Liuping Zhang

A preliminary study of mantle-derived fluids and their effects on oil/gas generation in sedimentary basins

Abstract This paper presents the geochemical features of mantle-derived fluids and their activities based on previous studies. To further reveal the features of the fluids and their effects on oil/gas generation in basins, we have investigated mantle-derived fluids in the Dongying Depression of the Bohai Bay Basin by means of isotopic geochemistry, organic geochemistry and thermodynamics. We found that two types of mantle-derived fluids, those rich in CO 2 (H 2 O+CO 2 +CH 4 ) and those rich in H (H 2 O+CH 4 +H 2 ), were injected into the Dongying Depression. Quantitative studies of thermal energy transfer by mantle-derived fluids ascendance show that the mantle-derived fluids are an effective heat carrier. The thermal anomalies in the Dongying Depression demonstrate the existence of thermal effects of mantle-derived fluids. It is usually suggested that hydrocarbon generation from kerogen degradation not only needs thermal energy but also requires hydrogen. Our experimental simulations show that kerogen hydrogenation can increase hydrocarbon generation greatly. For the sapropelic kerogen, the effect of hydrogenation becomes highly evident after the peak hydrocarbon generation and the hydrocarbon production rate can be increased by up to 147%. For the humic kerogen, hydrogenation can increase hydrocarbon production rate at every stage of the organic matter evolution. It was found that hydrogenation by H-rich fluids derived from the mantle might have happened in the Dongying Depression. We believe that mantle-derived fluids have important effects on hydrocarbon generation by providing both reaction energy and materials.

Geochemical characteristics of rare earth elements in petroleum and their responses to mantle-derived fluid: an example from the Dongying Depression, East China

This paper studies the characteristics of rare earth elements (REEs) in petroleum, in order to investigate the influence of mantle-derived fluid on petroleum. Oil and gas samples were collected from the petroliferous Dongying Depression, East China. Basic igneous rocks and mantle-derived fluid have been found along the fault belts that cut to the basement in the active areas of this depression. The mudstone-normalized REE patterns of petroleum from regions that have not been affected by mantle-derived fluids are evidently characterized by enrichments in a) Nd, Eu, Tb, and Ho, the middle REEs (MREEs) that contain an even number of f-electrons, and b) Er, Tm, Yb and Lu, the heavy REEs (HREEs). The introduction of mantle-derived fluids into the basin is supported by helium isotope composition of the natural gas from the region. MREE anormalies and HREE enrichment are diminished in these affected areas, as compared to oils from the unaffected areas. Mudstone-normalizd Eu/Eu* and Lu/Er were selected to describe this diminishment. The correlation between observed changes in REE patterns in the oils and helium isotope ratios in co-produced natural gases suggests that the changes were likely caused by the mantle-derived fluid and that REEs in petroleum may potentially act as a tracer for understanding the influence of the mantle-derived fluid on oil, when combined with 3He/4He. Further more, the relationship between mudstone-normalized Eu/Eu* and Lu/Er values REEs in oil may help to identify whether the mantle-derived fluids influence on oil or on hydrocarbon generation.

A novel molecular index for secondary oil migration distance

Determining oil migration distances from source rocks to reservoirs can greatly help in the search for new petroleum accumulations. Concentrations and ratios of polar organic compounds are known to change due to preferential sorption of these compounds in migrating oils onto immobile mineral surfaces. However, these compounds cannot be directly used as proxies for oil migration distances because of the influence of source variability. Here we show that for each source facies, the ratio of the concentration of a select polar organic compound to its initial concentration at a reference point is independent of source variability and correlates solely with migration distance from source rock to reservoir. Case studies serve to demonstrate that this new index provides a valid solution for determining source-reservoir distance and could lead to many applications in fundamental and applied petroleum geoscience studies.

