Description: Accurate palaeoenvironmental analysis is at the heart of producing reliable interpretations and depositional models. This study demonstrates a multivariate statistical approach to facies analysis based on relationships between grain size and quantitative palynology. Our methodology has the advantage that it can be used on small amounts of sample, such as core or well cuttings, as the basis for facies analysis. Proof of concept studies involving the collection of grain-size and palynological datasets from well-exposed outcrops of the Middle Jurassic, Lajas Formation of the Neuquén Basin, Argentina, demonstrate that canonical correspondence analysis can be used to consistently recognize facies and aid in the determination of depositional environments. This study demonstrates the link between depositional facies, grain-size distribution, palynomorph hydrodynamics and assemblage taphonomy of palynomorphs. This knowledge can be transferred into a semi-automated statistical facies prediction technique for the subsurface in complex depositional settings, particularly when calibrated against conventional sedimentary facies analysis. Supplementary material: The full set of grain-size data and statistical scores are available at: https://doi.org/10.6084/m9.figshare.c.3745481.v1

Description: There is great potential for hydrocarbon exploration in the Triassic Baikouquan Formation, in the Mahu Depression of the Junggar Basin, NW China. Controversy remains around the conglomerate categories in coarse-grained deltas, which has inhibited further exploration in this area. Based on a dataset of cores, test data, well logs and seismic, this study attempts to recognize different genetic types of conglomerates formed within coarse-grained deltas. The Baikouquan Formation conglomerate has been divided into nine facies on the basis of textures, grain size and sedimentary structures. From the associations of these nine facies, three conglomerate categories have been recognized: debris-flow conglomerate (DFC); hyperconcentrated-flow conglomerate (HCFC); and traction-flow conglomerate (TFC). These have diagnostic characteristics that are expressed in core, well logs, seismic character, grain-size distribution curves and maximum particle size/bed thickness analysis (MPS/BTh). A depositional model has been built after analysis of the sediment transport mechanisms to facilitate exploration of the Baikouquan Formation, in the Mahu Depression. The evaluation of reservoir characteristics, physical properties and pore-throat structures of these conglomerate categories has identified the HCFC reservoirs as the primary potential targets for exploration in the study area.

Description: Regional seismic reflection profiles and well data from the Uruguayan margin were integrated in order to analyse and illustrate its tectonic and stratigraphic evolution. The evolution of the Punta del Este and Pelotas basins is divided into four major phases: pre-rift (Palaeozoic); rift (Jurassic–Early Cretaceous); transition (Barremian–Aptian) and post-rift (Aptian–Present). Each of these phases is characterized by a specific structural configuration and a stratigraphic architecture related to basin type, tectonic elements, sediment supply and base-level changes. During the geological evolution of the Uruguayan continental margin, a NE migration of depocentres occurred. The Cretaceous depocentre is located in the Punta del Este Basin, while the Cenozoic depocentre is located in the Pelotas Basin.

Description: The SH-1 wildcat well, firstly encountering oil, proved the existence of a working petroleum system in the Tenere Basin, but the origin of oil and its generation history is not fully understood. In this regard, one crude oil and nine Upper Cretaceous source rock samples from the SH-1 wildcat well in the Tenere Basin were characterized by a variety of biomarker parameters and stable carbon isotopic composition. The oil from the Tenere Basin displays a low pristine/phytane (Pr/Ph) ratio, low C 29 /C 30 and C 35 /C 34 hopane ratios, low C 22 /C 21 and high C 24 /C 23 tricyclic terpane ratios, and high gammacerane index (gammacerane/C 30 hopane), suggesting shale source rocks deposited under an anoxic and relatively high-salinity environment. Relatively high C 29 sterane concentrations, a low value of the regular steranes/17 α (H)-hopanes ratio and a canonical variable from stable carbon isotopic composition larger than 0.47 indicate that the oil is mainly derived from terrestrial higher plants. Both aromatic maturity parameters and sterane isomerization parameters suggest that the oil was generated from an early mature source rock. Correlation between oils from the Tenere Basin and those from the Termit Basin confirms that these oils belong to the same genetic family. The oil–source rock correlation suggests that the oil from the Tenere Basin originated from the Upper Cretaceous marine source rocks, and the Yogou Formation can be considered as effective source potential.

