Immature Oil Research Articles

Multiplex hydrocarbon-generation system in Yangxin Sub-depression and its exploration potential

The shahejie Formation of Paleogene System is the most important hydrocarbon generation interval in Yangxin Sub-depression.Multiplex-set hydrocarbon generation series of strata being of different geological-geo- chemical characteristics were formed because of the staging property of the filling and evolution of Paleogene lake basin and the cyclicity of sedimentary process.The organic carbon content,the parameters of pyrolysis and biomar- ker characters of saturated hydrocarbon was investigated by using the means of Rock-Eval,GC-MS.The Sha-4 hy- drocarbon source rocks formed under the semi-saline to saline water environment,the content of organic matter (TOC) was 1.5% averagely,and most of them was being matured and formed into matured oil;The Sha-3 hydro- carbon source rocks formed under the deep-lake environment with fresh-slight saline water,the content of organic matter(TOC) was 3.5% averagely,and most of them was still immature and formed into immature oil;The Sha-1 immature hydrocarbon source rocks formed under the salt lake-saline water environment,the content of organic mat- ter(TOC) was 5.0% averagely,they are formed into biogas.

OILS FROM CENOZOIC RIFT‐BASINS IN CENTRAL AND NORTHERN THAILAND: SOURCE AND THERMAL MATURITY

Oil is produced from the Suphan Buri, Phitsanulok and Fang Basins onshore central and northern Thailand. Most of the Cenozoic rift‐basins onshore Thailand are 2–4 km deep, but the Phitsanulok Basin is the deepest with a basin‐fill up to 8 km thick. In this basin, the Sirikit field produces ∼18,000–24,000 bbl/day of crude oil. In the Suphan Buri Basin, about 400 bbl/day of crude oil is produced from the U Thong and Sang Kajai fields. Approximately 800 bbl/day of crude oil is produced from the Fang field (Fang Basin), which in reality consists of a number of minor structures including Ban Thi, Pong Nok, San Sai, Nong Yao and Mae Soon. A total of eight oil samples were collected from these structures and from the Sirikit, U Thong and Sang Kajai fields. The oils were subjected to MPLC and HPLC separation and were analysed by gas chromatography (GC) and gas chromatography‐mass spectrometry (GC‐MS and GC‐MS‐MS). The U Thong oil was investigated in more detail by separating the oil into a number of fractions suited for the analysis of various specific compounds.The Sirikit oil appears to be the most mature, whereas the Suphan Buri oils and the oil from the San Sai structure (Fang Basin) are the least mature. Apart from the San Sai oil, the other oils in the Fang Basin are of similar maturity. The oils contain small amounts of asphaltenes and the asphaltene‐free fractions are completely dominated by saturated hydrocarbons (generally >60%). Long‐chain n‐alkanes extend to at least C40 and the oils are thus highly waxy. In general the oils were generated from freshwater lacustrine source rocks containing a large proportion of algal material, as indicated by the presence of long‐chain n‐alkanes, low C3122R/C30 hopane ratios, the presence of 28‐Nor‐spergulane, T26/T25 (tricyclic triterpanes) ratios of 1.07–1.57 and tetracyclic polyprenoid (TTP) ratios close to 1. Occasional saline conditions may have occurred during deposition of the Sirikit, Ban Thi and Pong Nok source rocks. The Fang Basin oils were sourced from two different kitchens, one feeding the Ban Thi and Pong Nok structures and one feeding the Mae Soon, Nong Yao and San Sai structures.The presence ofcadalene, tetracyclic C24 compounds, oleanane, lupane, bicadinane and trace amounts ofnorpimarane or norisopimarane indicate a contribution from higher land plant organic matter to the oils. The terrestrial organic matter may occur disseminated in the lacustrine facies or concentrated in coal seams associated with the lacustrine mudstones. Thermally immature oil shales (lacustrine mudstones) and coals exposed in numerous basins in central and northern Thailand could upon maturation generate oils with a composition comparable to the investigated oils.

