Distribution Of Bitumen Research Articles

Effect of solid bitumen on pore structure of ultra-deep carbonate reservoir and its logging calculation method

An amount of highly evolved bitumen has been discovered in carbonate reservoirs during ultra-deep oil and gas exploration. This bitumen damages the pore structure and is difficult to dissolve using chemical reagents. To address the gap in studying highly evolved bitumen’s impact on carbonate reservoirs. This study utilized samples from the second member of the Dengying Formation (referred to as Deng 2) in Sichuan Basin. Using thin-section observation, and full-diameter CT scanning, were employed to establish two digital models: with bitumen and without. The study aims to observe bitumen distribution characteristics, compare physical properties and pore structures of two digital cores, clarify bitumen content and its impact on the reservoir, and establish a regression equation. The results indicate: (1) Bitumen in Deng 2 exhibits three states: semi-filled, fully filled, and combined cementation; (2) Properties of the bitumen-containing model decline, with significant reductions in pore and throat numbers. Bitumen primarily occupies large-pore spaces with high coordination numbers and blocks large-radius, long-throat spaces; (3) A bitumen calculation model was established using digital core analysis and well logging parameters, demonstrating good applicability. In conclusion, this approach offers guidance for studying the impact of bitumen on ultra-deep reservoirs and quantitative well logging analysis.

PETROGRAPHY OF PYROBITUMENS IN MIDDLE – LATE JURASSIC SANDSTONES FROM THERMALLY DEGRADED HYDROCARBON ACCUMULATIONS, EAST GREENLAND

A number of exhumed hydrocarbon traps have been described from the Traill Ø region of East Greenland. This study focuses on the Bjørnedal area where the distribution of bitumen has been mapped out. Bitumen staining clearly has a cross‐cutting relationship to stratigraphic units and can be shown to form distinct palaeo‐accumulations. Detailed petrographic studies show that bitumen occurs as late diagenetic phases in Middle to Late Jurassic sandstones, and is present both as both grain‐coating and pore‐filling phases. Geochemical analyses confirm that the bitumen is organic in composition and is composed largely of carbon and hydrogen. Both H/C ratios and bonds identified by FTIR behave as expected with increasing maturity measured using bitumen reflectance. Together, these results provide strong evidence that the material is pyrobitumen derived from the in situ thermal degradation of a liquid hydrocarbon precursor. On the basis of textures in transmitted and reflected light and quantitative bitumen reflectance distributions, two populations of pyrobitumen may be recognised in some samples.Two phases of Paleogene magmatism occurred in the Traill Ø region. The first late Paleocene – early Eocene phase was related to the opening of the northern North Atlantic in the earliest Eocene, and was experienced throughout East Greenland and the northwest European margin. The later magmatic phase was related to the gradual separation of the Jan Mayen microcontinent from East Greenland through the late Eocene – early Oligocene. A single pyrobitumen phase is recognised in accumulations only affected by the early magmatism, and a second phase is only observed in areas affected by both the early and later magmatism. This relationship is interpreted as evidence for a direct relationship between magmatic phases and bitumen generation. The presence of bitumen formed by the thermal degradation of liquid hydrocarbons during the later magmatic event suggests that a viable petroleum system remained active following the early magmatic event.

Computational fluid dynamics investigation of bitumen residues in oil sands tailings transport in an industrial horizontal pipe