Description: The hydrocarbon source rock potential of shales from the Oligocene–Early Miocene Dingqinghu Formation was evaluated using organic geochemistry (Rock-Eval pyrolysis, total organic carbon (TOC) and bitumen extraction) and organic petrology. Forty-one outcrop samples from Lunbori and Jiangria Co shales in the Lunpola Basin were analysed. At Lunbori, in the central part of the basin, the shales have very good source rock potential, and show TOC contents ranging from 3.72 to 11.2 wt%, with an average TOC value of 7.36 wt%, S 2 values of 33.4 – 99.5 mg HC/g rock (average 61.8 mg HC/g rock), and extractable organic matter contents varying from 513 to 4525 ppm (average 2105 ppm). The shales have very good source rock potential. Rock-Eval T max values of 433 – 437°C (average 435°C) and high carbon preference index (CPI) values (4.76 – 6.70) indicate that the organic matter is immature to early mature for oil generation. The yellow fluorescence of kerogen under UV light is in good agreement with T max and CPI data. High hydrogen indices (HI) (635 – 1057 mg HC/g TOC) and H/C ratios (1.60 – 1.72) indicate that the Lunbori shales contain oil-prone organic matter and are characterized by high yellow fluorescing amorphous sapropelite content. The Lunbori shales contain a high content of hydrogen-rich Type II and minor Type I oil-prone kerogen, and have dominant medium to long molecular weight n -alkanes (maximizing at C 25 or C 27 ) with marked odd carbon number preference in the n -C 23 – n -C 31 range, pristine/phytane (Pr/Ph) ratios of 0.06 – 0.14, high phytane/ n -C 18 ratios (10.95 – 30.63), abundant gammacerane (gammacerane/C 31 hopane: 0.44 – 1.80), and predominant regular sterane C 27 . These data indicate deposition in stratified, anoxic and saline depositional conditions with a kerogen predominatly formed from a lacustrine biota. The Jiangria Co shales in the western part of the Lunpola Basin contain some hydrogen-rich Type I and II oil-prone kerogens at a marginally mature level. They have TOC contents of up to 2.62 wt%, S 2 values of up to 17.2 mg HC/g rock and extractable organic matter contents of up to 1731 ppm, indicating good source rock potential for oil generation. This is supported by their high HI values (420 – 720 mg HC/g TOC) and H/C ratios (1.05 – 1.50). The biomarker distributions of the extracts are characterized by odd carbon number predominance (C 23 –C 31 ), low Pr/Ph ratios (0.05 – 0.09), high concentrations of phytane and high gammacerane content, indicating lacustrine reducing and saline conditions.

Description: High-pressure methane sorption isotherms were performed on a series of Upper Ordovician Wufeng and Lower Silurian Longmaxi shale samples in the Fuling shale gas field to investigate the effects of organic matter content, thermal maturity, clay minerals, pore structure, temperature and pressure on methane sorption capacity. A large number of micropores with a pore width of less than 10 nm are developed within the organic matter, with its abundant specific surface areas as the fundamental factor to enhance the methane sorption capacity. The total organic carbon (TOC)-normalized sorption capacity increases with an increasing equivalent vitrinite reflectance R o , but an opposite trend is observed when R o is in the highly over-mature stage. The TOC-normalized sorption capacity shows no correlation with the total clay content as well as individual clay minerals. Most of the excess sorption capacity of shales increases with an increasing pressure, exhibits a maxima in the pressure range of 15–17 MPa and then decreases. The sorption isotherms show an obvious decrease in excess sorption capacity with increasing temperature. Moreover, the Langmuir pressure exponentially decreases with the reciprocal of temperature. Based on the Langmuir adsorption model, an empirical formula is established to evaluate the absolute sorption capacity of shales as a function of TOC content, pressure and temperature. Supplementary material: Additional description of experimental data are available at https://doi.org/10.6084/m9.figshare.c.3714040.v2

