Molecular indicators of secondary oil migration distances

Influence of thermal maturity on carbazole distributions in coal source rocks during compaction pyrolysis experiments

Carbazole compounds are widely used in determining the direction of petroleum migration, but the effect of thermal maturity on carbazoles is still ambiguity. In this paper, using compaction pyrolysis simulation experiments, artificial mature samples with vitrinite reflectance (Ro) range from 0.38 to 3.0% were acquired. And the content and composition change characteristics of carbazole compounds were analyzed in coal source rocks. The experimental results showed that thermal maturity controls the generation of a large amount of carbazole compounds in coal rocks. Compared with the low mature stage, the content of carbazole compounds was about 10-100 times higher in the mature stage. With the increasing maturity, in the coal sample, the content of carbazole compounds showed a trend of first increasing and then decreasing. In derivatives of carbazole, the corresponding maturity for the maximum generation of ethylcarbazole (EC), dimethylcarbazole (DMCA), methylcarbazole (MCA), carbazole (CA) and benzocarbazole (BCA) performed the increasing sequence. With the increasing maturity, the relative abundance of 2-MCA, 1,7-DMCA and benzo[a]carbazole increased with the increasing maturity, while 4-MCA, 1,4-DMCA and benzo[c]carbazole gradually decreased. Benzocarbazole ratio [a]/[a] +[ c] varies only in a narrow range 0.36-0.61 in the entire maturity range, suggesting limited maturity dependence. The experimental conclusion provides more theoretical basis for future geochemical analysis using carbazole compounds.

Quantitative Characterization of Excess Pressure Gradient in the Upper Interval of Es4 Member of Dongying Depression and Its Indicative Significance for Oil Migration and Accumulation

Excess pressure is the main driving force of oil migration in the source-reservoir system of overpressured petroliferous basins. It can reflect the change in driving force for oil migration and its influence on oil accumulation in overpressure transport layers. The drilling stem test (DST) data, well logging data, and seismic velocity data are used to describe the plane distribution of the excess pressures in the Es4s member of the Dongying Depression. Then, the values and directions of the excess pressure gradient, which can indicate oil migration and accumulation, are calculated based on the plane distribution of the excess pressure in the Es4s member of the Dongying Depression. The results suggest that overpressure is widely developed in the Es4s member of the Dongying Depression, and the excess pressure gradually decreases from the center to the edge of each sag, while the variation in the excess pressure gradient is characterized by “low-high-low” in a circular band around the sags. The excess pressure in the sag areas exceeds 15 MPa, but the excess pressure gradient is mainly between 0 and 1 MPa/km. The excess pressure in the northern steep slope zone of the Lijin sag and the northern steep slope zone of the Minfeng sag are less than 15 MPa, while the excess pressure gradient ranges from 1 to 7 MPa/km. The excess pressure in the central anticline belt and the gentle slope belt in the south of the Niuzhuang sag are between 0 and 15 MPa, and the excess pressure gradient is from 0 MPa/km to 2 MPa/km. From geochemical evidence, local oil migration directions indicated by the excess pressure gradient are consistent with those indicated by the ratio parameters of carbazole compounds in crude oil samples, indicating that the direction of the excess pressure gradient can indicate the dominant direction of oil migration driven by excess pressure, and the oil from the Es4s source rock is mainly distributed in the areas with a high excess pressure gradient or the areas with a low excess pressure gradient and low excess pressure (area II).

Carbazoles in Oils, and Their Application in Identifying Oil Filling Pathways in Eocene Syn-Rift Fault Blocks in the Dongpu Depression, Bohai Bay Basin, East China

Carbazoles and dimethyl carbazoles (DMCs) are important nitrogen heterocyclic aromatic compounds in oils and sedimentary rock extracts. Based on both migration fractionation effects and differences in the thermal stability of their isomers, carbazoles can be used to trace oil migration orientations and filling pathways. Molecular biomarker compositions indicate that all oils and oil-bearing sand extracts in the Eocene fault-blocked reservoirs of the Huzhuangji area (Western Slope Belt) of the Dongpu Depression (Bohai Bay Basin, East China) belong to a single oil population. In this study, four geochemical indicators relating to carbazoles, namely (a) 1,8-/2,7-dimethyl carbazoles (1,8-/2,7-DMC); (b) 1,8-/2,5-dimethyl carbazoles (1,8-/2,5-DMC); (c) 1,8-/N-exposed dimethyl carbazoles (1,8-/N-exposed DMC); and (d) G1 N-shielded %, were applied to trace oil migration orientations and filling pathways. The results show that these parameter values gradually increase toward the Hu-5 fault block at the structural high. The measured values from the subsurface are consistent with the results calculated from the molecular adsorption modeling. Therefore, it is concluded that the relative parameters of dimethyl carbazoles are practical molecular indicators for tracing oil migration orientations and filling pathways in syn-rift fault-blocked reservoirs.

