Chemical Composition Of Crude Oil Research Articles

Assessment of the effect of short-term weathering on the molecular-level chemical composition of crude oils in contact with aquatic environments.

Multiple studies have focused on the effect of long-term weathering processes on oils after spill events, without considering the chemical compositional changes occurring shortly after the release of oil into the environment. Therefore, the present study provides a broad chemical characterization for understanding of the changes occurring in the chemical compositions of intermediate (°API = 27.0) and heavy (°API = 20.9) oils from the Sergipe-Alagoas basin submitted to two simulated situations, one under marine conditions and the other in a riverine environment. Samples of the oils were collected during the first 72 h of contact with the simulated environments, followed by evaluation of their chemical compositions. SARA fractionation was used to isolate the resins, which were characterized at the molecular level by UHRMS. The evaporation process was highlighted, with the GC-FID chromatographic profiles showing the disappearance of compounds from n-C10 until n-C16, as well as changes in the weathering indexes and pristane + n-C17/phytane + n-C18 ratios for the crude oils submitted to the riverine conditions. Analysis of the resins fraction showed that basic polar compounds underwent little or no alterations during the early stages of weathering. The marine environment was shown to be much less oxidative than the riverine environment. For both environments, a feature highlighted was an increase of acidic oxygenated compounds with the increase of weathering, especially for the crude oil with °API = 27.0.

The role of water in the laboratory thermal advancement of immature type I kerogen from the Cretaceous Qingshankou Formation in China

To understand variations in geochemistry, organic petrology, and chemical composition of crude oil and byproducts, an immature sample from the Cretaceous Qingshankou Formation in the Songliao Basin, China, was analyzed by anhydrous and hydrous pyrolysis (AHP/HP) at a wide range of temperatures ranging from 300 °C to 450 °C. The geochemical parameters: TOC, S2, HI, and Tmax obtained from Rock–Eval pyrolysis showed both a decrease and an increase as thermal maturity progressed under HP and AHP conditions. Gas chromatography (GC) analysis showed the presence of n-alkanes in the C14 to C36 range in both expelled and residual byproducts, a Delta-shaped configuration although many samples had a gradually reducing (tapering) trend toward the high range. Gas chromatography-mass spectrometry (GC–MS) analysis revealed both an increase and a decrease in biomarker and very small changes in aromatic compound variations with increasing temperature during pyrolysis. To be more specific, C29Ts biomarker increased with temperature for the expelled byproduct, while the opposite trend was observed for the residual one. Next, The Ts/Tm ratio initially increased and then decreased with temperature while the C29H/C30H ratio fluctuated for the expelled byproduct but increased for the residual. Moreover, the GI and C30 rearranged hopane to C30 hopane ratio remained unchanged whereas the C23 tricyclic terpane/C24 tetracyclic terpane ratio and the C23/C24 tricyclic terpane ratio showed varying trends with maturity alike the C19/C23 and C20/C23 tricyclic terpane. Ultimately, based on organic petrography observations, increasing the temperature resulted in higher bitumen reflectance (%Bro, r) and optical and structural alterations in the macerals. The findings of this study provide valuable insights for future exploration endeavors in the studied region. Moreover, they contribute to our understanding of the significant role of water in the generation and expulsion of petroleum and associated byproducts, thereby facilitating the development of updated models in this field.

Characteristics of light hydrocarbons under the superimposed influence of biodegradation and subsequent thermal maturation

Recently, several large light oil and condensate reservoirs were discovered in the Tarim and Junggar basins (China). In these superimposed petroliferous basins, the early formed palaeo-oil reservoirs experienced complex tectonic movements, resulting in complex secondary alterations (e.g., biodegradation and water washing), known to alter the chemical composition of crude oils. Thus, crude oils previously modified by biodegradation exhibit significantly different generation potential, chemical composition, and timing of generation (in terms of light oils/condensates), during subsequent thermal alteration. To elucidate the influence of biodegradation on crude oil cracking (to form light hydrocarbons), we artificially matured four crude oils of the same origin (with varying degrees of biodegradation) to various maturity levels, using programmed-temperature gold tube closed-system pyrolysis. The yields, molecular parameters, and stable carbon isotope ratios of the light hydrocarbon products derived from the matured oils were analysed and compared. The preferential removal of n-alkanes and branched alkanes during biodegradation significantly reduced the generation potential of light hydrocarbons. Under the superimposed influence of biodegradation and subsequent thermal alteration, referred to as superimposed secondary alteration (SSA), a few hydrocarbon ratios used for oil–oil correlations were severely modified. Most maturity-related light hydrocarbon parameters of the pyrolysis products derived from the biodegraded oils achieved maximum values at a lower maturity (Easy%Ro) than that observed for non-biodegraded crude oil. At 0.5–1.4 Easy%Ro, SSA caused the depletion of 13C in most light hydrocarbons, due to an increased sourcing from the biodegradation-resistant fractions of the oil. Such carbon isotopic differences decreased with increasing maturity.

