PETROLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF THE WEATHERING CRUST OF DONGPING BEDROCK IN QAIDAM BASIN

When the bedrock is strongly fragmented or cracked by faults, the bedrock can folin a good oil and gas enrichment area. The basement weathering crust oil and gas exploration in the Qaidam Basin has made some significant progress in recent years. In particular, the Dongping block in the Altun front zone is a favorable area for crystalline basement weathering crust oil and gas reservoirs. In order to analyze the geological characteristics of the bedrock reservoir in the Dongping area, we used the data of field outcrops, cores, rock slices, etc., by combining the analysis methods such as polarized light microscope, whole-rock X-ray diffraction, major element, and trace element testing to analyze the petrological and geochemical characteristics of rock weathering crust of the Dongping block. Research showed that the weathering crust of crystalline bedrock in Dongping block can be divided into five layers from top to bottom: soil layer, weathered layer, semi-weathered layer dissolution zone, semi-weathered layer disintegration zone, and unweathered bedrock. The soil layer is basically composed of clay minerals. It is often rich in organic matter. After oxidation, it is reddish-brown, and the thickness is generally less than 2m. The weathered layer is composed of weathered clay and small bedrock weathered residual breccia in situ. It is piled up (weathered clay content is greater than 50%), with a thickness of 0-15m. The semi-weathered layer dissolution zone develops a large number of dissolution pores and dissolution fractures, and the dissolution joints and structural fractures are interwoven into a network with a thickness of 30 similar to 200m. The weathered layer disintegration zone is dominated by weakly corroded joint fractures, supplemented by structural fractures. The dissolved pores are not well developed, with a thickness of about 300-1000m. The unweathered bedrock has only a few micro-cracks, the rock is dense, and the rock mass has little clay mineral content. The reservoir space of the crystalline bedrock weathering crust reservoir is mainly composed of matrix micropores, dissolved pores and fractures. The bedrock of the Dongping block is mainly monzogranite-potassium feldspar granite, which is characterized by strong peraluminous, high potassium and calcium alkaline S-type granite in geochemistry. The magma originates from the melting of crustal materials in the volcanic arc environment.