Identification of petroleum aromatic fraction by comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry

Comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC-TOFMS) is com-mercially available in the 1990s, with the characteristics of large peak capacity, high resolution, high sensitivity, etc. However, its application to the petroleum and geological analyses is just emerging in China and overseas. In this research, the analytical method for petroleum aromatic fraction using GC×GC-TOFMS is set up, via the choice of the column system and optimization of setting parameters, such as temperature programming, modulation time, hot pulse time, flow rate of carrier gas, data acquisition rate and data processing. The results indicate that different polar compounds of aromatic fraction distribute as bands on structured GC×GC chromatogram. Within each band, homologous compounds appear as a roof-tile structure based on the number of substituent residues. The aromatic compounds are identified and characterized according to the GC×GC chromatogram and mass spectra. According to the polarity and the number of rings, aromatic compounds are spatially present on one chromatogram, which directly reflects the distribution characteristics of complex compounds of aromatic hydrocarbons. In addition, quantitative analysis is favored as some overlapped peaks on traditional GC-MS chromatogram have been separated completely on GC×GC. Some heterocyclic atom aromatic compounds at trace level can be clearly identified using this method, for polarity differences from other interfered aromatic compounds. The development of this method and chromatogram recognition offer petroleum geologists a practical example for the application performance of GC×GC-TOFMS.     

Detection of 2-thiaadamantanes in the oil from Well TZ-83 in Tarim Basin and its geological implication

Thermochemical sulfate reduction （TSR） causes increasing of sulfur content in the oils, reduction of oil quality, and produces a toxic and corrosive H2S which may take great adverse effect on health and equipment. Although there are several ways to identify presence of TSR in gas field, yet in the case of oil with low H2S content, it is still very difficult to identify TSR. 2-thiaadamantanes is one of the most effective indicators of TSR. This paper for the first time discovers 2-thiaadamantanes in the oil from Well TZ-83 in Tarim Basin. A silver nitrate silica gel chromatography was used to enrich the oil from low 2-thiaadamantanes content to the level that GC/MS and GC/MS/MS can detect. 2-thiaadamantanes were assigned by comparison of mass spectra with published data, and the fragmental ions in mass spectra were confirmed by MRM mode of GC/MS/MS. H2S gas produced in Tazhong area is thought to be generated from TSR since 2-thiaadamantanes were present in the oil.     

The genesis of H2S in the Weiyuan Gas Field, Sichuan Basin and its evidence

The Sinian Dengying Formation gas pool in Weiyuan is the oldest large-scale sulfur-bearing gas field in China, which has a H2S content ranging from 0.8% to 1.4%. The Cambrian Xixiangchi Formation gas pool discovered recently above the Dengying Formation contains gas geochemical behaviors similar to those of Dengying Formation but different in sulfur isotopes of H2S. Investigations show that though these two Sinian and Cambrian gas pools are separate ones, they share the same Cambrian source rock. The higher dry coefficient, heavier carbon isotopes, sulfur isotopes of sulfide, lower filling of gas pools, formation water characteristics, reservoir properties and H2S distribution, indicate that H2S in both the Sinian and Cambrian gas pools originates from TSR. The sulfur isotopes of sulfates have shown that H2S was formed in respective pools, namely hydrocarbons charged into the pools reacted with the Dengying Formation and the Xixiangchi Formation gypsum （TSR）, respectively, to form H2S. Compared with sulfur isotopes of sulfates in each pool, δ^34S values of H2S are 8‰ lighter for the Dengying Formation pool and 12‰ lighter for the Xixiangchi Formation pool, respectively, which is attributed to the difference in temperatures of TSR occurrence. The reservoir temperature of the Xixiangchi Formation pool is about 40℃ lower than that of the Dengying Formation pool. Temperature plays a controlling role in both the sulfur isotopic fractionation and amounts of H2S generation during TSR.     

Control of coupling among three major factors for formation of high-efficiency gas reservoir——A case study on the oolitic beach gas reservoir in Feixianguan Formation in the northeast Sichuan Basin

Through a case study of the high-efficiency gas reservoir in Feixianguan Formation in the northeast Sichuan Basin, quantitative and semi-quantitative analyses of key elements such as hydrocarbon generation, migration and accumulation, and reservoir evolution as well as their interplay in the critical moment of reservoir formation controlled by the energy field were carried out, by means of numerical modeling of the energy field. It was found that the climax time for Permian hydrocarbon generation was Late Triassic-Early Jurassic and accumulation of oil and gas has resulted in large-scale paleoreservoirs in paleostructural traps in Feixianguan Formation, a process facilitated by fractures connecting the sources. The paleoreservoirs have been turned into high-efficiency gas kitchens due to pyrolysis, which resulted from deep burial at a temperature of 170―210 ℃ as induced by tremendously thick sedimentation in the foreland basin of Daba Mountain in Late Jurassic-Cretaceous period. Meanwhile, abundant acid gas like H2S produced from thermo-chemical sulfate reduction (TSR) at high temperatures leads to extensive dissolution of dolostone in the paleoreservoirs, which may in turn result in modification of the reservoirs and preservation of the reservoir rock porosity. The present distribution of gas reservoirs was ultimately determined in the processes of adjustment, cooling and decompression of the paleoreservoirs resulting from intense deformation in the front of Daba Mountain during the Himalayan orogeny.     

