Search

The Muglad Basin is one of the largest petroliferous basins within the Central and Western African Rift System (WCARS). A total of 42 oil samples including light, normal and heavy crude oils from the Cretaceous reservoirs of the Muglad Basin were systematically analyzed to fully understand their geochemical characteristics and possible origins. The results of oil–oil correlation show that these Cretaceous crude oils are genetically identical and derive from the same source kitchen. Differences in physical properties and geochemical compositions may be attributed to secondary alteration, such as biodegradation. Lightly, moderately and severely biodegraded oils can be divided on the basis of the relative abundance of n–alkanes and acyclic isoprenoids in the oils and the presence of “humps” on gas chromatogram baselines, as well as of the relative abundance of 25–norhopanes. The biodegradation degree of Cretaceous oils in the Muglad Basin is closely related to the burial depth of oil reservoirs and the distance between the locations of oil reservoirs and adjacent faults. This study may have practical application for further petroleum exploration and development in the Muglad Basin.

Systematic geochemical analysis of 39 crude oils from the Termit Basin, Niger, all belonging to the same oil family, reveals distinct differences in bulk properties and molecular compositions, which were undoubtedly caused by differences in the extent of biodegradation. Various degrees of microbial alteration were identified in crude oils, which were classified as non-, slightly, moderately, or heavily degraded oils, according to their physical properties, bulk chemical compositions, and certain molecular biomarker characteristics. Examination of the possible origins and geological backgrounds of the biodegraded oils shows that reservoirs with deeper burial depths (greater than 1800 m), thicker caprocks (>100 m), relatively higher stratigraphic temperatures (>80 °C), and which have been subject to the influence of silent faults had not suffered significant biodegradation. The burial depth of reservoirs is therefore considered to be the main factor controlling the biodegradation process, and may offset other factors if the depth is sufficient. The Paleogene reservoirs in the Dinga Step-Fault Zone (DSFZ) that meet all of these conditions are therefore dominated by normal oils. Degraded oils of varying degrees are widely distributed in other tectonic units where at least one of these factors is absent. This study may have practical application for further petroleum exploration, prediction of petroleum properties, and the reduction of production risk.

Based on the thermodynamic stabilities and molecular simulation, two trimethyldibenzothiophene molecular indicators, that is, 2,4,6-/(1,4,6 + 1,4,8 + 3,4,6)-trimethyldibenzothiophene (TMDBT) and (2,4,7 + 2,4,8)-/(1,4,6 + 1,4,8 + 3,4,6)-TMDBT (the numbers indicate the position of methyl-substitute), were proposed to trace subsurface oil migration orientations and reservoir filling pathways. In this paper, we successfully applied the aforesaid parameters both in an Ordovician carbonate reservoir of the Tuoputai region of the Tarim Basin (NW China), and in an Eocene clastic lacustrine reservoir of the Fushan Depression in the Beibuwan Basin, South China Sea. Besides the migration fractionation is proven the main control factor for the relative contents of TMDBT isomers in corresponding oils, which indicates that the trimethyldibenzothiophene molecular parameters can serve as the effective oil migration tracer in an oil reservoir, especially in high to over mature and severely biodegraded oils.

