Geological controls of shale gas accumulation and enrichment mechanism in Lower Cambrian Niutitang Formation of western Hubei, Middle Yangtze, China

The lower Cambrian Niutitang Formation is of crucial importance for shale gas target reservoirs in western Hubei, China; however, little work has been done in this field, and its shale gas accumulation and enrichment mechanism are still unclear. Based on survey wells, outcrop data, and large numbers of tests, the geological conditions of shale gas accumulation were studied; moreover, the factors that influence the gas content were thoroughly discussed. The results show that the Niutitang Formation (Є₁n) can be divided into three sections: the first section (Є₁n¹), the second section (Є₁n²), and the third section (Є₁n³). The Є₁n² is the main shale gas reservoir. The deep shelf facies is the main sedimentary facies and can be divided into three main lithofacies: argillaceous siltstone, carbonaceous shale and carbonaceous siliceous rock. The total organic carbon (TOC) content shows gentle growth trends until bottom of the Є₁n² and then decreases rapidly within the Є₁n¹, and the TOC content mainly ranges from 2% to 4% horizontally. The calcite and dolomite dissolution pores, clay intergranular pores and organic pores are the main pore types and the micropore types are clearly related to the mineral compositions and the TOC content. Vertically, the gas content is mainly affected by the TOC content. Horizontally, wells with high gas contents are distributed only southeast of the Huangling anticline, and the combination of structural styles, fault and fracture development, and the distribution of the regional unconformity boundary between the upper Sinian Dengying Formation (Z₂d) and the Є₁n² are the three most important factors affecting the gas content. The favorable areas must meet the following conditions: a deep shelf environment, the presence of the Є₁n¹, wide and gentle folds, far from large normal faults that are more than 5 km, moderate thermal evolution, and greater than 500 m burial depth; this includes the block with the YD2-ZD2 wells, and the block with the Y1 and YD4 wells, which are distributed in the southern portion of the Huangling anticline and northern portion of the Xiannvshan fault.