Origins and fates of CO2 in the Dongying Depression of the Bohai Bay Basin

CO2 gas reservoirs have been found in a series of rift basins in East China. The objective of this study is to reveal origins and fates of CO2 in the Dongying Depression in East China by using isotopic (He and C) and molecular compositions. 3He/4He, CO2/3He and δ13CCO2 values indicate that the CO2 gases with CO2 > 79% in the deep fault belts in the depression are predominantly mantle-derived. In contrast, CO2 in most gases in the relative stable areas, where source rocks are developed, is less than 10%. Based on helium isotope and δ13CCO2 ∼ CO2/3He diagram, it was found that 10−30% of these CO2 gases is mantle-derived, 28% to 68% is limestone-decomposed and 18% to 54% sedimentary organic in the relative stable areas. The mantle-derived and limestone-decomposed CO2 in the relative stable areas indicates that the mantle-derived geo-fluid met the source rocks. More hydrocarbons can be generated via its heat influence. As the previously published model is not suitable for study of CO2 fates at least in the case of thermogenic CO2 addition, we proposed two models for CO2 addition and loss: model I for CO2 loss and limestone-decomposed CO2 addition and model II for CO2 loss and addition of sedimentary organic CO2, based on a δ13CCO2 ∼ CO2/3He diagram. By using the new models, it was revealed that the loss rates of mantle-derived CO2 can be as high as 50–99.99% for most gases in the depression, the addition rates of limestone-decomposed CO2 range from 18% to 150%, and the addition rates of sedimentary organic CO2 from 11% to 230%. From this study, it was revealed that more mantle-derived CO2 entered the Dongying Depression than previously recognized.

A Method for Eliminating Caprock Thickness Influence on Anomaly Intensities in Geochemical Surface Survey for Hydrocarbons

All petroliferous basins exhibit near-surface anomalous signals. The successful use of these signals must lead to great development in petroleum exploration. However, oil/gas fields cannot be effectively predicted in the case of intensive influence of caprock thickness on intensities of anomalies in a geochemical survey for hydrocarbons. In this paper, we first study the features of the influence of caprock thickness and then establish digital conceptual models for the influence in one and two dimensions. Finally, we develop techniques for eliminating this influence with wavelet analysis, based on the digital conceptual models. The newly developed techniques are applied to the actual data of acid-extractable hydrocarbons of soils in the southern slope of the Dongying Depression, East China where the anomaly intensities are considerably influenced by caprock thickness. The results illustrate that this new approach enables us to satisfactorily eliminate the influence of caprock thickness on anomaly intensities and thus can greatly improve the predictive capability of the existing geochemical data.

Multi-origin alkanes related to CO2-rich, mantle-derived fluid in Dongying Sag, Bohai Bay Basin

With the newly obtained carbon isotope data for the natural gas, a pilot study of multiple-sourced alkanes related to the mantled-derived fluid is presented. The carbon isotope values of alkanes in the Dongying Sag possess the features indicating a general organic origin. However, there are two sub-populations in the isotopic data set, which reflect two specific types of origins. In general, the sub-population with high δ13CCH4 values is related to the CO2-rich, mantle-derived fluid, and it is distributed in the belts where mantle-derived fluid flow and basic volcanic activities have occurred. Geological and geochemical studies demonstrate that this variation of methane carbon isotope values in the Dongying Sag is unrelated with the basin bury and thermal histories, types of source rocks, and reactions between basin fluid and rocks. Mixing of mantle-derived fluid and organic sourced hydrocarbons is probably the cause for the variation.

Origins of N2 in the Dongtai Depression of the Subei Basin

Identification of N2 origins can help to reduce exploration risks and to improve the understanding of gas generation and accumulation. N2 from the mantle and organic matter in basins cannot be unraveled by using θ15 N ratios alone, due to overlapping θ15 N signatures. In this paper, we comprehensively studied N2, Ar, θ15 N, N2/3He, 3He/4He, θ13 C and their relationships of natural gases in the Dongtai Depression of the Subei Basin. Nitrogen contents of the gases in this depression range from 0.2 to 85.9%. Some samples have the air N2/Ar ratio but others are excess relative to this ratio. θ15N data of all the samples except one constitute two populations in the histogram. All these features reflect at least two origins of N2. One of these populations has θ15N values around 0‰ with N2/3He ratios of 1.0×107 to 9.5×109, whereas the other population has negative θ15N values from −7.3 to −2.2 ‰ with N2/3He of 1.1×107 to 9.4×109. It was obtained that the nitrogen of the former population is mainly atmospheric in origin. For the latter population, a considerable amount of nitrogen from organic matter in sedimentary rocks was identified, which coexist with the nitrogen from the air and mantle. In the Dongtai Depression of the Subei Basin, therefore, atmospheric, organic and mantle origins provided remarkable contributions to the natural gases.