Evaluation of the effectiveness of commercial demulsifiers based on polyoxyalkylated compounds in relation to oil and water emulsions of the Sarybulak oilfield

Water and oil emulsion formation is a natural process that takes place during oil recovery and processing. Heavy oils of Kazakhstan form highly stable oil emulsions mostly stabilized by a high content of asphaltenes, resins, and other surface-active components. Oil-in-water emulsions initiate the corrosion of equipment and cause transportation issues. Dewatering of oil emulsions is economically reasonable and requires universal techniques which could be applied to any sort of oil. In this study, the chemical composition of crude oil from the Sarybulak oilfield was determined, and commercial demulsifiers of Basorol brand were applied to these water-in-oil emulsions. The natural stabilizers content (asphaltenes and resins) was determined and correlated with IR-spectrum data. Finally, the effectiveness of demulsifiers is compared and explained according to their structures. It has been found that the higher the relative solubility number of the demulsifier, the better water-in-oil emulsion separation efficiency and dewatering mechanism was assumed. Results of water separation showed that Basorol PE-10400 and PE-10500 are the most effective, with DE of 96% and 91%, respectively, for 30% (vol.) water-in-oil emulsion at 60°C during 1-hour treatment.

Rheological behavior of crude oil and its dependence on the composition and chemical structure of oil components

In this article on the example of two crude oil with the same content of resins, asphaltene, and solid paraffin, it is shown that they have different rheological behavior at low shear rates. The chemical composition of crude oil's high molecular weight components (asphaltene and solid paraffin) was studied by EPR, IR spectroscopy, and chromatography. It is shown that their composition does not differ significantly. It was revealed that the reason for the different rheological behavior of the crude oils is the content of saturated and aromatic hydrocarbons. When the content of mono- and bi-aromatic hydrocarbons in crude oil increases, the degree of its non-Newton behavior increases. It is due to the formation of a coagulation structure in it. The data obtained have to take into account to predict crude oil viscosity when technologies for oil recovery and transportation developed.

On the applicability of regular solution and Flory‐Huggins theories to asphaltene liquid mixtures

AbstractWe critically review two classic, molecular‐based solution theories, the regular solution theory and the Flory‐Huggins theory, in the context of revealing their limited applicability to liquid mixtures containing asphaltenes, a significant fraction of crude oil. Asphaltene molecular structures, either as depicted by the Archipelago model or the island model, are non‐linear (not chain‐like) and rigid, making the application of the aforementioned theories to describe their phase behaviour problematic. In addition, since asphaltenes contain chemical moieties that are able to interact through directional dependent specific intermolecular interactions (e.g., hydrogen bonds), this renders the incorporation of Hildebrand solubility parameters into the theories inappropriate. As a result, the two theories do not always predict the onset and amount of asphaltene precipitation correctly for crude oil under different conditions (e.g., temperature, pressure, chemical composition of crude oil, solvent concentration). It is the hope of this author that researchers in the field channel their efforts into developing a brand‐new solution theory for asphaltene liquid mixtures by taking the key molecular attributes of asphaltenes into account.

A Comparative Analysis of Analytical Techniques for Rapid Oil Spill Identification.

