Sequential distribution of gas reservoirs in the Hangjinqi area, Ordos Basin, North China: Application of continuous and discontinuous hydrocarbon accumulation mode

Abstract

The gas accumulation mode can be divided into two types based on the migration and accumulation characteristics: discontinuous accumulation into the reservoir rocks and continuous accumulation within or adjacent to the source rocks. Correspondingly, there are three different gas reservoirs including conventional, unconventional, and those in a transitional state. To demonstrate the differences in the formation mechanisms and distribution features of the two accumulation types, we have scrutinized the continuous accumulation and discontinuous accumulation in the Hangjinqi area of the Ordos Basin in North China. Continuous gas accumulation (basin-centered gas and water-soluble gas, as well as potential coalbed methane and shale gas) and discontinuous accumulation (various conventional gas traps) are, respectively, distributed in the southern and northern regions of the Hangjinqi area, which may change with the source-rock quality, migration force, reservoir capacity, and trapping condition. We identified that the essential factors influencing the formation and current distribution of gas reservoirs are the hydrocarbon generation ability, porosity, and cap rock-trap combinations in the study area. An ideal complete gas reservoir sequence develops coalbed methane, shale gas, basin central gas, water soluble gas, and conventional gas reservoirs from the center to the edge of the basin. Understanding the distribution characteristics of continuous accumulation and discontinuous accumulation allows better prediction of the potential gas reservoir types with the knowledge of discovered reservoirs. We recommend exploring anticline gas reservoirs in areas north of the Boerjianghaizi fault and basin-centered gas reservoirs in areas south of the Boerjianghaizi fault. We also suggest performing further research on shale source rocks because the maturity and the burial depth of the marine-continental organic-rich shale in the Shanxi-Taiyuan Formations are recognized to be favorable for shale gas generation and preservation.