Structural Features of Weathering Crust of Granitic Basement Rock and Its Petroleum Geological Significance: A Case Study of Basement Weathering Crust of Dongping Area in Qaidam Basin

In recent years, many scholars have begun to pay attention to the basement reservoir, but there is no report about the study of weathering crust of basement. In this paper, structural characteristics of bedrock weathering crust and its relationship with oil and gas accumulate are systematically studied in Dongping area in the Qaidam Basin, combining with the geological data, logging and geochemical data. It is found that establishment of granitic bedrock weathering indexes based on Al₂O₃, CaO, Na₂O, and K₂O can ensure accurate identification of the ancient soil layer, elurium, semi-weathered layer, and unweathered layer. According to the imaging logging response characteristics and logging curves can well identify paleocrust characteristics, there are three types of tight reservoir space in weathering crusts of Dongping area: rock erosion holes, matrix mineral dissolved pores, and cracks in bedrock. Weathering crust on the base up gradually develops porosity, large semi-weathered rock porosity to the middle, and large pores mainly from semi-weathered rock to weathering exclusion soil, the pore size becomes smaller, and gradually to micro pore-based. In the longitudinal direction, porosity and degree of weathering and tectonic development, the burial depth and distance from the stress relief center have some relevance. The presence of weathering crust, greatly improves the reservoir properties, extends the bearing area, increases the gas-bearing system, which has important theoretical values and scientific significance to the granitic bedrock stratigraphic reservoirs and deep oil and gas exploration.