Tar Mat Research Articles

Reservoir screening using Iatroscan TLC-FID and identification of palaeo-oil zones, oil–water contacts, tar-mats and residual oil saturations in the Frøy and Rind petroleum accumulations

Combined thin layer chromatography (TLC) and flame ionization detection (FID) of the Iatroscan TLC-FID instrument were used for geochemical screening of 600 core samples from the Elf-operated Frøy Field and Rind Discovery on the Norwegian Continental Shelf. The results allowed a pseudo 3-D characterization of the bitumen distribution in the reservoirs. Calibration of the data to well log-defined oil saturations allowed much more detailed recognition of variation in residual oil saturations (ROS) and helped in determining present and palaeo-oil–water contacts (OWCs). The quantification of bitumen amount and composition between different reservoir units was used to define OWCs. Palaeo-oil zones within present condensate and water zones were identified using an empirical approach to measure bitumen content and composition from live oil zones, condensate zones and dry gas zones, respectively. In this way, the Iatroscan TLC-FID data can be used to reconstruct elements of the filling history of the reservoirs. The application of this technique for identifying compartments, barriers for fluid flow and tar mats is also demonstrated.

THE GEOLOGY OF NATURAL GAS IN THE PEOPLE'S REPUBLIC OF CHINA: A REVIEW

Commercial accumulations of natural gas in China have been generated by marine carbonates in Proterozoic and Palaeozoic cratonic basins, and by lacustrine shales and coal measures in Mesozoic‐Cenozoic continental basins. These accumulations occur in reservoir rocks of various types. The Chinese natural gas industry has developed rapidly over the past two decades, and proved reserves of gas have increased accordingly. To date, 138 gasfields and 110 gasfields with oil‐rims have been found in China.Based on its geological and geochemical characteristics, natural gas can be divided into three principal types: (i) biogenic (bacterial and biode graded) gas; (ii) wet associated gas, sourced from either sapropelic or humic organic matter; and (iii) dry thermal gas, likewise sourced from sapropelic or humic OM.Each type of gas has its own characteristics in terms of generation, migration, accumulation and distribution. Bacterial gas is concentrated in Quaternary strata (for example, in the Qaidam Basin of NW China); biodegraded gas occurs in shallow strata in the eastern oil provinces, often in association with tar mats. Associated gas occurs mainly in Mesozoic‐Cenozoic continental basins, whereas dry thermal gas is generally confined to Proterozoic and Palaeozoic cratonic basins, especially in West China. Proterozoic and Palaeozoic gas resources make up about one‐half of the Chinese total, and are set to become a major exploration target in the future.

Waxes and asphaltenes in crude oils

High molecular weight (HMW) hydrocarbons (>C 40) and asphaltenes are important constituents of petroleum and can cause problems related to crystallization and deposition of paraffin waxes during production and transportation as well as in the formation of tar mats. However, traditional methods to isolate asphaltene fractions, by adding 40 volumes in excess of low boiling point solvents such as pentane, hexane or heptane, can produce asphaltene fractions which are contaminated with a significant amount of microcrystalline waxes (>C 40). The presence of these microcrystalline waxes in the asphaltene fractions has the potential to provide misleading and ambiguous results in modeling and treatment programs. The sub-surface phase behaviour of an asphaltene fraction will be quite different from that of a wax-contaminated asphaltene fraction. Similarly accurate modelling of wax drop-out requires information on pure wax fractions and not asphaltene-dominated fractions. Hence the goal of this paper is to describe a novel method for the preparation of wax-free asphaltene fractions. In addition, this method provides a quantitative subdivision of the wax fraction into pentane soluble and insoluble waxes which, when correlated with physical properties of crude oil such as viscosity, pour point, cloud point, etc., may help explain causes of wax deposition during production, transportation and storage of petroleum.