Experimental Study of Decay Conditions of Organic Matter and its Significant for Immature Oil Generation

In order to study the decay conditions of organic matter in early stage of sedimentation, 23 samples were simulated in a low temperature (80°C), long-term condition (>1 year). Gypsum, metals, salt, were used as catalysts, and coniferae and algae were used as original source of oil generation in this experiment. The results indicate that coniferae and algae can produce oil in briny conditions in a very short time (<30 days), and oil was expelled from organic matter at different times dependent on the simulation conditions.

A REVIEW OF GEOLOGICAL DATA THAT CONFLICT WITH THE PARADIGM OF CATAGENIC GENERATION AND MIGRATION OF OIL

The majority of petroleum geologists today agree that the complex problems that surround the origin, generation, migration and accumulation of hydrocarbons can be resolved by accepting the geochemical conclusion that the process originates by catagenic generation in deeply‐buried organically‐rich source rocks. These limited source rock intervals are believed to expel hydrocarbons when they reach organic maturity in oil kitchens. The expelled oil and gas then follow migration pathways to traps at shallower levels. However, there are major geological obstacles that cast doubt upon this interpretation.The restriction of the source rock to a few organically rich levels in a basin forces the conclusion that the basin plumbing system is leaky and allows secondary horizontal and vertical migration through great thicknesses of consolidated sedimentary rocks in which there are numerous permeability barriers that are known to effectively prevent hydrocarbon escape from traps. The sourcing of lenticular traps points to the enclosing impermeable envelope as the logical origin of the trapped hydrocarbons.The lynch‐pin of the catagenic theory of hydrocarbon origin is the expulsion mechanism from deeply‐buried consolidated source rock under high confining pressures. This mechanism is not understood and is termed an “enigma”. Assuming that expulsion does occur, the pathways taken by the hydrocarbons to waiting traps can be ascertained by computer modelling of the basin. However, subsurface and field geological support for purported migration pathways has yet to be provided.Many oilfield studies have shown that oil and gas are preferentially trapped in synchronous highs that were formed during, or very shortly after, the deposition of the charged reservoir. An unresolved problem is how catagenically generated hydrocarbons, expelled during a long‐drawn‐out maturation period, can have filled synchronous highs but have avoided later traps along the assumed migration pathways.From many oilfield studies, it has also been shown that the presence of hydrocarbons inhibits diagenesis and compaction of the reservoir rock. This “Füchtbauer effect” points to not only the early charging of clastic and carbonate reservoirs, but also to the development of permeability barriers below the early‐formed accumulations. These barriers would prevent later hydrocarbon additions during the supposed extended period of expulsion from an oil kitchen.Early‐formed traps that have been sealed diagenetically will retain their charge even if the trap is opened by later structural tilting. Diagenetic traps have been discovered in clastic and carbonate provinces but their recognition as viable exploration targets is discouraged by present‐day assumptions of late hydrocarbon generation and a leaky basin plumbing system.Because there are so many geological realities that cast doubt upon the assumptions that devolve from the paradigm of catagenic generation, the alternative concept of early biogenic generation and accumulation of immature oil, with in‐reservoir cracking during burial, is again worthy of serious consideration. This concept envisages hydrocarbon generation by bacterial activity in many anoxic environments and the charging of synchronous highs from adjacent sources.The resolution of the fundamental problem of hydrocarbon generation and accumulation, which is critical to exploration strategies, should be sought in the light of a thorough knowledge of the geologic factors involved, rather than by computer modelling which may be guided by questionable geochemical assumptions.

Effects of Karanj (Pongamia pinnata Vent) Methanolic Seed Extracts and Fractions on Growth and Development of Plutella xylostella L. (Lep., Yponomeutidae)

ABSTRACT The effects of larval feeding for 48 h on cabbage leaves treated with different concentrations of karanj (Pongamia pinnata Vent) seed methanolic extracts and their fractions were investigated against the biology and food consumption-utilization indices of Plutella xylostella under laboratory conditions. The treatments included 2.5, 5.0, 7.5 and 10.0% of methanol extracts of mature and immature seeds and 0.5, 1.0, 1.5 and 2.0% concentrations of fractions (acetone, chloroform and hexane) to compare with karanj oil (1.0%), acetone and water as controls. The experiments revealed that all the treatments with extracts and fractions of mature and immature seeds and karanj oil (1.0%) adversely affected biological parameters of the pest. At similar concentrations, mature seed extracts and fractions proved better than immature seed and at higher concentrations effects were more pronounced against first than against third instar larvae. Consumption-utilization indices were also adversely affected by all the treatments particularly at higher concentrations.