The complex chemical composition of crude oils presents many challenges for rapid chemical characterization in the case of a spill. A number of approaches are currently used to "fingerprint" petroleum-derived samples. Gas chromatography coupled with mass spectrometry (GC-MS) is the most common, albeit not very rapid, technique; however, with GC-MS alone, it is difficult to resolve the complex substances in crude oils. The present study examined the potential application of ion mobility spectrometry-mass spectrometry (IMS-MS) coupled with chem-informatic analyses as an alternative high-throughput method for the chemical characterization of crude oils. We analyzed 19 crude oil samples from on- and offshore locations in the Gulf of Mexico region in the United States using both GC-MS (biomarkers, gasoline range hydrocarbons, and n-alkanes) and IMS-MS (untargeted analysis). Hierarchical clustering, principal component analysis, and nearest neighbor-based classification were used to examine sample similarity and geographical groupings. We found that direct-injection IMS-MS performed either equally or better than GC-MS in the classification of the origins of crude oils. In addition, IMS-MS greatly increased the sample analysis throughput (minutes vs hours per sample). Finally, a tabletop science-to-practice exercise, utilizing both the GC-MS and IMS-MS data, was conducted with emergency response experts from regulatory agencies and the oil industry. This activity showed that the stakeholders found the IMS-MS data to be highly informative for rapid chemical fingerprinting of complex substances in general and specifically advantageous for accurate and confident source-grouping of crude oils. Collectively, the present study shows the utility of IMS-MS as a technique for rapid fingerprinting of complex samples and demonstrates its advantages over traditional GC-MS-based analyses when used for decision-making in emergency situations. Environ Toxicol Chem 2021;40:1034-1049. © 2020 SETAC.

Spectroscopic Fingerprinting of Organic Material Extracted from Tight Chalk Core Samples of the North Sea.

The detailed chemical composition of crude oil in subsurface reservoirs provides important information about reservoir connectivity and can potentially play a very important role for the understanding of recovery processes. Relying on studying produced oil samples alone to understand the rock-fluid and fluid–fluid interactions is insufficient as the heavier polar components may be retained by tight reservoirs and not produced. These heavy and polar compounds that constitute the resin and asphaltene fractions of crude oil are typically present in low concentrations and yet are determining for the physical–chemical properties of the oil because of their polarity. In order to obtain a fingerprint analysis of oils including polar compounds from different wells, the oil content of drill cores has been extracted and analyzed. Infrared spectroscopy has been used to perform chemical fingerprinting of the oil extracted from drill cores sampled in different geographical locations of the Danish North Sea. Statistical analysis has been employed to identify the chemical differences within the sample set and explore the link between chemical composition and geographic location of the sample. A principal component analysis, based on spectral peak fitting in the 1800–1400 cm–1 range, has allowed for statistical grouping of the samples and identified the primary chemical feature characteristic of these groups. Statistically significant differences in the quantities of polar oxygen- and nitrogen-containing compounds were found between the oil wells. The results of this analysis have been used as guidelines and reference to establish an express statistical approach based on the full-range infrared spectra for a further expansion of the sample set. The chemical information presented in this work is discussed in relation to oil fingerprinting and geochemical analysis.

Impact of biodegradation of oil on the kinetics of gas hydrate formation and decomposition

Nowadays the biological methods based on utilization of bacteria for different purposes in oilfields are becoming more and more popular because these methods potentially may become a cheaper alternative to chemical treatments. Gas hydrates formation in pipelines is the one of the main flow assurance problems. In previous works it has been shown that some biodegraded oils can have components that may act as natural Anti-Agglomerants. These natural Anti-Agglomerants can prevent hydrate particles from agglomeration which allow the particles to flow downstream along with oil in form of slurry. But there is lack of understanding of how products of oil biodegradation affect the kinetic aspects of hydrate nucleation, formation and decomposition. This information might be useful for development of new approached of treatment of the hydrate related issues in oil pipelines.Impact of the changes in component composition of crude oil caused by biodegradation process on the kinetics of gas hydrates nucleation, formation and decomposition has been studied with different methods. It was demonstrated that the changes of chemical composition of crude oil caused by biodegradation can have a strong impact on the hydrate nucleation rates at static conditions. Almost twofold decrease of hydrate nucleation rate was observed in the experiments with the water-in-oil emulsion for the biodegraded oils compared to one obtained with emulsion in the original crude oil. At the same time, no significant effect of biodegradation on the hydrate film growth rates at static conditions was observed. Similar tangential and normal hydrate film growth rates were obtained in experiments with both crude oil and with biodegraded oil. Initial growth rates of hydrate in stirring emulsions were an order of magnitude higher for water emulsions in the biodegraded oil compared to ones for the original crude oil. The studies of the decomposition rates of hydrates at sub-zero temperatures and at dynamic conditions showed that the products of vital activity of bacteria promoted self-preservation of hydrates dispersed in the biodegraded oil.