Abstract: Tar Mat Formation Within the Hitch Oil Field, Seward County, Kansas 

Abstract: Tar Mat Formation Within the Hitch Oil Field, Seward County, Kansas 

The factors controlling the abundance and migration of heavy vs. light oils, as constrained by data from the Gulf of Suez. Part II. The significance of reservoir mass transport processes

Reservoir flow processes have a major impact on the oil quality and distribution of heavy vs. light oils, both within reservoirs and on the petroleum system scale. In the Gulf of Suez (GOS) gravitational segregation of black oil columns is a common phenomenon. An understanding of the mass transport processes in these reservoirs is important for evaluating their impact on the composition of migrating petroleum. 2D simulation results show that pressure diffusion is a too slow process to be the main cause for the vertical compositional gradients. A reservoir filling model, which is a refined version of the geological model of Gussow, linked to the composition of the GOS expelled petroleum fluids and their PVT behaviour and constrained by the GOS oil field data, is presented. This model gives viable explanations for (a) formation of gravitationally segregated oil columns, (b) tar mat occurrences and their affinity to low maturity, heavy oils and (c) wide spread occurrences of heavy oils and their relationship with light oils. In this model, continuous density overturning, by compositionally controlled bulk flow is the prime cause for the formation of gravitationally segregated columns, occurring when (a) the charge to the reservoir displays a maturity related decrease in density with time and (b) the reservoir volume, relative to the charge volume, is large enough for the reservoir to retain a wide maturity range. Lateral compositional variability in many reservoirs is today interpreted to image the initial filling of the reservoirs. It is demonstrated that such lateral gradients can form effectively from re-orientation of vertical gradients, via ordinary diffusion. These gradients have symmetry elements relative to the petroleum mass distribution in the reservoir. Fill and spill, from gravitationally segregated columns, enhance the efficiency of the migration of heavy oils.

The origin of pyrobitumens in upper Devonian Leduc formation gas reservoirs, Alberta, Canada: an optical and EDS study of oil to gas transformation

Pyrobitumens are common within deep basin, Devonian carbonate gas reservoirs of the Western Canada Sedimentary Basin. Incident light microscopy and SEM-EDS have been used to study the origin of pyrobitumens from Upper Devonian isolated reefs of the Leduc Formation, west central Alberta. Reflectance properties (%Ro max and %Ro min), coke microtextures and relative S/C ratios are used to classify the pyrobitumens. In portions of reservoir units where NSO-enriched, low aromaticity crudes may have been dominant, oil to gas transformations produced mainly low-reflecting isotropic to fine-grained mosaic coke microtextures within the reservoir pyrobitumens (Type A). In contrast, where ‘normal’ crudes were most likely dominant in the reservoir units prior to thermal cracking, pyrobitumens with high bireflectance and very anisotropic coarse flow to domain cokes dominate (Type B). Oil to gas cracking within some of the reservoir units resulted in overpressuring and is represented by highly deformed pyrobitumens with fibrous coke textures and high bireflectance. Relative concentrations of Type A and B pyrobitumens vary vertically and temporally in some of the reefal buildups. The variations in the dominant pyrobitumen types are attributed to crude oil alteration which preceded crude oil to gas transformation (e.g. tar mats) in some zones whereas other zones may have been altered contemporaneously with crude oil cracking. The later may have resulted from the oxidation and sulphur enrichment of the crude oil during thermochemical sulphate reduction. This reaction is prevalent within Devonian evaporite-carbonate strata of the Western Canada Sedimentary Basin, generating copious volumes of H 2S. The interaction between crude oil and sulphur-enriched, highly alkaline fluids is suggested by the presence of high S/C, very fine-grained, late stage calcined needle cokes within brecciated pyrobitumens.

Discovery of tar mats in terrestrial gas-condensate reservoirs and its geological significance

TAR mat is reservoir zone containing petroleum rich in asphaltene, and is situated in gascondensate reservoirs and light oil reservoirs. Tar mat is an organic barrier for oil and gas reservoirs. Recently, determination of tar mat has become reality as thin layer chromatography-flame ionization detection (TLC-FID) technique is applied to organic geochemical de-...

