Fault characteristics and their control on oil and gas accumulation in the southwestern Ordos Basin

3D seismic data recently acquired from the Ordos Basin shows three sets of regularly distributed fault systems,which overrides previous understanding that no faults were developed in this basin.Seismic interpretation suggests that the faults in the southwestern Ordos Basin have three basic characteristics,namely extreme micro-scale,distinct vertical stratification,and regularity of planar distribution.These NS-,NW-,and NE-trending fault systems developed in the Meso-Neoproterozoic e Lower Ordovician strata.Of these,the NS-trending fault system mainly consists of consequent and antithetic faults which show clear syndepositional deformation.The fault systems in the Carboniferous e Middle-Lower Triassic strata are not clear on seismic reflection profiles.The NW-and NE-trending fault systems are developed in the Upper Triassic e Middle Jurassic strata.Of these,the NW-trending fault system appears as a negative flower structure in sectional view and in an en echelon pattern in plan-view;they show transtensional deformation.A NE-trending fault system that developed in the Lower Cretaceous e Cenozoic strata shows a Y-shaped structural style and tension-shear properties.A comprehensive analysis of the regional stress fields at different geologic times is essential to determine the development,distribution direction,and intensity of the activity of fault systems in the Ordos Basin.Current exploration suggests three aspects in which the faults within the Ordos Basin are crucial to oil and gas accumulation.Firstly,these faults serve as vertical barriers that cause the formation of two sets of relatively independent petroleum systems in the Paleozoic and Mesozoic strata respectively;this is the basis for the‘upper oil and lower gas’distribution pattern.Secondly,the vertical communication of these faults is favorable for oil and gas migration,thus contributing to the typical characteristics of multiple oil and gas fields within the basin,i.e.oil and gas reservoirs with multiple superimposed strata.Finally,these faults and their associated fractures improve the permeability of Mesozoic tight reservoirs,providing favorable conditions for oil enrichment in areas around the fault systems.

Formation of hoodoo-upland on Ordovician karst slope and its significance in petroleum geology in Tahe area,Tarim Basin,NW China

Based on a large number of geological and geophysical data,the formation,fracture-caves types and hydrocarbon distribution of hoodoo-upland on the Ordovician karst slope in the Tahe area,Tarim Basin,are discussed by analyzing faults and strata thickness.The hoodoo-upland was made of high peaks and narrow valleys in the Ordovician karst slope during the Early Hercynian karst period,which were distributed along the NNE positive flower structure and had inherited evolution.The fault-fractures and fracture-vugs complex were extremely developed,with a thickness of 100 m.The cumulative oil production of 60% oil wells was more than 20×10^(4) t per well in the hoodoo-upland,where the residual thickness of the Ordovician Yingshan Formation was greater than karst depressions.Caves formed by the shelter of collapsed breccias were developed in the valleys.They were 1.6 to 13.5 m high,with a filling rate of 51.6%.The positive flower structure under the settings of strike-slip compression controlled the early formation of the hoodoo-upland on the karst slope,resulting in the differences of drainage distribution and karstification.Compared with the water-rich karst valley,the hoodoo-upland with lean water suffered weaker karstification,had thicker residual stratum,and was higher in terrain.In rainy season,the meteoric water flew and corrode along the cracks,forming a complex network of fractures and caves.Combined with inherited uplift and the effective match of the NNE deep faults,oil and gas continuously charged into the reservoir space in the upland,forming the hoodoo fracture-cave reservoir with vertically quasi continuous distribution,high hydrocarbon abundance and high production.

Ag_(2)S Quantum Dots Decorated on Porous Cubic-CdS Nanosheets-assembled Flowers for Photocatalytic CO_(2)Reduction

The production of renewable fossil fuels such as CH_(4) and CO by photocatalytic CO_(2)reduction has attracted more and more attention.However,single photocatalyst is less efficient for photocatalytic reduction of CO_(2)due to the fast recombination of photogenerated electron pairs.Herein,we successfully prepare CdS-Ag_(2)S composite by assembling the Ag_(2)S QDs cocatalyst on the surface of CdS nanosheet-assembled flower through oil-bath solvothermal method.This composite is prepared through a simple self-assembly strategy using cadmium chloride,ammonia and thiourea as precursors of the CdS nanosheet-assembled flower and silver nitrate and 3-mercaptopropionic acid as the precursors of Ag_(2)S QDs.The average diameter of Ag_(2)S QDs is apparently 6.0 nm.The light absorption edge of the composite is at around 560 nm,with the corresponding band gap at 2.14 eV.The CdS-Ag_(2)S QDs composite with 5 wt%Ag_(2)S QDs loaded achieves CO evolution rate of 16.6μmol·g^(-1)·h^(-1)without noble-metal cocatalysts.This strengthened photocatalytic performance and photocatalytic stability were attributed to the energy band broadening of Ag_(2)S QDs caused by quantum size effect and the large specific surface area due to the assembled flower.The mechanism underlying the enhanced photocatalytic CO_(2)reduction activity is further proposed.This study demonstrates that semiconductor-based quantum dots are strong candidates for excellent cocatalysts in photocatalysis.

Words of the Editor-in-Chief——Academic discussion is an effective measure to promote scientific development

Van Loon et al.＇s paper ＂The response of stromatolites to seismic shocks： Tomboliths from the Palaeoproterozoic Chaibasa Formation, E India＂ with a new term ＂tomboliths＂ and original viewpoints should be published, but some contents need to be discussed. Shanmugam＇s paper ＂The response of stromatolites to seismic shocks： Tomboliths from the Palaeoproterozoic Chaibasa Formation, E India： Discussion and liquefaction basics＂ pointed out some queries and problems about Van Loon et al.＇s paper. It is an academic discussion paper and should be published as well. However, some main problems, such as the new term＂tomboliths＂ and its origin of seismic shocks, ＂whether stromatolites or tomboliths are soft-sediment deformation structures or not＂, etc., also need to be discussed. Academic discussion is an effective measure to promote scientific development. The more thorough academic discussions are carried out regarding academic problems, the more scientific facts and truths will become clear. All participants in this discussion are contributors. It is active to carry out the policy of ＂A hundred flowers blossom and a hundred schools of thought contend＂ by our Journal of Palaeogeography.