Pipeline transport is commonly used in the oil sand industry to convey crushed oil sand ores and tailings. Bitumen residues in the oil sand tailings can be a threat to the environment that separating them from tailings before disposal is crucial. However, low bitumen concentration in the tailing slurry and the complex transport characteristics of the four-phase mixture make the process difficult. This study establishes an Eulerian–Eulerian (E–E) computational fluid dynamics model for an industrial-scale oil sand tailings pipeline. A comprehensive sensitivity analysis was conducted on the selection of carrier-solid and solid-bitumen drag models. The combination of small and large particle sizes (i.e., 75 and 700 μm) and bitumen droplet size (i.e., 400 μm) provided good agreement with field data in velocity profiles and pressure drop. The validated model was subsequently extended to investigate the influence of the secondary phase (i.e., bitumen droplets and bubbles) on flow characteristics in a tailing pipeline. The investigation covered a range of bitumen droplet size (100–400 μm), bitumen fraction (0.0025–0.1), bubble size (5–1000 μm), and bubble fraction (0.0025–0.3) and their influences on the velocity, solids, and bitumen distribution are revealed. For an optimum bubble size of 500 μm, a maximum recovery of 59% from the top 50% and 83% from the top 75% of the pipe cross section was obtained. The present study demonstrates the preferential distribution of bitumen and provides valuable insight into bitumen recovery from an industrial-scale tailing pipeline.

Characterization of crude oil charging of Xiaoerbulake dolomite in the Tarim basin: Insights from bitumen distribution

Understanding hydrocarbon charging mechanisms is essential for reservoir quality evaluation. In this study, a systematic investigation of micro-nano scale oil charging characteristics in the Lower Cambrian Xiaoerbulake Formation of the Tarim Basin was performed via mercury intrusion capillary pressure (MICP) on samples of four types of dolomitic lithofacies before and after bitumen removal. Additionally, to determine the pore structure and space that was occupied by the bitumen, a series of observations using both optical and field-emission scanning electron microscopy (FE-SEM), along with micro-computed tomography (micro-CT) were conducted. A combination of these micro-CT and MICP techniques enabled us to determine how bitumen is distributed in micro-nano pore space of the bulk sample. Imaging results show that three types of pores (intergranular pores, intercrystalline pores and intragranular pores) and microfractures are developed within these lithofacies and confirmed that bitumen has occupied pores >20 nm in diameters. Furthermore, a relationship between pore-throat threshold and charging force was established under the reservoir conditions. It was found that the oil charging force and pore-throat threshold are different for each lithofacies with the microbial dolomites demonstrating the highest charging force and smallest pore throat threshold; as a consequence, they are the most favorable lithofacies for the accumulation and migration of oil and gas which should be considered for future exploration efforts in dolomitic reservoirs.

АНАЛІЗ ФІЗИКО-МЕХАНІЧНИХ ХАРАКТЕРИСТИК ТЕПЛИХ АСФАЛЬТОБЕТОННИХ СУМІШЕЙ НА ОСНОВІ БІТУМУ З ДОДАВАННЯМ ЕНЕРГОЗБЕРІГАЮЧИХ ДОБАВОК ТА СПІНЕНОГО БІТУМУ

Introduction. The article investigates asphalt concrete mixtures made by standard technology according to [1], [2] and so-called warm asphalt concrete mixtures made at low technological temperatures on the basis of bitumen with the addition of energy-saving additives and foamed bitumen. Problem statement. Considering the trends of the modern world, the problem arises regarding the effective use of energy resources. In conditions of increase the volumes of road works one of actual questions which arises at preparation of asphalt concrete mix, is the energy saving for its manufacturing. The main direction in improving the technology of preparation of asphalt mixtures is to improve the adhesive properties that provide strong and stable adhesion to the pavement surface of mineral materials of different origins and adhesion between layers at low temperatures. Therefore, in the development of new technologies and methods of use of paving bitumen for the preparation of asphalt mixtures, the issue of their adhesion to the pavement surface of mineral materials has to be central. At that the adhesion means the connection or interaction (adhesion) between the surfaces of two dissimilar contact layers - bitumen and mineral material [9]. Improving the properties of bitumen under normal conditions is reached by using additional expensive surfactants, which complicate the manufacturing technology and increase the cost of asphalt concrete [9]. Improving of adhesion can be reached without use of expensive modifiers, using bitumen foaming technology, which reduces the surface tension of bitumen and thus facilitate the mixing process, promotes better coverage and uniform distribution of bitumen on the surface of mineral grains. This allows to reduce the temperature of the asphalt mixture preparation and as a result - to provide significant energy savings. The formation of thinner bitumen films on the mineral surface reduces the required volume of binder and thus reduces the cost of the asphalt mixture. Lower technological temperatures cause less thermal aging of bitumen, which in turn leads to improved quality of asphalt concrete and increased durability of pavements [9]. Purpose. The purpose of research was to study the feasibility of use of asphalt concrete mixtures made at low technological temperatures on the basis of bitumen with the addition of energy-saving additives and foamed bitumen. Materials and methods. Experimental comparative tests of traditional hot asphalt concrete mixtures with miscellaneous content of the output, foamed bitumen and bitumen with the addition of energy-saving additives were performed. Results. The feasibility of warm asphalt concrete mixtures use has been established. Recommendations on technological parameters of preparation, transportation, laying and compaction of such mixtures are presented. Conclusions. Performed studies have shown that the use of warm asphalt concrete technologies can reduce the temperature of hot asphalt concrete mixtures preparing by 20-40 ° C without deterioration of strength characteristics of the pavement in compare with traditional hot asphalt concrete that prepared on the same bitumen. This is reached due to the relatively new physical and chemical effects that lead to decrease in the shear resistance of the mixture during its preparation and compaction. Keywords: Road, asphalt mixtures, binder, energy saving additive, foamed bitumen, pavement, temperature, physical a