RECOVERY OF TARMATS USING COMMERCIAL SHRIMPING BOATS DURING THE BUFFALO 292 SPILL

ABSTRACT One of the greatest challenges facing responders to a recent intermediate fuel oil (IFO 380) spill was the recovery of tar mats and patties from the surface waters of the Gulf of Mexico before they could impact the barrier islands of Texas. When the marine barge Buffalo 292 spilled approximately 3000 barrels of IFO 380 in heavy weather conditions in Galveston Bay in March 1996, it is estimated that more than half of the spilled IFO 380 was swept out into the Gulf of Mexico by high northerly winds. There was great success in tracking the oil as it moved around the Gulf and eventually formed into large tar mats and patties. The overall weathered condition of the IFO 380 soon made use of conventional offshore skimmers ineffective, so responders began looking at ways to effectively recover the tar mats and patties before they could impact the barrier island beaches of Texas during the busy spring break season. Since commercial shrimping vessels could tow a net and were designed for slow operating speeds, it was decided to try using these vessels to recover the tar mats and patties. By modifying traditional nets and using a spotter aircraft to move the shrimping vessels to the greatest concentration of tar mats and patties, participants made the operation a success.

Petrophysical Characterization of the Carito Norte, Mulata and Carito Fields, Norte de Monagas, Eastern Venezuela: ABSTRACT

The Norte de Monagas trend is located in the Eastern Venezuelan Basin and is characterized by high temperature, high initial pressure and production rates, variable PVT properties, GORs, and asphaltene content, and the presence of a tar mat at the base of the oil column. These reservoirs were petrophysically characterized through the integration of core and log data. Productive sands are composed of very coarse- to medium-grained sands deposited primarily in deltaic to shallow marine environments. These sands are quartz-rich, with clay matrix averaging 3% in the Oligocene and 6% in the Cretaceous. The main difference is the presence of up to 30% feldspars in the Cretaceous. Clay mineralogy is dominated by illitic material in the Cretaceous and by kaolinite in the Oligocene. Post-depositional changes dominated by cementation, mechanical compaction and subsequent dissolution processes have resulted in marked improvement or degradation of the productive characteristics of these reservoirs. Rocks were divided into four petrophysical. categories based on grain size, pore throat radius, porosity and permeability. Mercury saturation vs. mercury saturation over capillary pressure plots show the dominant interconnected pore system which controls flow rates is best defined by 50% saturation. Plots constructed using R50 pore aperture correlation indicate the poremore » throats to be macro to mega porous. Average log calculated water saturation is 14% compared with estimated from capillary pressure data of 12% at 1200{prime} above free water level, and 15% from relative permeability data. Relative permeability data indicate the rocks behave preferably as water-wet.« less

Overview of the geochemistry of some tar mats from the North Sea and USA: implications for tar-mat origin

Abstract Tar mats and oil legs from several marine source-rock derived oil fields (Ula and Oseberg, North Sea and an unidentified North American example) have been geochemically characterized. Tar mats were derived from the same petroleum system as the oil-leg oils and are found in the most porous and permeable portions of the reservoir. It is possible to eliminate some potential tar-mat formation mechanisms using the data from case histories and subsurface asphaltene solubility characteristics. Adsorption of asphaltenes on reservoir clays and light to moderate levels of biodegradation can probably be eliminated as feasible tar-mat formation mechanisms. Thermal degradation of oil in the carrier and reservoir causing asphaltene precipitation may be a major contributing factor to formation of tar mats in the Ula Field, whereas increased gas solution in the oil leg may have contributed to deasphaltation in the Oseberg Field. Based on tentative mass-balance calculations, the extreme pressure dependence of asphaltene solubility in subsurface crude oils and the occurrence of tar mats in zones of elevated permeability, it is suggested that tar-mat formation is initiated during petroleum migration. While tentative, this model may provide a more generic initiation mechanism than others in the literature.