Experimental evidence for formation water promoting crude oil cracking to gas

Crude oil cracking to gas is the key to determining the exploration potential and strategy for deep hydrocarbon resources.Identifying the factors that affect the threshold and potential of crude oil cracking to gas as well as other possible influencing factors will provide the scientific basis for deep hydrocarbon exploration.A comparison of pyrolysis simulation experiments of crude oil,hydrous crude oil,and various water media under a constant temperature(350℃) and pressure(50 MPa) shows that water plays a large role in crude oil cracking to gas.(1) When water is added,the gas yields increase significantly,including those of alkane gases and non-hydrocarbon gases:the yield of alkane gases increases 1.8-3 times;the yields of H2 and CO2 also increase significantly.This means that water takes part in the process of crude oil cracking to gas,and supplies hydrogen.Therefore,the presence of water will dramatically enhance the potential of crude oil cracking to gas.(2) Mg2+ ions in the formation water promote the crude oil + water reaction to some extent and increase the total yield of alkane gases and the yields of both H2 and CO2 ;more interestingly,the i-C4/n-C4 and i-C5/n-C5 ratios increase significantly.This indicates that Mg2+ ions in formation water act as a catalyst,and a disproportionation reaction is involved in the crude oil + water reaction.This study helps us to understand the factors influencing crude oil cracked gas and to evaluate the hydrocarbon resources in deep sedimentary basins.     

Late Yanshan-Himalayan hydrocarbon reservoir adjustment and hydrotherrnal fluid activity in the central Tarim Basin

The adjustment of primary hydrocarbon reservoirs in marine formations is an important feature of the oil pools in the Tarim Basin. Large-scale hydrocarbon adjustment is related to the strong regional tectonic movements, which is always accompanied by extensive migration of basin fluids including diagenetic and mineralizing fluids. Organic fluid inclusions are well developed in hydrothermal minerals, such as fluorite, which have been found in the dissolution-enlarged fractures or karstification caves in the Ordovician in the central Tarim Basin. Proved by well drilling, the fluorite deposit is good reservoir for oil and gas. So the peculiar accompanied or superimposed relationship between fluorite hydrothermal fluid mineralization and hydrocarbon migration and accumulation exists in the Ordovician in the central Tarim Basin. Considering regional tectonic setting and mineralization condition,through different kinds of analytic methods including electron spin resonance dating, fluid inclusion laser Raman and colonial inclusions hydrocarbon fossil analysis, we proposed that extensive mineralizing fluids and hydrocarbon migration occurred in late Yanshan-Himalayan (110.4-30.8 Ma) period, and Himalayan, especially, is an important period for hydrocarbon accumulation from 34.3 Ma to present.     

Relationship between the later strong gas-charging and the improvement of the reservoir capacity in deep Ordovician carbonate reservoir in Tazhong area, Tarim Basin

Some large-scale oil-gas fields have recently been discovered in marine carbonate in China, especially the significant discoveries in deep reservoir that reveals a favorable exploration prospect. Tazhong area is the first-order tectonic unit in Tarim Basin, where there are nearly trillion cubic meters of natural gas resources in the Ordovician limestone reef flat complex in Lianglitage Formation. The reservoir is shelf edge reef flat complex, characterized by ultra-low porosity, low permeability and strong heterogeneous, with a current burial depth of 4500―6500 m. Studies find that the formation and distribution of deep reservoir of the Lianglitage Formation were controlled not only by the early high-energy sedimentary facies and corrosion, but the fracture network formed by the strong gas-charging process since the Himalayan epoch, which played an important role in optimizing and improving reservoir properties. This paper discusses the relationship between the strong later gas-charging and the improvement of the reservoir capacity in deep Ordovician carbonate reservoir, and also builts the corresponding mechanisms and modes, which is favorable for the prediction and evaluation of the advantageous exploration targets.     

Discussion on marine source rocks thermal evolvement patterns in the Tarim Basin and Sichuan Basin, west China

Taking marine source rocks of lower Paleozoic in the Tarim Basin and Paleozoic ones in the Sichuan Basin as examples, their sedimentation process could be classified into four styles continuous subsldence with deep sedimentation in early stage, continuous subsidence with deep sedimentation in later stage, that deeply buried-uplift-shallowly buried, and that shallowly buried-uplift-deeply buried.Unlike that in East China, the marine source rocks evolvement patterns did not accord with sedimentation styles one by one in superimposed basins in west China. Taking local geothermal field into account, four types of source rock evolvement patterns were built that evolved fast in early stage,evolved fast in middle stage, evolved continuously and evolved in multistage. Among them, the 1st pattern contributed little to the present industrial oil pools directly, but paleo-oil reservoirs and gases cracked from crude oils were main exploration targets. Although some gases were found in the 2nd pattern, the scale was not big enough. For the 3rd and 4th patterns, the hydrocarbon potential depended on organic matters maturity in early stage. For relatively low mature rocks, it was possible to generate some oils in later stage; otherwise the main products were gases. Paleo-oil reservoirs remained fairly well in the Sichuan Basin, and most source rocks underwent kerogen-oil-gas processes,which was useful reference to gas exploration in the Tarim Basin.