Geochemistry and genesis of heavy oil in the Erlian Basin

Heavy oil is commonly distributed in the Erlian Basin, North China. It can be divided into two genetic types: the primary and the secondary. Primary heavy oil is generated directly by Cretaceous immature or low-mature hydrocarbon sources, or is derived from differentiation of crude oil during migration. Oil of this type has viscosity varying in the range of 100–5000 mPa · s. By contrast, secondary heavy oil, which has been thickened to various degrees, is resultant from a combination of biodegradation, water-leaching and oxidation of primary heavy oil. Based upon a large number of determinations on the physical properties of crude oil, GC analyses of saturated and aromatic hydrocarbons, and GC-MS analyses of steranes and terpanes, the characteristics of heavy oil of various types have been described. Differences in inspissation degree have made it possible the grouping of heavy oil of the Erlian Basin into an inspissation series, i. e., low maturity and immature heavy oil with viscosity of 100–2000 mPa · s, weakly biodegradated heavy oil with viscosity of 500–5000 mPa · s, moderately biodegradated heavy oil with viscosity of 1000–50000 mPa · s and highly biodegradated heavy oil with viscosity over 50000 mPa · s. With the progress of inspissation, the products vary from common heavy to over-heavy oil. Slope and uplifted areas inside a reservoir basin are most favorable to the inspissation and formation of heavy oil. Interaction of several mechanisms of inspissation has resulted in a widespread distribution of heavy oil within the Erlian Basin.

The evolution characteristics and fractionation mechanism of carbon isotopes in the process of “multi-stage hydrocarbon generation”

The Jiyang Sag and the Liaohe Basin are the two important areas where immature oil resources are distributed in China. From these two areas immature-low mature to mature oil samples were collected for carbon isotopic analysis. The extracts of source rocks are dominant in the Jiyang Sag while crude oils are dominant in the Liaohe Basin. The maturity index, Ro, for source rocks varies from 0.25% (immature) to 0.65% (mature). Studies have shown that within this range of Ro values the extracts of source rocks and crude oils, as well as their fraction components, have experienced observable carbon isotope fractionation. The carbon isotopic values tend to increase with burial depth, the oils become from immature-low mature to mature, and the rules of evolution of oils show a three-stage evolution pattern, i. e., light→heavy→light→heavy oils. Such variation trend seems to be related to the occurrence of two hydrocarbon-generating processes and the main hydrocarbon-forming materials being correspondingly non-hydrocarbons and possessing MAB characteristics, lower thermodynamic effects and other factors. In the process towards the mature stage, with increasing thermodynamic effects, the thermal degradation of kerogens into oil has become the leading factor, and correspondingly the bond-breaking ratio of12C-13C also increases, making the relatively12C-rich materials at the low mature stage evolve again towards13C enrichment.

THE GENERATION OF IMMATURE OILS IN THE LACUSTRINE JIYANG MEGA‐DEPRESSION, BOHAI BAY BASIN, CHINA

The lacustrine Jiyang mega‐depression (Bohai Bay Basin, eastern China) produces important volumes of immature oils. These oils were generated at shallow burial depths by source rocks which have not entered the conventional oil window. Oil‐source correlation studies have determined that the oils were mainly generated by source rock intervals in the Es4 and Es1 Members of the Paleogene Shahejie Formation, which were deposited in saline‐ to brackish‐water and brackish‐water lacustrine conditions, respectively. Minor quantities of oil came from Es3 Member source rocks deposited in freshwater conditions.Samples of the Es4, Es3 and Es1 source rocks underwent artificial maturation at low temperatures (100–400°C). For the Es4 sample, the hydrocarbon generation characteristics of soluble organic matter and kerogen were investigated. The results of the experiments show that soluble organic matter made a greater contribution than kerogen to the immature oil (76% versus 24%). A comparison of the results of the thermal simulation experiments with the naturally‐occurring maturation profile shows that various factors influenced the generation of the immature oils. Firstly, the Es4 and Es1 Members include intervals enriched in algal material. Secondly, most of the immature oil is derived from soluble organic matter. Finally, the saline depositional environment was favorable for the preservation of organic matter.