Asphaltene-rich residues in sandstone reservoirs of Haltenbanken province, mid-Norwegian continental shelf

Thin zones of asphaltene-rich residue are present near the crests of two adjacent oil- and gas-filled structures. In one structure, the Smørbukk Sør Field, multiple thin (5–50 cm) dark coloured zones are observed within the lower 20 m of the 88 m thick Garn Formation. These dark coloured zones tend to occur immediately above stylolitized mica-clay laminations and become gradually lighter upwards, apparently reflecting gradual upward decrease in residue abundance. In the other structure, the Smørbukk Field, the lowermost 1 m of the formation is dark coloured and the sandstone gradually becomes lighter in colour upwards for a distance of about 3 m. Similar, but more diffuse, residues are also present throughout the oil zone in two other fields nearby. Organic geochemical analyses show that the residue-rich (dark coloured) zones differ from the sorrounding lighter coloured sandstone mainly in having a higher content of asphaltenes, whereas the total content of extractable organic matter is often similar. Thus the darker rocks might be regarded more as zones of conspicuous oil staining due to asphaltene deposition, rather than as small tar mats. The residues typically appear to have accumulated under the influence of gravity at points where the downward flow was hindered by a lithological discontinuity. Present day zonation (upward-decreasing gas to oil ratio) in the oil column of the Smørbukk Sør reservoir is consistent with a model of asphaltene precipitation in the lower part of the reservoir in response to upward leakage of gas from an underlying gas condensate reservoir in the Ile Formation. This is also consistent with the available organic geochemical data. In thin section, the asphaltene-rich material is seen to preferentially coat feldspar and clay but not quartz surfaces, thus illustrating why sandstone wettability should be expected to depend on both mineralogy and the asphaltene content of the petroleum involved.

Distribution and Quantification of Thin Tar Mats in a North Sea Field: ABSTRACT

Distribution and Quantification of Thin Tar Mats in a North Sea Field: ABSTRACT

New methods to detect tar mats in petroleum reservoirs

The occurrence of tar mats, their lateral extent and volume in place is important for reservoir assessment since it may provide information to improve reservoir management, help to assess the producible reserves and my help to increase the oil recovery. We discuss here currently available tools to locate tar mats in petroleum reservoirs. Dominantly core based detection methods are discussed and several log based methods will be briefly described. A newly developed core based method and a new log method are introduced: The low cost core density approach is based on routinely determined core petrophysical data (porosity, grain density). Porosity and rock density are usually measured on plugs which have been rinsed using organic solvents. In tar mat intervals the petroleum present in the core plug is frequently incompletely extracted and the petrophysical data result in artificially lower porosities and grain densities. The core density approach uses the difference between the real grain density and the calculated grain density to detect the presence of heavy oil or tar mat. The dipmeter approach makes use of the high vertical resolution resistivity measurements to detect thin tar mats which are characterized by a very high resistivity. Both new tar mat detection methods, i.e., the core based core density approach and the log based dipmeter approach allow tar mats to be readily identified and may serve as exploratory screening tools for more detailed organic geochemical analysis.

Origin of tar mats in petroleum reservoirs. Part I: introduction and case studies

Three case studies of petroleum reservoirs containing tar mats (Oseberg field and Ula field, both from the North Sea, and a North American field) have been studied using a variety of organic geochemical techniques with special emphasis on the geochemistry of the asphaltenes. The results were evaluated in combination with the available geological data and with numerical simulations to evaluate possible mechanisms of tar mat formation (Part II; Wilhelms and Larter, 1994). In Part I the petroleum geochemistry of three sets of tar mats is described. The tar mat extracts were characterized by a sharp increase in the asphaltene content (20–60 wt.%) compared with the oil leg extracts (1–5 wt.%). The extract yields (kg EOM/t rock) in the tar mats (ca. 4–46 for the Oseberg field, 8–18 for the Ula field and 8–43 for the North American case study) are significantly higher than in the corresponding oil leg extracts, which have extract yields of ca. 1–14 kg/t rock. The tar mat extracts are geochemically related to the pooled oil as revealed by geochemical fingerprinting, including biomarker analysis. The tar mats are situated in high porosity and high horizontal permeability sandstone layers above permeability barriers. Several zones of low extract yield and immature petroleum geochemical characteristics were observed within the oil legs and between tar mats, which might represent minor amounts of petroleum generated locally from indigenous organic matter.

