Abstract:

The bedrock weathering crust reservoirs in the eastern segment of the Altun piedmont area are dominated by magmatic weathering crust and metamorphic weathering crust. In order to understand the differences between the two kinds of weathering crust reservoirs, the weathering crust structures of the bedrock are identified through core observation, conventional logging curves and FMI image logging. The weathering crust structures in typical wells are classified and well-tie sections are mapped. Based on core observation, thin section identification, scanning electron microscopy, elemental energy spectrum logging, conventional logging, constant element testing and helium porosity-air permeability testing data, the differences in mineral composition, geochemical response, conventional logging, reservoir spaces and characteristics between the two kinds of weathering crust reservoirs are summarized, and the development model of weathering crust reservoirs of magmatic and metamorphic rocks are established. The results show that the weathering crust of bedrock can be divided into four layers according to its weathering degree, namely soil layer, full-weathered layer, semi-weathered layer and unweathered layer, and the semi-weather layer is further divided into dissolved zone and disintegrated zone. There is little difference in the vertical distribution of magmatic minerals. The contents of mafic minerals and clay in the metamorphic rocks from shallow to deep decrease as a whole. It can be discovered from SiO₂/Al₂O₃ that the bedrock in the study area is mostly in semi-weathering state, the SiO₂/Al₂O₃ of granite ranges from 5.13 to 6.50 and that of the metamorphic rock is 3.01-6.53. In the magmatic rock, the reservoir space of the dissolved zone in the semi-weathered layer is fractured-vuggy type, and the reservoir space of the disintegrated zone in the semi-weathered layer is fractured type; in the metamorphic rock, the reservoir space of dissolution zone in the semi-weathered layer belongs to vuggy type, and that of the disintegrated zone of semi-weathered layer is vuggy-fractured type. The porosities of the dissolved zones of the semi-weathered layer in the metamorphic rock and magmatic rock are 0.413%-8.509% and 0.926%-7.152%, respectively; the porosities of the disintegrated zones of the semi-weathered layers in the metamorphic rock and magmatic rock are 1.367%-5.211% and 1.429%-8.572%, respectively. In the magmatic rock and metamorphic rock, the correlation between porosity and permeability of the disintegrated zones in the semi-weathered layers are better than that of the dissolved zones in the semi-weathered layers. The dissolved zone of the semi-weathered layer in the metamorphic rock is the most important reservoir of the weathered crust, and is also the key factor for stable oil production, followed by the disintegrated zone of the semi-weathered layer. The disintegrated zone of the semi-weathered layer is the most important reservoir of weathered crust reservoir in the magmatic rock, followed by the dissolved zone of the semi-weathered layer. The reservoir performance of the metamorphic weathering crust reservoir is better than that of magmatic weathering crust reservoir.

Key words: Altun Mountains, bedrock weathering crust, reservoir, magmatic rock, metamorphic rock, bedrock weathering crust structure, reservoir development model