Petroleum Charging History of the Paleogene Sandstone Reservoirs in the Huangtong Sag of the Fushan Depression, South China Sea

The Paleogene sandstone reservoir in the Huangtong Sag of the Fushan Depression is one of the most commercially prolific oil accumulations in the Beibuwan Basin, South China Sea. In this study, twenty-seven crude oil samples from four oilfields in the sag were geochemically analyzed and classified into three oil groups. Oils in the Meitai and Hongguang oilfields show similar geochemical characteristics, with relatively abundant C30 4α-methyl-24-ethylcholestanes and low contents of oleanane and C19+20 tricyclic terpanes (TT), and therefore may be derived from the same source kitchen. Oils from the Yong’an oilfield have a greater proportion of oleanane and C19+20 TT compounds. Oils from the Chaoyang oilfield have intermediate contents of these biomarkers and proportionate values of other related parameters. The unimodal distribution pattern of homogenization temperatures of fluid inclusions found in all the Paleogene reservoirs indicates a single episode of charging. Combining this understanding with the reconstruction of stratigraphy-burial and geothermal histories by 1-D basin modeling, the petroleum charging time was determined to be between 8–2 Ma (the end of the middle Miocene to the early Pliocene). Hydrocarbon migration orientation and charging pathways were traced using molecular indicators (4-/1-methyldibenzothiophene and 1-/4-methodibenzofuran). In the Chaoyang oilfield, the values of these geochemical parameters decreased with decreasing burial depth, indicating vertical oil migration along faults, which are plentiful in the sag. Sandstone bodies with deeper burial depths may therefore be the most promising exploration targets in the Chaoyang area. The Yong’an oils generally migrate from the northwest toward the southeast, suggesting that the source kitchen for the Yong’an oilfield is in the northwest. By similar inference, the hydrocarbon source kitchen for the Hongguang-Meitai area is likely to be on the north side of the Hongguang-Meitai area. By tracing these hydrocarbon charging pathways, the northwest of the Yong’an area and the northern part of the Hongguang-Meitai area are identified as the most promising areas for exploration.

High resolution nanoscale chemical analysis of bitumen surface microstructures

Surface microstructures of bitumen are key sites in atmospheric photo-oxidation leading to changes in the mechanical properties and finally resulting in cracking and rutting of the material. Investigations at the nanoscale remain challenging. Conventional combination of optical microscopy and spectroscopy cannot resolve the submicrostructures due to the Abbe restriction. For the first time, we report here respective surface domains, namely catana, peri and para phases, correlated to distinct molecules using combinations of atomic force microscopy with infrared spectroscopy and with correlative time of flight-secondary ion mass spectrometry. Chemical heterogeneities on the surface lead to selective oxidation due to their varying susceptibility to photo-oxidation. It was found, that highly oxidized compounds, are preferentially situated in the para phase, which are mainly asphaltenes, emphasising their high oxidizability. This is an impressive example how chemical visualization allows elucidation of the submicrostructures and explains their response to reactive oxygen species from the atmosphere.

Relative sorption coefficient: Key to tracing petroleum migration and other subsurface fluids

The accumulation and spatial distribution of economically important petroleum in sedimentary basins are primarily controlled by its migration from source rocks through permeable carrier beds to reservoirs. Tracing petroleum migration entails the use of molecular indices established according to sorption capacities of polar molecules in migrating petroleum. However, little is known about molecular sorption capacities in natural migration systems, rendering these indices unreliable. Here, we present a new approach based on a novel concept of relative sorption coefficient for quantitatively assessing sorption capacities of polar molecules during natural petroleum migration. Using this approach, we discovered previously unrecognized “stripping” and “impeding” effects that significantly reduce the sorption capacities of polar compounds. These discoveries provide new insights into the behaviors of polar compounds and can easily explain why traditional molecular indices yield incorrect information about petroleum migration. In light of these new findings, we established new molecular indices for tracing petroleum migration. We demonstrate via case studies that the newly established indices, unlike traditional molecular indices, are reliable and effective in tracing petroleum migration. Our approach can be applied to diverse basins around the world to reveal distribution patterns of petroleum, which would decrease environmental risks of exploration by reducing unsuccessful wells.