Frequency domain electromagnetic and electrical resistivity geophysical investigation of tar sands deposits in the Ijebu Waterside area, Eastern Dahomey Basin, southwestern Nigeria

The use of electromagnetic conductivity and electrical resistivity geophysical techniques has been employed in this study to investigate the occurrence, thickness, and spatial distribution of bitumen deposit around Makun-Omi in the Ijebu Waterside area of southwestern Nigeria. Subsurface apparent conductivity distribution data obtained using Geonic 34-3 equipment along fifteen traverses which sounded from different depths of 7.5, 15, 30 and 60 m at inter-coil separation of 10, 20 and 40 m in vertical and horizontal coupling modes and ground resistivity distribution measurements, obtained using Geopulse Tiger Allied resistivity meter at fourteen Vertical Electrical Sounding (VES) stations, were processed and employed to characterize the subsurface in terms of tar sands distribution. The conductivity and resistivity distribution in the form of a 3D earth model, iso-depth maps, 2D sections generated from the processed conductivity and resistivity data indicate relatively low conductivity <20 mS/m and high resistivity >1300 Ωm values around tar/bitumen impregnated sands which mostly occurred in the southern part of the study area. Clearly defined conductivity and resistivity anomalies which delineate the lateral and vertical occurrence of tar impregnated sands underscore the efficacy of integrating electromagnetic and electrical resistivity geophysical techniques to identify occurrence of economic deposits of tar sands in parts of southwestern Nigeria.

Quick-traffic slurry surfacing mix with orthophosphoric acid

Abstract Bitumen emulsions for slurry surfacing mix technology using oxidized bitumen and hydrochloric and orthophosphoric acids on laboratory DenimoTech bitumen-emulsion plant are made in the work. Methylene blue adsorption index of granite aggregate for use in slurry surfacing mix was investigated. Comparatively mix time and cohesion strength build-up of selected compositions slurry surfacing mix with hydrochloric and orthophosphoric acids depending on the variable content of bitumen emulsion. The advantage of using orthophosphoric acid in slurry surfacing mix according to the cohesion strength build-up criterion has been proved. The importance of correct distribution of bitumen drops in the emulsion was confirmed using a sedimentograph Mastersizer 2000. Two emulsions of the same component composition were compared, which differed in particle size. It has been established that it is not possible to design a slurry surfacing mix using the mix time criterion with the help of coarse bitumen emulsion.