Dibenzothiophenes (DBTs) and benzo[ b ]naphthothiophenes (BNTs) are important sulfur heterocyclic aromatic compounds in oils and sedimentary rock extracts. Based on both migration fractionation effects and differences in thermal stability of different isomers, DBTs and BNTs can be used to trace oil migration orientations and filling pathways. They are present in significant abundances in oils, including light oils and condensates, in which biomarkers are commonly absent or present only in extremely low concentrations. Molecular biomarker compositions indicate that all oils and condensates in the Eocene clastic reservoir of the Fushan Depression (Beibuwan Basin, South China Sea) belong to a single oil population. In this study, three geochemical indicators relating to DBTs and BNTs, namely; (1), 4-/1-methyldibenzothiophene (4-/1-MDBT); (2), total content of DBTs; and (3), benzo[ b ]naphtha[2,1- d ]thiophene/(benzo[ b ]naphtha[2,1- d ]thiophene+benzo[ b ]naphtho[1,2- d ]thiophene) ([1,2]BNT/([1,2]BNT+[1,2]BNT)) were applied to trace oil migration orientations and filling pathways. The results show that these parameter values gradually decrease from northeast to southwest, towards the Huachang Uplift, and from north to south towards the Bailian Faulted-Belts. Therefore, the source kitchen for identified oil accumulations in the Huachang Uplift is likely to be located in the Bailian Sag, which lies to the northeast of the Huachang Uplift. Zones in the Bailian Sag, situated upstream of the preferred oil filling pathways are likely to be the most favorable prospecting regions. It is concluded that relative parameters of DBTs and BNTs are practical molecular indicators for tracing oil migration orientations and filling pathways.

The well RP3–1 lies in the Tarim Basin, which is located in the southern Xinjiang Uygur Autonomous Region in northwest China. This region is one of the world's largest frontier basins. Drilling data that we collected on the well RP3–1 reveal a 2398–7066 m thick sedimentary sequence, containing Ordovician to Tertiary strata. The Yijianfang (O2yj) formation of the Middle Ordovician sequence is one of the most important reservoir stratums. The O2yj formation consists mainly of bioclast and intraclast limestones. Two different fluid types are identified by the associations they have with veinlets, which are colors caused by petroleum inclusions under UV light. Early-stage petroleum fluids are characterized by a blue-white color under UV-epifluorescence microscopy, and early-stage aqueous fluid inclusions are mainly homogenized between 54 and 102 °C, with salinities ranging from 12.0 to 16.7 wt% NaCl equiv. Late-stage petroleum fluids are characterized by a yellow color under UV-epifluorescence microscopy, and late-stage, aqueous-fluid inclusions are mainly homogenized between 85 and 135 °C, with salinities ranging from 2.0 to 8.0 wt% NaCl equiv. In order to obtain accurate measurements of both the trapped temperature (Ttrap) and pressure (Ptrap) of these fluids, we use 10 pairs of petroleum inclusions and coeval aqueous fluid inclusions to measure the intersection of a number of isochors. The Ttrap of early-stage fluids we surveyed were between 82 and 97 °C, with the Ptrap ranging from 97 to 259 bars. In contrast, the Ttrap of late-stage fluids were between 141 and 170 °C, with Ptrap values ranging from 430 to 608 bars. When combined with the regional buried history, the early-stage fluids charged from 422 Ma to 412 Ma BP, and the late-stage fluids charged from 20 Ma to 6 Ma BP.

The Tabei Uplift is one of the largest petroleum accumulation region in China. After decades of exploration and development a number of commercial oil reservoirs have been discovered in the Mesozoic formations suggesting a great potential for hydrocarbons. The Mesozoic oils were sampled from 18 wells in the Tabei Uplift. A total of 6 Ordovician oil samples were also collected for contrast analysis. On the basis of oil–oil correlation, all of the 24 crude oils belong to the same oil population and probably derived from the same source kitchen. Despite the coexistence of “UCM”,25-norhopane and intact n -alkanes which is generally interpreted as twice oil filling history, the homogenization temperature ( T h ) of brine inclusion indicates a single oil filling event which happened at 4–3 Ma, with the major frequency ranging from 110 °C to 120 °C. By means of T s /( T s + T m ), MPI−1, 4-/1-MDBT and total concentration of carbazole compounds, the oil filling orientation in Yakela and Yuqi areas were from NEE to SWW and NW to SE, respectively. The Luntai fault is believed to be an oil-migrating fault connecting the Ordovician reservoirs with Mesozoic reservoirs. Based on the physical properties, the geochemical characteristics and the oil filling histories, it can be concluded that the Mesozoic reservoirs in the Tabei Uplift were of secondary oil accumulations derived from the deeper Ordovician reservoirs.