Origin of tar mats in petroleum reservoirs. Part II: formation mechanisms for tar mats

Tar mats and oil legs from several marine source rock derived oil fields, including the Ula and Oseberg fields in the North Sea and an unidentified North American example, have been analysed geochemically. In all cases the tar mats were derived from the same petroleum system as the oil leg oils and were found in the most porous and permeable portions of the reservoir. Comparing case history data with published data on the subsurface solubility characteristics of asphaltenes in petroleums, it is possible to eliminate some potential tar mat formation mechanisms and suggest the most likely causative mechanisms. Thus the adsorption of asphaltenes on reservoir clays, and light to moderate levels of biodegradation, are considered unlikely to contribute to tar mat formation. In the Ula field thermal degradation of the oil in the carrier and reservoir causing asphaltene precipitation may be a major contributing factor to the tar mats there. In the Oseberg field increased gas solution in the oil leg may have contributed to deasphaltation. Tentative mass balance calculations, the extreme pressure dependence of asphaltene solubility in subsurface crude oils, the occurrence of tar mats in zones of elevated permeability, plus a simple transport model, suggest that during secondary migration an asphaltene-rich petroleum phase may be produced in dipping carrier beds and may be carried into the reservoir, contributing in some instances to tar mat formation.

Characterization of asphaltenes by pyrolysis-field ionization mass spectrometry—some observations

Pyrolysis-field ionization mass spectrometry (Py-FIMS) has been applied in an exploratory study to petroleum asphaltenes derived from (a) oils sourced from different source rock types, abd (b) reservoir core extracts from tar mat containing petroleum reservoirs. Between 50 and 75% of the asphaltenes were volatilized under the conditions used (heating from 50 to 750°C at 1°C/s in a high vacuum of 10 −3 Pa) Weight average molecular weights of the asphaltene pyrolysates, as determined by Py-FIMS, range from 451 to 704 Dalton, number average molecular weights ranging from 394 to 599 Dalton. Molecular weights of asphaltene pyrolysates (from Py-FIMS) correlate weakly with molecular wweight data of asphaltenes determined in solution by gel permeation chromatography. The Py-FIMS pyrolysate data were compared with elemental analysis, quantitative flash pyrolysis and gel-permeation chromatography molecular weight data for the isolated asphaltenes and with biomarker parameters determined from the saturated hydrocarbon fractions of the respective oils or core extracts. Very general inverse correlations between asphaltene pyrolysate molecular weight as determined by Py-FIMS and routinely determined biomarker maturity parameters were found, tentatively suggesting maturity dependencies of the molecular weights of asphaltenes and their pyrolysates, i.e. generally decreasing weight average molecular weight with increasing thermal maturity.

CHEMICAL COMPOSITION AND SOURCE FINGERPRINTING OF DEPOSITIONAL OIL FROM THE KUWAIT OIL FIRES

ABSTRACT Depositional oil and soot derived from the 1991 Kuwait oil field fires produced huge tar mats in the desert. The concentration of oil in the upper 5 cm of the desert surface ranged from 1.16 to 8.92 percent oil by weight as determined by TPH analysis over a 10 km transect in southern Kuwait. The mean concentration of asphaltenes in the tar mat samples was estimated at 62 percent; a greater than 17-fold increase in concentration over unweathered Kuwait crude oil. The remaining oil forming these tar mats is believed to be derived from petroleum that escaped combustion within the well fires. Results from GC/MS analyses characterize the depositional oil as being significantly altered by evaporative weathering. We suspect that evaporation of the depositional oil will continue, further consolidating the weathered oil, sand, and soot into asphalt-like zones which will remain for a very long time as features of the Kuwait desert. Significant quantities of oil residue from the fires also fell into the Arabian Gulf, contributing to what may be the largest man-made oil spill.