Biodegraded bitumens dispersed in Vendian (Neoproterozoic) rocks of the Khatyspyt Formation, Northeastern Siberia

The organic matter of the Vendian (Neoproterozoic) Khatyspyt Formation was investigated. The new data obtained from the GS and GS-MS analyses include: high UCM humps andheightened peaks of steranes and terpanes against the backdrop of C27+ alkanes on chromatograms of saturated hydrocarbons fractions of five samples; predominance of С21-С26 in n-alkanes distribution; the presence of 12- and 13-monomethylalkanes on chromatograms and demethylatedterpanes on filtered chromatograms (m/z 191 and 177). Besides, dispersed bitumens with biodegraded hydrocarbons have been for the first time identified in carbonate and carbonate-siliceous rocks of the Khatyspyt Formation. Also 8,14-sekohopanes resistant to biodegradation and gammacerane were established. The revealed features of the composition and distribution of hydrocarbons showed that their source was the autochthonous (syngeneic) organic matter of the Khatyspyt Formation,one of the potential hydrocarbon source rock of Northeastern Siberia. The composition and content of bitumen are found to be controlled by the inputs of primary organisms (eukaryotes and prokaryotes)which lived in Vendian seas, by changing redox conditions of sediment deposition and transformation, as well as by relatively little effect of temperature and high degree of biological oxidation in hypergenesis. The patterns of bitumen distribution throughout the section show, that besides the overlying deposits,their accumulations canbe discoveredin the Khatyspyt Formation.

Exhumed hydrocarbon traps on the North Atlantic margin: Stratigraphy, palaeontology, provenance and bitumen distribution, an integrated approach

AbstractPrevious work has demonstrated the presence of a number of exhumed hydrocarbon traps in central East Greenland. Re‐evaluation of the stratigraphy alongside detailed investigation of the occurrence of bitumen within the Mols Bjerge and Laplace Bjerg exhumed hydrocarbon traps provides new perspectives on these structures, as well as the petroleum geology of East Greenland and the wider North Atlantic. Sedimentological and stratigraphic studies, augmented with palynological and provenance investigations, have constrained the dating and correlation of the strata exposed in the Mols Bjerge and Laplace Bjerg. Petrographic analysis, alongside analysis of the bitumen identified, has highlighted a much wider distribution of hydrocarbon than previously recognized. It was previously considered that Jurassic strata formed the main reservoir interval within the Mols Bjerge and Laplace Bjerg exhumed hydrocarbon traps. It is shown here that the reservoir intervals in the Laplace Bjerg trap lie within the Late Triassic Ørsted Dal and Vega Sund members, which contain up to 18% pyrobitumen and were previously misidentified as Jurassic. The Jurassic Bristol Elv Formation is the most extensively bitumen stained unit in the Mols Bjerge trap. However, occurrences of pyrobitumen (up to 3%) are recorded throughout the Triassic stratigraphy, including the Early Triassic Wordie Creek Formation. Faults, thick calcrete development and regionally continuous mudstone units play an important role in compartmentalizing the palaeohydrocarbon accumulations.

Features of the natural bitumen distribution in the Kazan deposits on the territory of the Melekess depression in Tatarstan

This article presents the distribution of natural bitumen in the sediments of the Kazan tier of the middle Permian. Natural bitumens, concentrated in the Permian sediments of the Republic of Tatarstan, occur at shallow depths and are heavy, highly viscous, hypergenically converted oils. In the tectonic relation, the area of maximum Kazan bitumen occurrence is confined to the Melekess depression and the adjacent slope of the South Tatar arch.

Methods of step extraction of rocks of high-carbon formations for the study of the component distribution of bitumen and variability of their basic geochemical parameters

The article describes organic-rich rocks sequential extraction method for the bitumen composition difference. The method step-by-step extracts bitumen first from open and then from closed pores using three organic solvents with increasing polarity. The released bitumen differs in malthens/asphaltenes ratio, the distribution of n-alkanes, the content of terpanes and steranes, and, thus, the values of the biomarker parameters. These parameters show difference in the degree of organic matter thermal maturity, which is relates to different types of rock pore space. At the same time different parameters show different maturity ratio between bitumen from open and closed pores. Changes in the composition of bitumen allow to estimate the migration of hydrocarbon compounds through the well, to investigate the effect of sedimentation conditions on the pore space and the distribution of bitumen. The results show that investigation of bitumen received by one-step extraction from powder can lead to incorrect interpret of obtained results.