The Paleogene Xingouzui Formation is one of the exploration targets in the Jiangling Depression from Jianghan Basin, Central China. However, the origin and source of crude oils remains controversial. In the present work, a total of 16 fresh source rocks and 23 oil samples were collected and geochemically investigated to clarify the source of organic matters, sedimentary environment and evolution degree. The Xingouzui Formation source rock samples are mainly of fair−good source rocks, with the TOC content, S1 and S2 of 0.36%–1.36% (mean of 0.74%), 0.14–0.38 (mean of 0.27 mg HC/g rock) and (mean of 1.12 mg HC/g rock), respectively. And these rocks are dominated by type Ⅱ2 kerogen within “oil window” maturity stage, with the Ro value of 0.6%–1.2% (mean of 0.9%). The biomarker results indicate that the source rocks of the Xingouzui Formation are mainly deposited in anoxic reducing environment, which main source materials are the mixture of terrestrial higher plants and lower marine organisms. The Pr/Ph values of the oils and source rocks range of 0.47–0.64 and 0.23–1.07, respectively. And the δ13C values of the oils and source rocks range from −34.93‰ to −26.41‰ and from −28.3‰ to −23.8‰, respectively. Family-I and family-Ⅱ oils were separated based on the results of biomarkers and stable carbon isotope, indicating that family-I oils, mainly occurring in the Wancheng fault zone and Gonggan slope zone, are characterized by relatively lower homohopane series, higher mature, and less input of terrestrial higher plants. Comparatively, family-Ⅱ oils mainly occur in the Jingzhou anticlinal zone, with the characterization of small amount of homohopane series, less mature and more terrigenous organic matter input. Family-I oils mainly originate from of the Ⅲ and Ⅱ groups of the lower Xingouzui Formation (Ex1-Ⅲ and Ex1-Ⅱ) source rocks in the Wancheng fault zone and less contribution from Ex1-Ⅱ source rocks of the Shiqiao monoclinal zone; while family-Ⅱ oils are mainly derived from the Ex1-Ⅲ source rocks in the north of Jingzhou anticlinal, and a small amount comes from the Ex1-Ⅱ source rocks of the Shiqiao monoclinal zone. In addition, Ex1-Ⅱ and Ex1-Ⅲ source rocks in the Wancheng fault zone show some contribution to family-Ⅱ2 oils. This work could provide scientific theoretical guidance for petroleum exploration and exploitation in the Jiangling depression.

Carbazole (CA) and alkylated carbazoles are common nitrogen-containing heterocyclic aromatic compounds widely found in crude oils and sedimentary rocks extracts. Their geochemical significances in petroleum exploration have been widely investigated in previous studies. Using density functional theory (DFT) and molecular dynamics (MD) simulation, we calculated the polarity of alkylated carbazole isomers/homologues and their adsorption energy with water and α-quartz, respectively. The calculated results show that there are significant differences in migration fractionation effects for alkylated carbazole isomers/homologues. This study confirmed the validity of previously proposed carbazole migration indicators, i.e. 4-/1-MCA (the relative abundance of 4-methylcarbazole to 1-methylcarbazole), 2,5-/1,8-DMCA (2,5-dimethylcarbazole to 1,8-dimethylcarbazole) and 2,4-/1,8-DMCA (2,4-dimethylcarbazole to 1,8-dimethylcarbazole) on the basis of quantum chemistry and molecular simulation. All these three parameters have been successfully applied in a lacustrine sandstone reservoir in the Fushan Depression, Beibuwan Basin (South China Sea). The calculations of Gibbs free energy show that the differences in thermal stability between the isomers of alkylcarbazoles can be negligible. The source input and depositional environment have no significant effect on these indicators for oils from a same oil family. Hence, the values of these parameters are mainly controlled by subsurface petroleum migration processes. Therefore, the molecular parameters relative to alkylated carbazoles can be used as practically effective geochemical markers to trace the oil filling orientation and pathways for subsurface oil migration.