Reservoir space and potential reservoir-formation areas in deep bedrock gas reservoirs in Altun forelands, Qaidam Basin: Recognition and discussion

摘要

The discovery of deep bedrock gas reservoirs in Altun forelands in the Qaidam Basin has expanded a new field of natural gas exploration and development in China. Since then, it has always been believed that the storage space of this kind of gas reservoirs is composed of well-developed dual media of matrix pores and fractures, but the practices of such gas reservoir development began to be in contradiction with this cognition. In order to achieve a better understanding of the storage space and the main controlling factors of reservoir formation in this bedrock gas reservoir, it is necessary to investigate the dissolved increased pores characteristics and the forced fractures characteristics in the bedrock gas reservoir. Then, based on the data such as cores and cast thin sections in the bedrock intervals in the Dongping 1 and Jiantan 1 blocks, the main storage space types of deep bedrock gas reservoirs in Altun forelands were analyzed, then the main controlling factors of favorable target reservoirs in these blocks were put forward, and in combination with the regional geological background, the geological understandings of favorable target reservoirs and potential reserves were deepened in this study area. The following research results were achieved. (1) The lithology of the bedrock gas reservoirs in this area consists of calc-alkaline igneous rocks and gneiss suite regional metamorphic rocks. Matrix pores are not developed, and their development degree is mainly controlled by faults. Main effective storage spaces and permeable channels are structural fractures and dissolution fractures. (2) The main controlling factors of target reservoir formation include lithology, tectonic effect, weathering, intrusive dikes, and on the whole the rule of ternary-control is followed, namely prevalent lithology–dominant stresses–hydrocarbon accumulations in the structural higher parts. (3) The potential zones of reservoir formation include the following 5 types: tectonically stress concentrated tension–torsional zones, contact zones between lithologic interfaces of intrusive body, weathering zones of compressing uplift, slope sediment zones near circumscribed erosion area, para-conformity or unconformity interface.