Adsorption of Polar Organic Components onto Sandstone Rock Minerals and Its Effect on Wettability and Enhanced Oil Recovery Potential by Smart Water

It is generally accepted that reservoir wettability is one of the most important parameters in oil recovery processes. In the published literature, it is believed that the state of reservoir wettability mainly depends on the adsorption or precipitation of oxygen and nitrogen compounds present in the heavy-end fractions of crude oil. However, the establishment of reservoir wetting is a more complex process that involves chemical interactions between all phases of the reservoir: rock mineral surfaces, formation water, and surface-active components in the crude oil. In this study, dynamic adsorption tests were performed by flooding modified crude oils with a low asphaltene content through outcrop sandstone cores. Adsorption of crude oil components was analyzed by comparing base number (BN) and acid number (AN) of the effluent oil samples with the known initial BN and AN of the crude oil. The experimental results showed that crude oil bases are more active than acids toward the silicate rock mineral surfaces. Within the pore volumes flooded, it was not possible to achieve equilibrium BN values because of the continuous adsorption of basic components. Spontaneous imbibition (SI) tests showed that the core sample behaved slightly water-wet after crude oil flooding. Ion-modified smart water as an imbibition fluid in tertiary mode has previously shown potential for wettability alteration and improved oil recovery. With an increase in the amount of injected crude oil through the core, a decrease in oil recovery and a decrease in smart water-enhanced oil recovery (EOR) potential were observed. SI oil recovery results indicate reduced positive capillary forces and a change in wetting toward a less water-wet state. Thus, the chemical composition of crude oil should be considered as an important parameter for a reliable estimation of the reservoir wettability state and EOR potential by smart water injection.

Chemical Prevention of Asphaltene Flocculation in Oil Production System

The change of pressure, temperature or chemical composition of crude oil during oil production constitute the factors that may destabilize the heaviest fraction of crude oil presented in asphaltenes leading to many problems in the production system including the alteration of reservoir rock wettability, permeability reduction, plugging of tubing and production aerial facilities which generate a high production cost because of loss in the well productivity and the need of corrective measures. In this work, a crude oil sample was taken from an Algerian oil field to study the efficiency of two commercial inhibitors. The flocculation onset point of asphaltenes was measured by a solid detection system which is based on the light transmittance of flocculated particles when n-heptane is added. The effectiveness of two inhibitors was evaluated by the solid detection system at different concentration to select the best inhibitor, however, the best additive should provide a good performance at low concentration. Meanwhile, the effect of thermodynamic conditions has been also established in this work, however, the operator conditions of pressure and temperature have affected the efficiency of the added inhibitor on the flocculation of asphaltenes.

Quantification of wettability characteristics for carbonates using different salinities

Low salinity water flooding has received strong interest recently. It has been reported that injecting low salinity water can improve oil recovery in carbonate reservoirs through wettability alteration, whose mechanism however remains unclear, especially regarding the contributions from adsorbed polar organic components. This work investigates systematically the key parameters that govern the complex interactions of rock/brine/crude oil in carbonate reservoirs. The effects of crude oil composition and salinity on surface wettability alteration are studied by using three types of dead oil from Norwegian and North Sea with various acid numbers and two types of carbonate rocks (outcrop-calcite and subsurface-dolomite). Ion-substitution-adsorption is evaluated using the infrared spectrometer (IFR) and energy-dispersive X-ray spectroscopy (EDX). Oil/brine interactions by means of the interfacial tension (IFT) and contact angle as a function of oil composition, rock type, and salinity are experimentally investigated. The IFR and EDX data indicate a compositional change when the carbonate minerals are in touch with saline solutions, supporting the proposed rock-brine interaction mechanism. The salinity effect is more salient at the liquid/rock interface than the liquid/liquid interface, and the response to the brine composition is highly dominated by the oil composition with respect to its content of polar organic components. There is a reduction of contact angle as the seawater water is diluted twice while its influence on the IFT is small, and the effect is more pronounced for dolomite rock, showing different results for different crude oils used. This suggests that the wettability alteration of carbonate rock is an intricate phenomenon that is influenced not only by the salinity but also by the chemical composition of crude oil and rock mineralogy. A combination of three possible mechanisms, including multiple ion exchange, EDL expansion (electrokinetics repulsive), and salting-out, is discussed to illustrate the mechanism of wettability alteration during low salinity injection.