Formation mechanism of the Donghe Sandstone residual oil zone in the Tarim Basin, NW China

The residual oil zone (ROZ) with near-residual oil saturation is a naturally water-flooded interval adjacent to an oil-bearing reservoir. It has become a promising development target for the technical progress of enhanced oil recovery. The hydrocarbon show, bitumen distribution and grains containing oil inclusions all indicate the presence of ROZs in the Donghe Sandstone Member of the Tazhong4 and Hade Oilfields in the Tarim Basin. Analysis of the two ROZ examples reveals that a stable paleoreservoir and structural adjustment are significant elements of the ROZ-forming process. The ROZ of the Tazhong4 Oilfield is defined as a reactivated fault ROZ (RFROZ) lying between the paleo oil-water contact (OWC) and producing-OWC. Paleo-hydrocarbons leaked through the reactivated faults, and the current oil saturation has a positive proportion to baffle frequency. The ROZ in the paleo trap northwest of the current reservoir in the Hade Oilfield is defined as a migratory trap ROZ (MTROZ). Paleo-hydrocarbons migrated through connected sand bodies. The current oil saturation in the MTROZ has a positive proportion to baffle frequency and negative proportion to petrophysical properties. The microscopic occurrence state of the residual oil in ROZs was analyzed with direct computed tomography (CT) scanning. The displacement experiments coupled with CT scanning aimed to reveal the different occurrence states of the residual oil in the major stage of natural water flooding in the ROZ-forming process. In the microscopic view, the formation of both RFROZ and MTROZ is a process of clustered flow converting to discontinuous flow, and the clustered flow can be scattered to a larger extent in a high permeability zone. The ROZ development feasibility was preliminarily researched. Low-salinity water flooding after the displacement of the formation water in the etching model provided experimental corroboration for remobilizing the residual oil.

Evaluation of bitumen intervals in the carbonate reservoir, case study in Y oilfield of Iran

ABSTRACTThe bitumen intervals, which were drilled at Y oilfield of Iran, often reduce drilling efficiency. The characteristics of high natural gamma radioactive(GR), low uranium-free gamma(KTH), and poor GR-KTH relationship have been found to detect the bitumen by logging. Conventional porosity logs, Poisson's ratio, the P & S-wave velocity ratio and the wave impedance, derived from logging, can be used to identify the heavy oil bitumen. The bitumen distribution can be predicted by the log interpretation. Implementation of log evaluation allows better solving the bitumen intervals identify problems, and also to helps in enhancing development efficiency in the similar petroleum reservoirs.

Three-dimensional optical tomography of bitumen and clay association in oil sands tailings

Alberta has one of the largest oil reserves in the world. Large-scale commercial oil production from oil sands in Alberta for the past 40years has led to accumulation of tailings water in tailings ponds covering areas ranging over 150km2. Less than 1% of this area has been certified as reclaimed leading to both economic and environmental consequences. Research is underway to reduce tailings ponds reclamation time from decades to weeks by developing new polymer flocculants, better tailings treatment methods and recovering bitumen from tailings. Information about impact of residual bitumen on the shear strength, trafficability, densification, hydraulic conductivity, consolidation, post-reclamation settlement for oil sands tailings is insufficient. Outstanding challenges exist in understanding bitumen and clay interaction in tailings to help with the development of techniques which accelerate clay sedimentation and enhance bitumen recovery. To shed light on the bitumen-clay interactions, here we develop advanced three-dimensional optical tomography approaches approaching sub-micron resolution. In this paper, we report, the first ever Total Internal Reflection Fluorescence (TIRF) microscope tomography for Mature Fine Tailings (MFT) samples to reveal bitumen distribution on clay in MFT. We employ a unique evanescent wave illumination approach as opposed to conventional fluorescence microscopy with enhanced axial resolution and high signal-to-noise ratio. The resolution of TIRF is further improved by using an Axial Super-Resolution Evanescent-wave Tomography (AxSET) technique. The information obtained from this study not only gives evidence of the presence of hydrophilic and oleophilic clays but with aid of 3D reconstruction using advance image processing also validates that bitumen is partially coating some of clay surfaces, thus verifying the presence of biwettable clays in oil sands MFT. The advances from our imaging work can aid the development of bitumen recovery techniques for environmental and economic impact.