Description: An analysis of the characteristics of ultra-low-permeability reservoir rocks and the processes for permeability loss form the basis for establishing high-resolution well log interpretation models. Moreover, this type of analysis is a prerequisite for the effective development of ultra-low-permeability reservoirs. The Chang 7-1 sandstone members of the Triassic Yanchang Group in the Longdong region of the SW Ordos Basin are typical ultra-low-permeability reservoirs and are studied in detail in this paper. A comprehensive methodology of evaluating the ultra-low-permeability reservoir is developed based on geological information and well logging data. First, pore structure characteristics are analysed in depth using core observations, cast thin sections, scanning electron microscopy and mercury injection porosimetry. The mechanism behind the formation of ultra-low permeability in a reservoir rock, and its controlling factors, are also clearly determined. Second, from the above results and the pore structure characteristics in the study area, the flow-zone indicator (FZI) is selected as the key reservoir modelling parameter to establish permeability models that can reflect different pore structure types and improve precision compared to non-classification pore structure models. In addition, conductivity experiments with water-saturated cores confirm that the additional conduction phenomenon of clay minerals has little impact in terms of reducing reservoir resistivity and, consequently, the Archie formula is found to be reliable for ultra-low-permeability rocks with porosities greater than 5% in the Chang 7-1 reservoir. Three types of well log interpretation models, such as permeability, oil saturation and porosity, are established for the ultra-low-permeability reservoirs using FZI as a classifier in the research area. These models reduce the relative errors to less than 10% and performed well for the ultra-low-permeability Chang 7-1 sandstones. This study contributes to the evaluation of ultra-low-permeability reservoir rocks and also generates fit-for-purpose log evaluation models to guide completion interval selection.

Description: This thematic set emerged out of the 3rd EAGE Sustainable Earth Science Conference, held in the picturesque town of Celle in Lower Saxony, Germany, on 13 – 15 October 2015. This conference series was established to develop and promote an emerging group of applied geoscience topics focused on technologies for sustainable use of the subsurface to serve the energy transition; and the focus so far has been mainly on geothermal energy, CO 2 storage and energy storage. Inspired by the research emerging from this conference, the conveners proposed a thematic set on the common ground between long-term geological storage of CO 2 and the shorter-term cyclic storage of energy. The resulting set of papers is very rewarding, since they capture examples of all these topics and illustrate the complementary nature of subsurface studies on the storage of energy and CO 2 . We have arranged the set to start with two review papers, followed by three papers on energy storage and then a further three on CO 2 storage.

Description: Human society needs to achieve a low-carbon energy mix this century. To achieve this, we need: (a) an appreciation of the value of Earth's atmosphere; and (b) a sustainable approach for low-carbon energy. For sustainable developments, three pillars need to work together: the environment, social equity and economics. To address the societal aspects of the low-carbon energy transition, we need to appreciate that our future depends on protecting the Earth's atmosphere. By reviewing the discovery of the greenhouse gas effect over the last 200 years, we establish the essential motivation for changing human behaviour with regard to energy use. From this basis, we consider the challenge of how to achieve this energy transition or, more specifically, how to overcome the dissonances related to societal acceptance, economic hurdles and lack of progress with deployment of low-carbon energy options. The last decade has seen a significant growth in the renewable energy and natural gas sectors: however, CCS has made limited progress. This has to change if the human population is to significantly reduce greenhouse gas emissions. In order to accelerate reductions in global CO 2 emissions, all low-carbon energy options must be deployed at an increasing rate in the coming decades.

Description: Large-scale energy storage in the geological subsurface (e.g. by storing hydrogen gas) may help to mitigate effects of a fluctuating energy production arising from the extensive use of renewable energy sources. The applicability of hydrogen (H 2 ) storage in a porous sandstone formation is investigated by defining a usage scenario and a subsequent numerical simulation of a storage operation at an existing anticlinal structure in the North German Basin. A facies modelling approach is used to obtain 25 heterogeneous and realistic parameter sets. The storage operation consists of the initial filling with nitrogen used as cushion gas, the initial filling with H 2 , and six withdrawal periods with successive refilling and shut-in periods. It is found that, on average, the storage can sustain a continuous power output of 245 MW for 1 week when using five storage wells, while peak performance can be as high as 363 MW, indicating that the storage is mainly limited by the achievable extraction rates. The median of the maximum pressure perturbation caused by this storage is around 3 bars and can be observed at a distance of 5 km from the wells.