Quantitative Assessment of Hydrocarbon Expulsion of Petroleum Systems in the Niuzhuang Sag, Bohai Bay Basin, East China

Abstract Based on a detailed survey of the distribution and organic geochemical characteristics of potential source rocks in the South Slope of the Niuzhuang Sag, Bohai Bay Basin, eastern China, a new approach to assess the amount of hydrocarbons generated and expelled has been developed. The approach is applicable to evaluate hydrocarbons with different genetic mechanisms. The results show that the models for hydrocarbon generation and expulsion vary with potential source rocks, depending on thermal maturity, types of organic matter and paleoenvironment. Hydrocarbons are mostly generated and expelled from source rocks within the normal oil window. It was calculated that the special interval (algal‐rich shales of the Es4 member formed in brackish environments) in the South Slope of the Niuzhuang Sag has a much higher potential of immature oil generation than the other intervals in the area. This suggests that hydrocarbons can definitely be generated in early diagenesis, especially under certain special geological settings. The proportion of hydrocarbons generated and expelled from the Es4 shales in the early diagenetic stage is up to 26.75% and 17.36%, respectively. It was also observed that laminated shales have a much higher expulsion efficiency than massive mudstones. In contrast, the special interval of the Es4 shales proposed from previous studies is probably not the whole rock for oil in the South Slope of the Niuzhuang Sag because of the small proportion of the gross volume and corresponding low percentage of hydrocarbons generated and expelled. A much lower expulsion efficiency of the source rock during the early stage relative to that within the normal oil window has been calculated. Our results indicate that the Es4 mudstones rather than the shales deposited in the Niuzhuang and Guangli Sag are the main source rocks for the oil discovered.

Organic petrological studies on immature source rocks

Organic petrology is a marginal science that is quite practicable. At present, it has developed into a routine research tool that is widely applied in petroleum exploration and assessment. Based on several years’ research of the authors, this paper presents the advances in organic petrological studies on immature source rocks, including the classification and characteristics of macerais, the composition of macerais and types of organic matter, the abundance and evolution of organic matter, oil-prone macerais, hydrocarbon generation and expulsion. All these results show that organic petrology is of considerable value pertaining to its application in the assessment of immature oil and gas. The immature source rocks consist of various macerais with obvious heterogeneity, contain different hydrocarbon-generating macerais with different oil thresholds and oil peaks, and show a two-staged evolutionary pattern of organic matter.

Catalytic decarboxylations of fatty acids in immature oil source rocks

Catalytic decarboxylations of fatty acids in immature oil source rocks

Study of coal chemical group composition. Part (II). Optimization of coal dissolution system : Tian, Y. et al. Meitan Zhuanhua, 2002, 25, (1), 28–32. (In Chinese)

Organic-rich samples derived from a Middle Cambrian Formation in the Georgina Basin, and from the Middle Proterozoic of the McArthur Basin in northern and central Australia, yielded alginite ranging from immature oil shale material to overmature residue. A maturation scale has been developed based on the thermal evolution of alginite as determined from reflectance and fluorescence. The coalification path of alginite is marked by jumps in contrast to the linear path of wood-derived vitrinite. Six zones have been recognised, ranging from undermature (zone I), through the mature (zones II/III), followed by a stable stage of no change (zone IV) to the overmature (zones V and VI). The onset of oil generation in alginite as evident from the present study is at 0.3% Ro Alg. and is expressed in a change of fluorescence from yellow to brown, and a coalification jump from 0.3 to 0.6% Ro of Alg. In many boreholes zone III can be distinguished between 0.6 and 0.8% Ro of Alg. where subsequent oil generation occurs. Zones II and III represent the oil window.A zone of little or no change designated zone IV, at 0.91.0% Ro of alginite follows zones II/III. A marked coalification jump characterises zone V, where a pronounced change in reflectance occurs to >1.0% Ro Alg., signifying peak gas generation. The border of oil preservation lies at the transition of zone V and VI, at 1.6% Ro Alg. In zone VI gas generation only occurs.Comparison of reflectance results with experimental and geochemical pyrolysis data supports high activation energies for hydrocarbon generation from alginite, and therefore a later onset of oil generation than other liptinite macerals (i.e. cutinite, exinite, resinite) as well as a narrow oil window.Transmission electron microscopy (TEM) confirms that alginite does not go through a distinct intermediate stage but that the percentage of unreacted organic matter decreases as maturation proceeds. A clear distinction can be made in TEM between immature alginite, alginite after oil generation, and alginite residue following gas generation. Alginite beyond 1.6% Ro acquires very high densities and the appearance of inertinite in TEM.Bitumens/pyrobitumens make a pronounced contribution to the organic matter throughout the basins and have been shown to effect pyrolysis results by suppressing Tmax. The bitumens/pyrobitumens have been divided into four groups, based on their reflectance and morphology, which in turn appears to be an expression of their genetic history. Their significance is in aiding the understanding of the basins' thermal history, and the timing of oil and gas generation.

