Gas-Source Comparison and Hydrocarbon Generation and Expulsion Histories of the Main Source Rocks of the Middle Jurassic Shaximiao Formation, Western Sichuan Basin.

With the discovery and exploration of tight sandstone gas reservoirs in the Sichuan Basin, the Middle Jurassic Shaximiao Formation has gradually become the focus of exploration. However, the gas source of the Shaximiao Formation in the Western Sichuan Basin is still controversial, and research on the hydrocarbon generation and expulsion histories of the primary source rocks lags behind, which restricts the study of tight sandstone gas reservoir dynamics within the Shaximiao Formation in the Western Sichuan Basin and further affects the oil and gas exploration process. This paper, utilizing geochemical parameters, including 82 natural gas components and 68 carbon isotopes, conducted an investigation into the origin identification of natural gas and gas-source comparisons within the Shaximiao Formation in the Western Sichuan Basin. By utilizing basin modeling technology, we reconstructed the histories of hydrocarbon generation and expulsion of the primary source rocks and identified the potential exploration areas. The findings indicate that the natural gas within the Shaximiao Formation in the Western Sichuan Basin is characterized by a typical high-maturity wet gas-dry gas reservoir. It is a thermogenic coal-type gas produced by kerogen cracking, and the gas source is primarily type-III kerogen. The main source of natural gas is the coal-measure source rock of the Upper Triassic Xujiahe 5 Formation, which has mainly experienced two stages of increasing maturity, characterized by "two stages of gas generation and one stage of gas expulsion". The hydrocarbon generation and expulsion histories for the source rocks within the Xujiahe 5 Formation is shown as follows: the source rocks of the Xujiahe 5 Formation reached the hydrocarbon generation threshold by the end of the Early Jurassic and peaked in hydrocarbon generation by the end of the Late Jurassic, and then the natural gas migrated to the paleostructural highs of the Shaximiao Formation to form the paleo-gas reservoir. From the end of the Early Cretaceous, the paleo-gas reservoir underwent adjustments and transformations to evolve into the present-day gas reservoir. The source rocks exhibit substantial potential for the forming of large-medium and extralarge gas fields, with favorable exploration zones concentrated in the southern and central portions of the study area. The research findings can provide a scientific foundation for the next exploration and deployment of natural gas resources within the Shaximiao Formation in the Western Sichuan Basin.

Present geothermal field of the Santos Basin, Brazil.

The Santos Basin, located in the southeastern waters of Brazil, is a passive continental margin basin with the most abundant deepwater petroleum resources in the world discovered to date. However, few studies have been conducted on the present geothermal fields of the Santos Basin, which severely restricts the oil and gas resource evaluation of the basin. This study first utilizes 35 temperature data from 16 post-salt drilling wells and 370 temperature data from 31 pre-salt drilling wells to calculate the post-salt and pre-salt geothermal gradients and terrestrial heat flows in the Santos Basin. Then, the basin simulation software BasinMod 1D is used to quantitatively evaluate the impacts of salt rock sedimentation on the present geothermal fields and the maturity of pre-salt hydrocarbon source rocks. The results demonstrate that the present post-salt geothermal gradient in the Santos Basin is 2.20-3.97 °C/100 m, with an average value of 2.99 °C /100 m, and the post-salt terrestrial heat flow is 54.00-97.32 mW/m2, with an average value of 73.36 mW/m2, while the present pre-salt geothermal gradient is 2.21-2.95 °C/100 m, with an average value of 2.53 °C/100 m, and the pre-salt terrestrial heat flow is 61.85-82.59 mW/m2, with an average value of 70.69 mW/m2. These values are characteristic of a low-temperature geothermal field in a zone with a stable structure. The sedimentation of the salt rock causes a decrease in the temperature of the pre-salt strata, which inhibits pre-salt hydrocarbon source rock maturity, with an inhibition rate of up to 1.32%. The inhibition degree decreases with increasing salt rock thickness. At the same time, the salt rock thickness is positively correlated with the present surface heat flow. The unique distribution of the salt rock and related salt structures lead to present terrestrial heat flow differences among different structural units in the basin. This study is of great significance for evaluating and exploring the pre-salt oil and gas resources in the Santos Basin.

Hydrocarbon Generation and Expulsion Histories of the Upper Permian Longtan Formation in the Eastern Sichuan Basin, Southwest China.

The Upper Permian Longtan Formation is the main source rock of the Lower Triassic Jialingjiang Formation in the Eastern Sichuan Basin, Southwest China. However, studies of its maturity evolution and oil generation and expulsion histories are lacking, which are not conducive to the accumulation dynamics of the Jialingjiang Formation in the Eastern Sichuan Basin. Based on the study of tectono-thermal history and geochemical parameter data of the source rock, this paper uses basin modeling technology to simulate the maturity evolution and hydrocarbon generation and expulsion histories of the Upper Permian Longtan Formation in the Eastern Sichuan Basin. The results show that the Longtan Formation source rock in the Eastern Sichuan Basin entered the oil generation threshold at the middle stage of the Early Jurassic and reached the high-maturity stage in the north and central regions at the late stage of the Early Jurassic, and the maturity did not increase after the late stage of the Middle Jurassic. The source rock had the characteristic of "one-stage oil generation and one-stage oil expulsion"; the corresponding period of high oil expulsion was 182-174 Ma (the late stage of the Early Jurassic), which was later than the formation time of the trap of the Jialingjiang Formation, possibly providing oil sources for the paleo-oil reservoirs of the Jialingjiang Formation. The results are of great significance to the gas accumulation process and exploration decision-making in the Eastern Sichuan Basin.

