean effective stress (i.e. mean stress minus pore pressure) is much lower and the pore pressure is usually higher than under hydrostatic pressure conditions. As a result, the progressive molecular ordering of the vitrinite, such as the aromatization, is retarded, and the vitrinite reflectance can hardly be increased. The reflectance values predicted by T–P–Ro model as it has been applied to the Well LD3011 at Yinggehai Basin in South China Sea, are similar to the measured ones. The kinetic model predicts that overpressure can cause the oil window floor and the peak oil generation to shift to greater burial depth. In the case of Yinggehai Basin, the local geological conditions are prone to generate light hydrocarbons and natural gas (instead of oil), hence they constitute the predominant products in this high temperature (heating rate) basin.

EasyDelta: A spreadsheet for kinetic modeling of the stable carbon isotope composition of natural gases

A new kinetic model and an Excel^(C) spreadsheet program for modeling the stable carbon isotope composition of natural gases is provided in this paper. The model and spreadsheet could be used to describe and predict the variances in stable carbon isotope of natural gases under both experimental and geological conditions with heating temperature or geological time. It is a user-friendly convenient tool for the modeling of isotope variation with time under experimental and geological conditions. The spreadsheet, based on experimental data, requires the input of the kinetic parameters of gaseous hydrocarbons generation. Some assumptions are made in this model: (1)the conventional (non-isotope species) kinetic parameters represent the light isotope species; (2)the initial isotopic value is the same for all parallel chemical reaction of gaseous hydrocarbons generation for simplicity, (3)the re-exponential factor ratio, ^1^3A/^1^2A, is a constant, and (4)both heavy and light isotope species have similar activation energy distribution. These assumptions are common in modeling of isotope ratios. The spreadsheet is used for searching the best kinetic parameters of the heavy isotope species to reach the minimum errors compared with experimental data, and then extrapolating isotopic changes to the thermal history of sedimentary basins. A short calculation example on the variation in @d^1^3C values of methane is provided in this paper to show application to geological conditions. ple on the variation in @d^1^3C values of methane is provided in this paper to show application to geological conditions.

The Cenomanian-Turonian anoxic event in southern Tibet: A study of organic geochemistry

The Cenomanian-Turonian oceanic anoxic event (C/T OAE) is developed in southern Tibet. Organic geochemical study of the Cenomanian-Turonian sediments from the Gamba and Tingri areas shows that the mid-Cretaceous black shales in southern Tibet are enriched in organic carbon. The molecular analyses of organic matter indicate marine organic matter was derived from algae and bacteria. In the Gamba area, the organic matter is characterized by abundant tricyclic terpanes and pregane, which are predominant in 191 and 217 mass chromatograms, respectively. Pristane/phytane (Pr/Ph) ratios in the C/T OAE sediments are less than 1, demonstrating the domination of phytane. The presence of carotane can be regarded as a special biomarker indicating oxygen depletion in the C/T OAE sediments in the Tethyan Himalayas. In anoxic sediments, β-carotane and γ-carotane are very abundant. The β- and γ-carotane ratios relative to nC17 in the Cenomanian-Turonian anoxic sediments vary from 32.28 ∼42.87 and 5.10∼11.01.

Composition of Closed-System Fischer-Tropsch Synthesis Gases and the Constraint Factors

Biogenic gases were reported to exhibit a ‘‘normal’’ carbon isotope order, whereas abiogenic gaseous hydrocarbons in igneous rocks and meteorites exhibit a reversed order. In this study, closed-system Fischer–Tropsch synthesis was conducted at 350°C and 380°C under 30 MPa and 390°C under 30, 100, and 200 MPa, respectively, with magnetite as a catalyst. The results do not show an expected reversed order, even though the features of partially reversed order in carbon isotope values can still be commonly observed. Studies have shown that the essential constraint factor on the carbon isotopic pattern of abiogenic gases is the molar ratio of H2/CO2.