Catalytic decarboxylations of fatty acids in immature oil source rocks

Catalytic decarboxylations of fatty acids in immature oil source rock samples were examined in this study. The rock samples were obtained from seven oil fields in China. In order to clarify the effect of each mineral matter in the rock samples, both the Fe Mossbauer effect and the X-ray diffraction (XRD) were used to determine the relative content of each mineral in the rock samples, and the catalytic activities of several minerals like clays, carbonates and pyrite were determined. The Fe Mossbauer effect and the XRD studies show that clays are the main mineral components in the rock samples except for the samples from Biyang and Jianghan in which the main mineral component is ankerite. The other mineral components include calcite, plagioclase, quartz, feldspar, siderite, aragonite, pyrite, analcime, pyroxene and anhydrite. The studies of the catalytic decarboxylations of fatty acids suggest that carbonates and pyrite can make much greater contributions to the catalytic activities of the rock samples than clays. It is found that the overall catalytic activities of the rock samples are well related to the relative contents and the catalytic activities of clays, carbonates and pyrite in the rock samples.

Distribution of aromatic biomarkers in pyrolysates of coccolithophore

Analyses of the original and pyrolytic products of coccolithophore at various temperatures suggest that the contribution of coccolithophore to the formation of large immature oil reservoirs should attract keen interest. Through biochemical processes algae can change inorganic sulphur into organic sulphur, which could be one of the most important precursors of organic sulphur compounds in oil and source rocks. When Methylphenanthrene Index (MPI) and Methylphenanthrene Ratio (MPR) indices were used to evaluate the evolution degree of source rocks and oil maturity, other maturity indices must be used together for correction. In low maturity, the relative abundances of dibenzothiophene (DBT), fluorine (F) and dibenzofuran (DBF) can be used to identify the oxidation-reduction environments.

Carbon isotopic composition of an isoprenoid-rich oil and its potential source rock

Despite the multiple controls on the carbon isotopic composition of organic matter, δ13C values for saturated and aromatic hydrocarbon fractions for over 300 Norwegian North Sea (Central Graben) oils range from −26 to −31‰ and from −24 to −30‰, respectively. However, for one oil (well 2/2-5), δ13C values for the saturate and aromatic fractions are −21.5‰ and −22‰, significantly enriched in 13C relative to the isotopic compositions of all oil fractions investigated. The saturate hydrocarbon fraction of the 2/2-5 oil is comprised predominantly of low-molecular-weight (<C20), primarily regular, acyclic isoprenoids. Also present are C36–C40 irregular acyclic isoprenoids; these compounds are composed of various combinations of tail-to-tail condensed C15–C20 regular isoprenoid subunits, resulting in pseudo-homologous series with six to nine methyl groups. The isotopic compositions of both high- and low-molecular-weight isoprenoids are around −19‰. This is in marked contrast to n-alkane δ13C values (ca. −31‰), and it is clear that isoprenoid δ13C values dictate the δ13C value of the saturate fraction. The same 13C-enriched isoprenoids are present in a thin (<1 m) immature oil shale facies (hudlestoni biozone) of the Kimmeridge clay formation (KCF; Dorset, UK). Moreover, the relatively immature KCF sediment contains unique high-molecular-weight isoprenoid thiophenes and thianes, whose structures indicate that sulfur has reacted with functional groups in the isoprenoid precursor's interiors. This suggests that these novel high-molecular-weight isoprenoids were probably biosynthesized as such. Reasons for their profound 13C-enrichment and predominance are discussed. Although the KCF facies, located in the southern UK, cannot be a direct source of the North Sea 2/2-5 oil, a related North Sea facies seems plausible since an isotopically enriched TOC zone has been reported in a well also from the Central Graben.

