Weathered volcanic crust and its petroleum geological significance: A case study of the Carboniferous volcanic crust in northern Xinjiang, NW China

The weathered Carboniferous volcanic crust in the northern Xinjiang was analyzed in this paper, using outcrop and core chemical analysis, oil testing and logging data, in order to clarify the characteristics, distribution and petroleum geological significance of weathered volcanic crusts. A complete weathered volcanic crust consists of a 6 layered structure, comprising of a soil layer, hydrolysis zone, leached zone, disintegration zone I, disintegration zone II and parent rock, all of which have different characteristics and markers. The effective reservoir is developed mainly within the leached zone and disintegration zone I. The duration of weathering controls the thickness of the weathered crust. The crust thickness can reach 550 m deep and dates back to approximately 46.3Ma. The lithology, lithofacies and weathering degree control the properties of the weathered crust reservoir. Faults control the seepage of the weathered crust reservoir: the closer a reservoir is to a fault, the larger is its flow radius and seepage. Weathered volcanic crusts were mostly found at palaeogeomorphic high points, slope zones and fault-developed low-lying points, and oil layers near faults have a larger vertical span. We have established a model to evaluate the degree of weathering and effective reservoir generation of volcanic rock, while the well logging analysis could be used to evaluate weathered crust reservoir effectiveness for wells without core data.