Advances in our Knowledge of Biodegradation of Hydrocarbons in Reservoirs

Biodegradation of hydrocarbons in reservoirs is a widespread phenomenon that is currently observed by petroleum organic geochemists in most sedimentary basins. This basic phenomenon is responsible for the occurrence of large, heavy oil deposits referred to as tar mats or tar belts. Biodegradation of crude oils takes place in reservoirs in which oil-eating bacteria may thrive. For this reason, effective and present biodegradation effects are not observed at subsurface temperatures higher than 70-80[degrees]C. Significant compositional changes, especially at a molecular level, still remain linked to the aerobic biodegradation of crude oils. Under favorable circumstances, both alkanes and aromatics are degraded, but when nutrients (N, P, O[sup 2]) are impoverished, aromatics seem to be preferentially removed. Biodegradation extends also to sulfur-bearing aromatics with a preferential removal of alkylated structures. Changes in molecular patterns are used to assess degrees of biodegradation in crude oils. The most bacterially resistant structures are polycyclic alkanes and aromatics. The in-reservoir biodegradation of hydrocarbons does not generate new hydrocarbons, e.g., 25-norhopanes as proposed by several authors. In fact, the selective removal of less resistant structures concentrates preexisting minor families that were not detected on the unaltered crude due to their low absolute concentration. Consequently, the molecular spectrummore » found in severely biodegraded oils may be considered as highly diagnostic of a part of the primary genetic spectrum of each oil. In outcrop samples, biodegradation is associated with other complementary phenomena such as photooxidation, oxidation, inspissation, evaporation, water washing, etc. Of particular importance are weathering effects linked to oxidation, which entail drastic compositional changes, with neogenesis of resins, asphaltenes, and even insoluble residue.« less

Intertidal clean-up activities and natural regeneration on the Gulf coast of Saudi Arabia from 1991 to 1992 after the 1991 Gulf oil spill

Various clean-up techniques, some of which were done on a trial basis, are described. These were applied to various habitats in the Dawhat ad Dafi and Dawhat al Musallamiya area on the Gulf coast of Saudi Arabia (the study area) and the offshore islands ofKaran and Jana in the Gulf. The results from these clean-up activities are compared with the natural recovery taking place in the area. At present there is little to indicate any biological improvement on the trial clean-up sites over the natural recovery taking place in the impacted area. Natural recovery could be encouraged by certain clean-up activities such as the removal of the tar mats which encrust the upper shores of many sediment shores, precluding the settlement by typical upper shore fauna into these areas. Results from a rapid assessment survey of the Gulf coast of Saudi Arabia, from Kuwait to Dammam, are discussed briefly. Current clean-up activities (January 1993) taking place in the study area are also briefly described.

Petroleum Column Heterogeneity as a Geological Tool: Origin and Significance of Tar Mats

Compositional heterogeneity in liquid petroleum columns can be related to variation in (1) the maturities and facies of the source rocks feeding the field, (2) the complexity of the carrier beds acting as filling conduits, (3) the controls of physical mixing, diffusion and in-reservoir adsorption on the homogenization of heterogeneities inherited by the field during its filling, and (4) alteration of petroleum composition post trapping. The absence of convection in most liquid petroleum columns plus the inability of diffusive mixing in petroleum columns to homogenize large scale (1 km) heterogeneity even on a geological time scale, allows the petroleum geologist/geochemist to use these heterogeneities as geological tools. Tar mats, or viscous oil barriers, are commonly found in paraffinic oil reservoirs (as, e.g., in the North Sea Oseberg and Ula fields, and in Prudoe Bay field, Alaska). They commonly represent zones of from 1-25 m of asphaltene-rich oil (typically around 50% asphaltenes) sometimes at oil/water contacts with sharp compositional contrast and contact with the rest of the oil column. They are significant in that they represent potential flow barriers, often petroleum columns with tar mats have production and safety problems associated with deasphalting during production. Although there has been much debatemore » on their origin, most field evidence is circumstantial. The authors present a new model of tar-mat formation based on physical-chemical considerations and an improved chemical structural model of asphaltenes, and they show with case histories how this information can be used to aid the geologist in geologically interpreting reservoir geochemical information relating to field-charging processes.« less