CS2/NMP extraction of immature source rock concentrates

Eleven Chinese immature source rock concentrates from the immature oil formations in four different depressions were extracted ultrasonically with a mixture of CS2 and N-methyl-2-pyrrolidinone (CS2/NMP) at room temperature. The samples were also extracted with CHCl3 and a mixture of methanol/acetone/chloroform (MAC) for comparison. The solvent system CS2/NMP is very efficient for the extraction of immature source rock concentrates, giving much higher extraction yields than CHCl3 or MAC. The composition of the extracts using different solvent mixtures is also different. Model compound tests indicate that no chemical reactions have taken place between the NMP and the substrates in the extraction. These results suggest that there are abundant non-covalent bond interactions in the organic matter of the immature source rock concentrates. The fact that CS2/NMP mixed solvent extracts more than MAC and CHCl3 is not only because it can dissolve higher molecular weight fractions, but also because it has stronger ability to disrupt the complex interactions existing between the soluble and insoluble fractions. Biomarker distributions in the saturated hydrocarbon fractions are different for different solvent systems, suggesting that care should be taken when comparing the biomarker parameters in source rocks when using different solvents for extraction.

OCCURRENCE AND DISTRIBUTION OF n-ALKANESAND n-FATTY ACIDS IN SAUDI ARABIAN CRUDE OILS

ABSTRACT Ten crude oil samples, covering wide range of maturity (API gravity = 18·5–36·1), were assembled from Safaniya, Abqiq, Ain Dar, Wafra. Marjan and Zuluf oil fields in the area of Arabian Gulf. n-Alkanes of were separated from the petroleum distillate ((150°C–450°C) of the crude oils by urea adduction. n-Fatty acids were separated from the fraction of n-alkanes by treatment with aqueous solution of KOH. Distribution of n-alkanes and n-fatty acids has been investigated by means of gas chromatography. The studied crude oils showed symmetrical distribution curves of n-paraffins and fatty acids of low molecular weight were abundant as compared with n-paraffins. The n-paraffins distribution curve of Wafra/Iucene biodegraded immature crude oil showed three maxima at C17, C19, and C31 whereas the maxima of n-fatty acids are located at C14,C22and C24. The results were interpreted in terms of origin, maturation and depositional environments of the crude oils.

Immature crude oils in the salt lake depositional environment are related to organic matter precipitated at stage of carbonate in salt lake sedimentation sequences

The present paper studies the mineralogy and geochemistry of a geological section in Jianghan basin. Widely distributed immature oils in this basin have been shown to relate to the carbonate depositional stage of salt lake sedimentation cycle. It is concluded that only when the content of organic carbon in the rocks is taken into account, is it meaningful to calculate the immature oil in the source rocks.

Catalytic decarboxylation of fatty acid by iron-containing minerals in immature oil source rocks at low temperature

The catalytic activities of seven immature source rocks in China on fatty acid decarboxylation for generating hydrocarbons have been evaluated by determinating the CO2 content from decarboxylation reaction of stearic acid. It has been found that the catalytic activities of immature source rocks mainly depend on the structure type of the minerals. In general, the catalytic activities of carbonates are higher than those of clays at low temperature. The higher its iron content, the higher the catalytic activity of carbonate.

Simulation experiment of immature oil genetic mechanism in lake facies of semi-salt water

The Immature calcareous shale in the south slope of Dongying sag, Jiyang depression, plays a significant role in immature oil genesis, and is sampled to make simulation experiment. The geochemical characters of the products of soluble organic matter and kerogen are researched respectively. The yields of the two components and their relations are made out. The contribution ratio between soluble organic matter and kerogen undergoing earty degradation in lake facies of semi-salt water is figured out for the first time, so the research into the genetic mechanism of immature oil is developed.