Characterization of Nitrogen-Containing Polycyclic Aromatic Heterocycles in Crude Oils and Refined Petroleum Products

A large amount of polycyclic aromatic hydrocarbons (PAHs) and their heterocyclic analogues (N, S, O) are released to the marine environment from natural oil seeps, oil spills, bilge discharges and input of land-based sources. Many of these compounds are toxic and have a deleterious effect on marine biota. Nitrogen-containing compounds in crude oils are typically present as cyclic compounds such as polycyclic aromatic nitrogen heterocycles (PANHs) and are generally classified into the two categories of nonbasic (N-PANHs) and basic compounds (B-PANHs). Chromatographic analyses of PANHs are easily to be interfered by other oil components without proper sample preparation prior to instrumental analysis. In this work, dual solid phase extraction columns of 3-(isocyanato)propyl-functionalized silica gel (Si-NCO) and silica gel were employed to efficiently separate both N-PANHs and B-PANHs from saturated and aromatic petroleum hydrocarbons, which enable simultaneous accurate analyses of these groups with single sample preparation. Crude oils studied contain various concentrations of N-PANHs including carbazole, benzocarbazole and B-PANHs including quinolone, acridine and benzoacridine as well as their alkylated homologues. These compounds in light fuel and lubricating oil are generally not detected or are only in trace concentration, but have considerable abundance in heavy fuel oils. Crude oils from different sources and various petroleum products have their unique absolute concentrations and relative distribution patterns of PANHs. Chemical fingerprints of PANHs can provide valuable information for forensic oil spill identification and improve the understanding of the fate, behaviour and chemical degradation of spilled crude oil.

Distribution of coal and coal combustion related organic pollutants in the environment of the Upper Silesian Industrial Region

In this study, a large sample set (276) was separated into up to 15 groups, including coal, fly ash, total particulate matter, coal wastes, river sediments, and different water types. Grouping the sample set into these categories helped to identify the typical features of combustion or water-washing and compare them using newly developed polycyclic aromatic hydrocarbon diagnostic ratios. A wide range of organic pollutants were identified in samples, including aromatic and polycyclic hydrocarbons, nitrogen-heterocycles, sulphur-heterocycles + trithiolane, and polycyclic aromatic hydrocarbons substituted with oxygen functional groups. The distribution of compounds was significantly influenced by water washing or combustion. During the self-heating of coal wastes, secondary compounds such as chlorinated aromatics (chlorobenzene, chloroanthracene, etc.) or light sulphur compounds (e.g. benzenethiol and benzo[b]thiophene) were formed (synthesised). Since these compounds are generally absent in sedimentary organic matter, their origin may be connected with high-temperature formation in burning coal dumps. These compounds should be identified as persistent organic pollutants (POPs) in the environment. The newly defined diagnostic ratios have worked well in separating samples (petrogenic and pyrogenic) and have pointed out the effect of incomplete combustion on self-heated coal waste, ash from domestic furnaces, or water washing and biodegradation of the studied compounds.

Carbon and hydrogen isotopic compositions of n-alkanes as a tool in petroleum exploration

Compound-specific isotope analysis (CSIA) of individual organic compounds is a powerful but underutilized tool in petroleum exploration. When integrated with other organic geochemical methodologies it can provide evidence of fluid histories including source, maturity, charge history and reservoir processes that can support field development planning and exploration efforts. The purpose of this chapter is to provide a review of the methodologies used for generating carbon and hydrogen isotope data for mid- and high-molecular-weight n-alkanes.