Rapid hydrocarbon group-type semi-quantification in crude oils by comprehensive two-dimensional gas chromatography

Petroleum is the main source of energy used in the world. The oil industry faces challenges in trying to understand the chemical composition of crude oils, quantitatively and qualitatively. The application of specific and sophisticated techniques allows a detailed characterization and provides important information that impacts from the exploration to production, transportation, and crude oil refining. A set of twelve crude oil samples with different API gravity were analyzed via comprehensive two-dimensional (2D) gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-TOFMS) aimed at the detection, identification and semi-quantification of hydrocarbon classes using extracted ion chromatogram (EIC) and total ion chromatogram (TIC). The use of the polar/non-polar column configuration provided better separation between the hydrocarbon classes which allowed the identification and semi-quantification of the same. Therefore, series of n- and iso-alkanes, alkyl-cyclohexanes, alkyl-cyclopentanes, alkyl-decalines, alkyl-naphthalenes, alkyl-phenanthrenes and alkyl-9H-fluorenes were characterized. In the present study, a semi-quantification approach to data assessment using TIC is proposed. Comprehensive evaluation provided the chromatographic fingerprint of each sample in a single analysis followed by the semi-quantification of the hydrocarbon classes. The oil samples presented completely different hydrocarbon class data, even when presenting similar API gravity values. Branched alkanes and n-alkanes were the major semi-quantified compounds, and branched alkane concentrations were higher than n-alkanes in the majority of samples. These results afford valuable information for the petrochemical industry. GC × GC-TOFMS was applied for speciation and group-type semi-quantification using a single oil drop without a fractionation or clean up step.

Effect of Low Salinity on the Oil Desorption Efficiency from Calcite and Silica Surfaces

Low-salinity water flooding has been suggested to improve oil recovery in sandstone reservoirs. However, application in carbonate reservoirs is still controversial. In this work, a quartz crystal microbalance (QCM) was used to investigate the impact of the chemical composition of crude oils and crystal surfaces on the adsorption/desorption efficiency of oil components upon exposure to various salinity solutions to reveal the potential of enhancing oil recovery from carbonate reservoirs. The surface charge of carbonate rock is characterized by ζ potential measurements. For QCM-D measurements, Norwegian and North Sea crude oils with various acid numbers were used, while calcite and silica crystals were used to mimic carbonate and sandstone reservoirs. The results reveal that the amount of adsorption is correlated to the concentration of polar organic components present in the oil and the type of surface mineral. Subsequent desorption showed that low amounts of desorption are reported upon exposure to seawat...

Correlations between geochemical characteristics of source rocks and chemical compositions of crude oils from LD field, block 15-1/05, Cuu Long basin

LD field is the second oil discovery on offshore Block 15-1/05, which is located 15 kilometres East-North East of the first discovery (LDN field). The major aim of this study is to evaluate correlations between accumulated hydrocarbons and source rocks of LD structure, in order to verify their potential for generating oil and gas. Therefore, the authors have synthesized and analyzed geochemical and biomarker characteristics, structures, and chemical compositions of crude oils and source rock extracts by gas chromatography–mass spectrometry analysis to interpret hydrocarbon origins of the LD field. Oil samples and source rock hydrocarbon extracts were from LD-1X/LD-1Xst and LD-3X/LD-3Xbis well of the LD field. Based on biomarker distributions, five oil samples as well as nineteen studied extracts from source rocks indicated predominant non-marine algal organic substances as well as contributions of bacterial and higher plant input. Such samples are characterized by the presence of oleananes, high concentration of C27 steranes compared to C29 and C28 steranes, and the presence of 4-methyl C30-steranes low to moderate concentration. Those evidences show that the oil samples are derived from a single source unit.

A Comparative study of the chemical structure of asphaltenes from Algerian petroleum collected at different stages of extraction and processing

Abstract The chemical composition of crude oil, and hence its physicochemical properties, may change during extraction, transportation and treatment. This study characterizes the asphaltene fraction of algerian petroleum collected at three stages of production, namely at the petroleum well, from deposits resulted after petroleum storage, and from the vacuum residuum obtained during the refining process. Mass spectrometry (LDI-TOF), 1 H NMR spectroscopy and infrared absorption (FTIR) were used, along with elemental analysis to monitor the composition and average molecular structure of the asphaltene samples. Noticeable differences in chemical structure are found among the three types of samples. The asphaltenes from the storage deposit are particularly differentiated with respect to the other two ones, featuring a higher polarity, a greater heteroatom content, and a lower aromaticity involving a smaller number of substituted aromatic structures. These features must be taken into account when designing strategies to prevent precipitation throughout the production process.