Differential fluid migration behaviour and tectonic movement in Lower Silurian and Lower Cambrian shale gas systems in China using isotope geochemistry

Isotope geochemistry has been introduced as a means to trace origin of the hydrocarbon and characterize highly productive shale gas systems recently. To assess the impact of tectonic movement and the sealing of shale gas systems, isotope geochemistry, pressure coefficients and the distribution of bitumens are analysed. Many samples yield isotope geochemical data with typical carbon isotopic reversals (δ13C1 > δ13C2) and hydrogen isotopic reversals (δ13DC2H6 > δ13DCH4) in the Lower Silurian shale gas. Isotopically reversed gases are considered to originate in sealed, self-contained petroleum systems. Besides, isotope “reversals order” degree of shale gas has positive correlation with gas production. Isotopically normal gases from the Lower Cambrian indicate that this formation was a continued relatively open petroleum system when oil and gas generated. The pressure coefficients of the Lower Silurian shale gas reservoir range from 1.45 to 2.03, indicating that the reservoir is overpressurized, whereas the Lower Cambrian shale gas reservoir possesses a normal pressure system. Overpressurization of the Lower Silurian shale gas reservoir also indicates that it is a well-sealed system. The distribution and isotope geochemistry of bitumens in the Sinian dolomite and Cambrian shale suggests that the source rock of the Sinian hydrocarbon is the Cambrian shale. An unconformity induced from tectonic movement during the Tongwan period is interpreted to be the fluid migration tunnel and the cause of the differential shale gas content and production. Finally, the isotopic reversals associated with maturity, pressure coefficients and tectonic evolution can both assess the preservation conditions of the reservoir and explain the differential fluid migration behaviour.

Chemical composition and distribution of the components of valley peat from Amur oblast

The chemical composition of sedge peat from the Egor’evskoe deposit as lowland peat from Amur oblast was characterized. The elemental composition of organic matter and mineral macro- and microcomponents was studied, and the distribution of bitumens, their fractions, and humic acids in the depth of occurrence was revealed. It was found that peat genesis processes occur over the entire vertical profile, and the most converted organic matter is formed at a depth of to 90 cm. The individual composition of alkane fractions consists of C13–C36 normal structure homologues with a carbon preference index somewhat higher than unity; the presence of pristane, phytane, and farnezane was detected. In the fractions of fatty acids, predominantly even compounds from C10 to C30 were identified with special distribution features at the separate stages of peat formation. The analysis of the individual composition of n-alkanes and fatty acids made it possible to supplement botanical characteristics with information on the contribution of macrophytes, algal material, and microorganisms to the formation of the organic matter of peat.