We discuss the factors that control stable carbon and hydrogen isotope compositions of n-alkanes and related compounds in sedimentary and petroleum systems and review current and future applications of this methodology for petroleum exploration. We discuss basin-specific case studies that demonstrate the usefulness of CSIA either when addressing particular aspects of petroleum exploration (e.g. charge evaluation, source rock–oil correlation, and investigation of maturity and in-reservoir processes) or when this technique is used to corroborate interpretations from integrated petroleum systems analysis, providing unique insights which may not be revealed when using other methods. CSIA of n-alkanes and related n-alkyl structures can provide independent data to strengthen petroleum systems concepts from generation and expulsion of fluids from source rock, to charge history, connectivity, and in-reservoir processes.

Characterizing aliphatic moieties in hydrocarbons with atomic force microscopy

We designed and studied hydrocarbon model compounds by high-resolution noncontact atomic force microscopy. In addition to planar polycyclic aromatic moieties, these novel model compounds feature linear alkyl and cycloaliphatic motifs that exist in most hydrocarbon resources - particularly in petroleum asphaltenes and other petroleum fractions - or in lipids in biological samples. We demonstrate successful intact deposition by sublimation of the alkyl-aromatics, and differentiate aliphatic moieties from their aromatic counterparts which were generated from the former by atomic manipulation. The characterization by AFM in combination with atomic manipulation provides clear fingerprints of the aromatic and aliphatic moieties that will facilitate their assignment in a priori unknown samples.

A Simple Geotracer Compositional Correlation Analysis Reveals Oil Charge and Migration Pathways

A novel approach, based on geotracer compositional correlation analysis is reported, which reveals the oil charge sequence and migration pathways for five oil fields in Saudi Arabia. The geotracers utilised are carbazoles, a family of neutral pyrrolic nitrogen compounds known to occur naturally in crude oils. The approach is based on the concept that closely related fields, with respect to filling sequence, will show a higher carbazole compositional correlation, than those fields that are less related. That is, carbazole compositional correlation coefficients can quantify the charge and filling relationships among different fields. Consequently, oil migration pathways can be defined based on the established filling relationships. The compositional correlation coefficients of isomers of C₁ and C₂ carbazoles, and benzo[a]carbazole for all different combination pairs of the five fields were found to vary extremely widely (0.28 to 0.94). A wide range of compositional correlation coefficients allows adequate differentiation of separate filling relationships. Based on the established filling relationships, three distinct migration pathways were inferred, with each apparently being charged from a different part of a common source kitchen. The recognition of these charge and migration pathways will greatly aid the search for new accumulations.

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.

Dense non-aqueous phase liquids at former manufactured gas plants: challenges to modeling and remediation

The remediation of dense non-aqueous phase liquids (DNAPLs) in porous media continues to be one of the most challenging problems facing environmental scientists and engineers. Of all the environmentally relevant DNAPLs, tars in the subsurface at former manufactured gas plants (FMGPs) pose one of the biggest challenges due to their complex chemical composition and tendency to alter wettability. To further our understanding of these complex materials, we consulted historic documentation to evaluate the impact of gas manufacturing on the composition and physicochemical nature of the resulting tars. In the recent literature, most work to date has been focused in a relatively narrow portion of the expected range of tar materials, which has yielded a bias toward samples of relatively low viscosity and density. In this work, we consider the dissolution and movement of tars in the subsurface, models used to predict these phenomena, and approaches used for remediation. We also explore the open issues and detail important gaps in our fundamental understanding of these extraordinarily complex systems that must be resolved to reach a mature level of understanding.

Determination of alkylphenols after derivatization to ferrocenecarboxylic acid esters with gas chromatography-atomic emission detection

A method is described for the rapid determination of alkylphenols in nonpolar matrixes. The alkylphenols are derivatized with ferrocenecarboxylic acid chloride so that every phenol molecule is labeled with one iron atom. The resulting esters are analyzed by gas chromatography with atomic emission detection (AED) in the iron-selective detection mode. This method utilizes the AED's low detection limit (0.05 pg/s) for iron and the high selectivity versus carbon (3.5 x 10(6)) for the detection of the alkylphenols. Because the derivatization is performed before the first step of sample preparation, the risk of analyte loss by adsorption or volatilization is minimized. The total recoveries in the lower ppm concentration range vary between 79 and 125%. The quantification of 20 C0-C3-alkylphenols in crude oils is demonstrated by analyzing a shale oil (SRM 1580) and a petroleum crude oil (SRM 1582). The complete workup is easily carried out in only 45 min/sample.