Chemical Aspects of Onshore Crude Oils from the Carmópolis Field, Sergipe-Alagoas Basin, Brazil: A Case Study on the Industrial Process for Water–Oil Separation

The importance of the Carmópolis field in Brazil for the exploration of onshore crude oil dates back to 1963. Currently still in operation, this is a major field, serving as a model for enhanced oil recovery tests. In this field, the differences in the physicochemical characteristics have awakened an interest in understanding the implications of the chemical composition of crude oils. Four blends of crude oil from this field were characterized using the SARA (saturates, aromatics, resins and asphaltenes) method and advanced methods such as energy dispersive X-ray fluorescence spectroscopy (EDX), laser desorption/ionization mass spectrometry (LDI-MS), infrared (IR), ultraviolet (UV), and thermogravimetric (TG) analysis. Of the four blends investigated, CEOL3 had the greatest affinity for water. The results for the SARA analysis did not reveal significant differences between the blends. The analysis of CEOL3 asphaltenes by IR spectroscopy revealed greater O–H stretching associated with water, and the TG indicated a more advanced state of thermal evolution compared with the other samples investigated in this study. Similarly, according to the EDX analysis, this oil sample had a greater affinity with produced water. Also, based on the UV analysis, the asphaltenes present in CEOL3 were of higher molecular mass compared with the other samples. Finally, the LDI-MS analysis of the blends showed a higher content of naphthenic acids and carbazoles in the CEOL3 sample which, together with the asphaltene content, is related to the stabilization of emulsions.

Constant Asphaltene Molecular and Nanoaggregate Mass in a Gravitationally Segregated Reservoir

Spatial gradients in the chemical composition of crude oil are now routinely observed; oils near the top of a reservoir can be enriched in lighter ends, whereas oils near the bottom of a reservoir ...

Chemical composition of crude oil from the seeds of pumpkin (Cucurbita spp.) and mamey sapota (Pouteria sapota Jacq.) grown in Yucatan, Mexico

The objective of this work was to determine the chemical composition of crude oil from the seeds of pumpkin and mamey fruits grown in the Yucatan, Mexico. Through the extraction method utilized, it was possible to obtain 50 and 40% of crude oil from pumpkin and mamey seeds, respectively. The refractive index at 40°C and peroxide index of crude oil from mamey seeds were similar to those reported for pumpkin var. Xnuuk K'úum, but significantly higher (P < 0.05) than those reported for pumpkin var. Xmejen K'úum. The acidity and iodine indexes were significantly higher in crude oil obtained from mamey seeds in comparison with the oils obtained from pumpkin seeds. The main fatty acid identified in mamey seed oil was oleic acid (60.32%), whereas in pumpkin seed oils from var. Xnuuk K'úum and var. Xmejen K'úum it was linoleic acid (69.12% and 63.11% respectively). The results achieved suggest that the seeds of mamey and pumpkin might be an important source of vegetable oil for industrial uses.

Positive Effects of Bacterial Diversity on Ecosystem Functioning Driven by Complementarity Effects in a Bioremediation Context

Despite their importance as ecosystem drivers, our understanding of the influence of bacterial diversity on ecosystem functioning is limited. After identifying twelve bacterial strains from two petroleum-contaminated sites, we experimentally explored the impact of biodiversity on total density by manipulating the number of strains in culture. Irrespective of the origin of the bacteria relative to the contaminant, biodiversity positively influenced total density. However, bacteria cultured in the crude oil of their origin (autochthonous) reached higher densities than bacteria from another origin (allochthonous) and the relationship between diversity and density was stronger for autochthonous bacteria. By measuring the relative contribution of each strain to total density we showed that the observed positive effect of increasing diversity on total density was mainly due to positive interactions among species and not the presence of a particular species. Our findings can be explained by the complex chemical composition of crude oil and the necessity of a diverse array of organisms with complementary enzymatic capacities to achieve its degradation. The long term exposure to a contaminant may have allowed different bacteria to become adapted to the use of different fractions of the crude, resulting in higher complementarity in resource use in autochthonous bacteria compared to allochthonous ones. Our results could help improve the success of bioaugmentation as a bioremediation technique by suggesting the use of a diversified set of autochthonous organisms.