Search

In order to accelerate the exploration and development of shale gas, this paper elaborates on the micro mechanism of shale gas preservation based on shale self-sealing. The self-sealing capacity of shale formations is mainly related to the low connectivity of nano-throats, low speed diffusion caused by bound water occurrence, capillary force sealing, and breakthrough pressure along the layer direction under burial conditions. Results of analysis based on shale pore morphology and connectivity show that the organic pores in shale are mainly composed of nano-throats with poor connectivity and significant retention effects. At the same time, the stacking of multiple relatively dense sealing layers within the top, bottom, and shale layers is conducive to self-sealing in longitudinal direction. Based on experimental and molecular dynamics simulation analysis of the influence of bound water on the diffusion and breakthrough pressure of the shale matrix, it is revealed that the bound water occurrence significantly reduces the effective diffusion capacity of shale matrix pores and could cause high capillary forces, leading to effective sealing of the gas stored in organic pores. In the study, the permeability-breakthrough pressure evolution relationship was constructed, revealing that under deep burial conditions, the effective closure of shale bedding fractures along the layer direction can form high breakthrough pressure sealing. During the uplift stage, under relatively weak tectonic compression, the shale could still maintain a high sealing capacity along the layer direction, which is conducive to shale gas preservation. However, under strong tectonic compression, the shale bedding fractures open and communicate with open fault surface, causing the failure of preservation capacity and large-scale loss of shale gas. This study elucidates the microscopic mechanism of shale gas preservation through experiments and molecular dynamics simulations, which could provide useful guidance for marine shale gas exploration in complex structural areas.

A basic research program on deep hydrocarbon resource accumulation mechanisms and key exploitation technologies was initiated by the National Natural Science Foundation of China at the beginning of 2020, to meet the theoretical and technological demands of deep hydrocarbon exploration and development in the Tarim, Sichuan and Ordos basins. This paper provided a review of the recent program progresses on (1) the reconstruction of prototypes and the transformation histories of three ancient cratons in China; (2) the petroleum accumulation and flow mechanisms in deep marine carbonate reservoirs and deep shale gas systems; (3) the geophysical theories and methods for imaging the deep structures and predicting the deep petroleum reservoirs; and (4) the new protocols and downhole tools for deep well drilling and completion. Brief comments were also made on the preliminary application of the recent program results in facilitating several breakthroughs in the deep petroleum exploration of the Tarim and Sichuan basins.

Marine strata in South China are mainly distributed in the Paleozoic and Mesozoic. The strata are old and the thermal evolution degree is high. It was controlled by the multi-cycle sedimentary and multi-stage structural evolution history. The petroleum basic geological conditions are superior, but the distribution of oil and gas is complicated. Oil and gas exploration has mainly gone through three stages: general survey, structural reservoir, massive lithologic gas reservoir and shale gas exploration. Although major breakthroughs have been made in many fields, the overall detection rate is low. A systematic summary of the early discovery of large gas fields and the new progress of exploration was made in order to clarify the prospects for oil and gas exploration. There is a large potential for marine petroleum exploration in South China. Deep and ultra-deep strata as well as new areas are important breakthrough directions in the Sichuan Basin. The unconventional shale gas reservoirs in the Upper Ordovician Wufeng-Lower Silurian Longmaxi formations and the conventional gas reservoirs in the platform margin zone of the Upper Sinian Dengying Formation in Langzhong-Yuanba, the platform inner beach facies in the Sinian-Cambrian in Tongnanba, the pre-salt strata in the lower assemblage in Qijiang in the southern Sichuan and the reef beach facies in the Permian-Triassic have a total resource amount to trillions of cubic meters, which is expected to achieve a new round of breakthroughs and discoveries. Besides, unconventional shale gas in the Permian and Jurassic and some new targets for conventional gas such as hydrothermal dolomites, karst fissure group, stucco limestones and sedimentary tuffs in the Permian show a great exploration potential. New discoveries continue to be made in complex basin-margin regions in the Sichuan Basin. Shale gas in the residual syncline area and conventional piedmont areas in the basin margin are the main exploration directions. The periphery of ancient uplifts and the progressive deformation area in the southern periphery have weak structural deformation, and the preservation conditions are favorable with a certain exploration potential, and are expected to become a strategic succession area for oil and gas.