Strata temperatures and geothermal resource evaluation in the Dongpu Depression, Bohai Bay Basin, North China.

The development of geothermal resources in the Dongpu Depression can improve not only the economic benefits of the oilfield but also the ecological environment. Therefore, it is necessary to evaluate the geothermal resources in the region. Based on the heat flow, geothermal gradient and thermal properties, the temperatures and their distribution in different strata are calculated using geothermal methods, and the geothermal resource types of the Dongpu Depression are identified. The results show that the geothermal resources include low-temperature, medium-temperature and high-temperature geothermal resources in the Dongpu Depression. The Minghuazhen and Guantao Formations mainly include low-temperature and medium-temperature geothermal resources; the Dongying and Shahejie Formations include low-temperature, medium-temperature and high-temperature geothermal resources; the Ordovician rocks mainly include medium-temperature and high-temperature geothermal resources. The Minghuazhen, Guantao and Dongying Formations can form good geothermal reservoirs and are favorable layers for exploring low-temperature and medium-temperature geothermal resources. The geothermal reservoir of the Shahejie Formation is relatively poor, and the thermal reservoirs may be developed in the western slope zone and the central uplift. The Ordovician carbonate strata can provide thermal reservoirs for geothermal resources, and the Cenozoic bottom temperature is more than 150 °C except for most of the western gentle slope zone. In addition, for the same stratum, the geothermal temperatures in the southern Dongpu Depression are higher than those in the northern depression.

Geochemical Characteristics and Distribution of the Subsalt Source Rocks in the Santos Basin, Brazil.

The Santos Basin in Brazil is a hot area of oil and gas exploration in recent years, and its subsalt lacustrine mudstones are the main source rock of the basin. However, there is a lack of studies on the source rocks of the subsalt Picarras and Itapema formations, which is not conducive to the accurate evaluation of the source rock characteristics. Based on logging data of 51 wells and geochemical data of 16 wells, this paper makes detailed evaluations of the organic matter abundance, type, maturity, and distribution characteristics of source rocks of the subsalt Picarras Formation and Itapema Formation in the Santos Basin. The results show that the characteristics of source rocks of the Itapema and Picarras formations are similar, both of which have a high abundance of organic matter. The types of organic matter are mainly type I and II1, and the maturities are in the low-maturity to the high-maturity stage, which meets the standard of good source rocks. The total organic carbon content of the source rocks of the Picarras Formation ranges from 0.4 to 4.0%, much lower than that of the source rocks of the Itapema Formation, 0.8-5.6%. In addition, the hydrogen index average value of the source rocks of the Itapema Formation is 712.8 mg/g TOC, higher than that of the Picarras Formation, 697.5 mg/g TOC, both revealing a great hydrocarbon potential. The quality of source rocks of the Itapema Formation is better than that of the Picarras Formation. The two sets of source rocks have great hydrocarbon generation potential and are mainly developed in the eastern and western sags of the central depression. Therefore, the surrounding uplift areas will be the target for further oil and gas exploration.

Hydrocarbon Kitchen Evolution in the Early Cretaceous Bayingebi 2 Formation in the Chagan Depression, Yingen-Ejinaqi Basin, North Central China.

The Chagan Depression is an important oil and gas exploration tectonic unit in the Yingen-Ejinaqi Basin, north central China. It has been revealed that the Chagan Depression has abundant oil and gas resources, but the study of hydrocarbon kitchens has not been carried out. The Early Cretaceous Bayingebi 2 Formation has the most important source rocks in the Chagan Depression. In this paper, the Bayingebi 2 Formation was selected to study the hydrocarbon kitchen evolution. The thermal maturity evolution of the source rocks and the locations and geological time of the development of hydrocarbon kitchens were revealed. The results show that the maturity of source rocks in the Bayingebi 2 Formation has reached the maximum during the middle depositional period of the Yingen Formation, and the hydrocarbon generation has ceased since the Late Cretaceous. The source rocks of the Bayingebi 2 Formation in the Chagan Depression have two hydrocarbon kitchens, namely, the western subdepression and the eastern subdepression hydrocarbon kitchens. The western subdepression hydrocarbon kitchen was formed in the Suhongtu 1 Formation depositional period and ended in the Yingen Formation depositional period. The location of the hydrocarbon kitchen was relatively stable and developed in the central and southern parts. However, the eastern subdepression hydrocarbon kitchen developed only during the Yingen Formation depositional period and was located in the north subsag of the Hantamiao sag zone. Finally, the evolution of the hydrocarbon kitchen reveals that oil and gas exploration still needs to be carried out around the western subdepression hydrocarbon kitchen and it may be considered to abandon the exploration in the eastern subdepression.