The lower Cambrian Niutitang Formation in northwestern Hunan, South China, has already reached its high or over matured stage, and is formed with hydrothermal activity and deposition. Thus, it is extreme difficult to predict the total organic carbon (TOC) content accurately by common methods with well logging data. In order to solve this problem, we try to use artificial neural networks for predicting the TOC of the black shales in the study case. We got the input vectors through the principal component analysis, and based on the relationships and the logging response mechanism between TOC and logging data. In the BP algorithm, some important parameters including the sample databases, the number of hidden layer nodes, transfer function and weight value adjustment etc, were all optimized correctly in the networks. Then we built the mathematical model through a large number of learning sample datum and the error function between the actual and expected outputs, and found that the results are good by many indicators which judge the performance of the model. In testing samples, mean absolute and relative errors are all reduced probably due to the datum ranges and features are focused, but the accuracy is also dropping when the numbers of participating samples are reduced. Through redefining the TOC ≥ 1.0% samples database, we gained more accurately value for high TOC source rock. Finally, we think the results suggest that the method is suitable for the shale gas resource exploration under similar geological conditions and data characteristics.

We report on the effects of magmatic hydrothermal fluids on the mineralogical and geochemical compositions of 12U and 12L Coals from the Meitian Mine in the Meitian Coalfield, southern China. The minerals in 12U Coal are predominantly chlorite, quartz, and calcite, while the minerals in 12L Coal consist mainly of illite, quartz, chlorite, kaolinite, and mixed-layer illite/smectite (I/S). The vesicle- and fracture-filling illite, chlorite, I/S, pyrite, and fluorite, cleat- and fracture-filling carbonate minerals (i.e., calcite, and dolomite), and cleat-filling tremolite, diopside, and talc have epigenetic hydrothermal origins. Tremolite, diopside, and talc were probably formed from the reaction between dolomite and Si-rich magmatic hydrothermal fluids. Elevated Pb–Zn–Sn–Cd assemblages are characteristic for the 12U Coal, while 12L Coal is enriched in W, Rb, Cs, Th, V, Zn, and Zr, most notably W, Rb, and Cs. REY (Rare Earth Elements and Yttrium) plots for almost all coals, partings and host rocks are similar, showing an M-type REY distribution, Gd-maximum, positive Y anomalies, and negative Ce anomalies, suggesting acid hydrothermal circulation in the coal-bearing strata. Rubidium and cesium in the coal is clearly associated with K-rich clay minerals (illite + I/S), and to a lesser extent with silicate minerals that were precipitated from hydrothermal solutions. W in the coals mainly occurs in the inorganic constituents of illite and pyrite, especially illite. Enrichment of W, Rb, and Cs in the coal and host rocks is genetically associated with magmatic hydrothermal fluids. Specifically, magmatic hydrothermal fluids of relatively high temperatures that are rich in volatile matter can extract abundant W, Rb, and Cs from granitic melts. The enrichment of these rare metals in the coal is mainly related to illitization. Our study results suggest that, for coal intruded by magmatic rocks, the type of hydrothermal alteration may greatly influence the enrichment of elements.

There are many hydrothermal activities in the early Cambrian in northwestern Hunan, and Zhangjiajie and Xinhuang-Tianzhu are the concentration region which brings much influence during this time. Based on the total organic carbon, mineral composition, element analysis, and so on, the wells have no or little hydrothermal activity in HY1, XJD1, and XAD1 and may also have limited effects by the hydrothermal activity. However, the hydrothermal sediments can come to the wells during the sedimentary period of Niutitang formation. XJD1 has a clear diminishing trend on the intensity of hydrothermal activity. It also shows the high element enrichment by the hydrothermal activity. XAD1 show little influence in contrast, and HY1 is between them. The enrichment of the element is a result of the combination of effects of hydrothermal activity, anoxic/euxinic conditions, biological forces, etc. The hydrothermal activity can improve the paleoproductivity and especially enhance the reducibility in the sedimentary environment. Reducibility is one of the major factors affecting organic matter enrichment. Finally, the sedimentary model with hydrothermal activity can be established in the Ediacaran-Cambrian transition. That will help to understand the distribution laws of high-quality source rocks and provide evidences on shale gas exploration.

The gas adsorption of shale is one of the most important factors influencing shale gas accumulation and production. Different from other shale gas fields, the shale in Northwestern Hunan province with strong structural deformation has especially high portion of adsorbed gas. The adsorption abilities of shale reservoir play a significant role in the success of exploration and production of shale gas in Northwestern Hunan. Therefore, based on many experiments including field emission scanning electron microscopy, N₂/CO₂ adsorption, excess CH₄ adsorption with constant temperature and others, this paper found that: (1) the shale pores were identified with mineral matrix pores, organic-matter pores and fracture pores, and the pores can be further classified according to their shape features and forming mechanism; (2) after analyses of the distribution of the pore size, mesoporous is the largest contributor and widely distributed, and micropores provide more contribution on surface area but limited contribution to pore volume. Meanwhile, depth has little effect on the properties of shale reservoirs; (3) based on the fractal geometry theory, surface roughness is positively correlated to microstructure irregularity, which means the effects between the gas adsorption by Van der Waals force and by multilayer or capillary condensation in single pore system are consistent during the N₂ adsorption process. (4) The abilities of gas storage are apparently sensitive to the surface roughness of shale pores and nonsensitive to microstructure irregularity. Total organic carbon has strong positive correlation with the surface roughness, specific surface area, total pore volume, and Langmuir volume, which means TOC can improve the adsorption of shale reservoir in many ways. Pyrite can offer much contribution to the ability of gas adsorption.

Using 2D/3D seismic data and a large number of drilling and logging data and applying sequence stratigraphy, seismic sedimentology, and petroleum geology concepts, the characteristics of the sedimentary sequence of the forced regression have been analysed, the migration trajectory of the coastline have been reconstructed, the evolution model of the forced regression have been presented, and the significance for petroleum geology of the forced regressive sandbodies have been discussed. The falling stage systems tract (FSST) of the Zhujiang Formation present offlap high-angle oblique foreset reflection structure in the seismic profiles of the depositional trends and turbidite fan deposits with strong amplitude mound reflection structure are developed in the downdip direction of its front. The trajectory of migration of the shoreline shows a terraced downtrend in the direction of basin. The FSST is characterized by the shelf-edge delta without topset beds. The FSST was formed in the fall of relative sea-level. Five sets of foreset beds controlled by high-frequency relative eustatic were developed, therefore ordinal regressive overlap can be observed for the five sets of shelf-edge deltas in the depositional trends. The favourable reservoirs which were located close to the upper boundary of the falling stage systems tract and the basal surface of forced regression are sandbodies of the shelf-edge delta front and wave-dominated shoreface sands and the sandbodies of the turbidite fan. Those sandbodies favour the formation of lithologic oil–gas reservoirs by means of good trap sealing conditions, excellent oil–gas reserving performance, and effective oil source communication of fracture system.