Precipitation of anionic emulsifier with ordinary Portland cement

Cement has traditionally been used to accelerate bitumen emulsion breaking in cold mix asphalt and cold recycling asphalt. For cold emulsion mixtures, the mixing stability of bitumen emulsion is a crucial property, because it determines the distribution of bitumen and eventually affects the microstructure and the strength development of asphalt mixtures. Recent studies have proven that the interaction between cement and emulsifiers causes the destabilization of bitumen emulsions.The objective of this study is to understand the interaction between cement particles and rosin emulsifiers. For this purpose, the Ca2+ ions and rosin emulsifier concentration after filtration were measured to identify the interaction between cement and rosin emulsifiers. The consumed emulsifier increases linearly with the amount of added cement or CaCl2 concentration in the case of diluted rosin emulsifier solutions in which the rosin emulsifier concentration is below the CMC (critical micelle concentration). In the case of concentrated rosin emulsifier solutions (above the CMC), the rosin emulsifier concentration shows a sharp decrease when a certain amount of cement or CaCl2 is added.This study indicates that cement destabilizes anionic bitumen emulsion due to the precipitation of rosin emulsifiers caused by Ca2+ ions which are released by early cement hydration. Further studies on precipitation behavior have shown that micelles of rosin emulsifier can complex Ca2+ ions but do not precipitate. These findings explain why slow-setting bitumen emulsions, which contain a higher concentration of emulsifier, show better mixing stability.

The influence of bitumen on reservoir properties and hydrocarbon accumulation in the Santai Area of the Junggar Basin, North Western China

ABSTRACTBitumen's influence on reservoir properties and hydrocarbon accumulation in Santai area was studied by experiment measures and geochemical evidences. The porosity and permeability of the sandstones containing bitumen increased significantly by dissolving experiments, implying that bitumen strongly influences the reservoir properties. The analysis reveals that there were two stages of hydrocarbon accumulation. In the first stage oil migrated into coarse-grained sandstone and then degraded to bitumen, which deteriorated the reservoir properties. The second stage oil had to migrate into fine-grained sandstones, or the residual pores of the bitumen-containing coarse-grained sandstone. Exploration in this area should avoid the bitumen distribution zone.

Heterogeneous distribution of pyrobitumen attributable to oil cracking and its effect on carbonate reservoirs: Feixianguan Formation in the Jiannan gas field, China

Few previous studies have focused on solid bitumen distribution and its effect on gas reservoir quality during oil cracking. Solid bitumen is commonly found in both gas and nongas reservoirs of the Triassic Feixianguan Formation in the Jiannan gas field. The natural gases are mainly secondary cracking gases of oil generated from source rock of the Permian Wujiaping Formation , and the reservoir experienced temperatures above 150°C (302°F) for about 35 m.y. A relatively narrow range of values and a wide range of values and widespread solid bitumen indicate that oil cracking took place in the gas field. Low concentrations of (commonly 8.0%) can be high-quality gas reservoirs after oil cracking and should be favorable targets for future gas exploration in the northeastern Sichuan Basin and adjacent areas.

Processes involved in the origin and accumulation of hydrocarbon gases in the Yuanba gas field, Sichuan Basin, southwest China

Natural gases in the superimposed Sichuan Basin commonly experienced a history of remigration in marine carbonate reservoirs since the late Cretaceous. The reservoir in the Changxing Formation (P2c) in the Yuanba gas field in the Sichuan Basin is characterized by a great burial depth of 6200–7000 m and a high temperature about 165 °C. The gas dryness is 99.73–99.99%, and δ13C values of methane and ethane are −31.0 to −28.9‰ and −29.9 to −25.6‰, respectively. The chemical and isotopic compositions of natural gases, abundant reservoir solid bitumen, and high reservoir temperature (maximum to 240 °C) indicate that the P2c gases are of sapropelic origin and are derived from oil cracking. The paleo-oil layers, recognized by solid bitumen distribution, were mainly developed in high position traps when the paleo-oil accumulated during the early Jurassic. Reconstructed structural evolution shows the northwest was uplifted sharply and southern part dipped gently to the north in the gas field after oil cracking. Fluid potential analyses based on changes in the structural configuration imply that gas should re-migrate mainly to the northwest. The observations that paleo-oil-water contacts are mainly above the present day gas-water contacts in the northwest traps, and are below present day gas-water contacts in the middle and eastern traps also confirm the gas remigration trend. Currently, high gas production wells are mainly located in northwest traps and in high positions in the middle and eastern traps. Systematic analyses on early paleo-oil accumulation and late gas remigration processes can reduce the economic risks associated with natural gas exploration in the northeastern Sichuan Basin.