Large Language Models (LLMs) are increasingly used to support scientific writing, but their tendency to produce hallucinated content threatens academic reliability. Existing benchmarks have addressed hallucination detection in general-domain tasks, such as fact-checking or question answering, but they do not reflect the fine-grained, domain-specific needs of scientific communication. We introduce SciHallu, a dataset for identifying hallucinations in academic text at three levels of granularity: token, sentence, and paragraph. To establish a reliable ground truth, we select source passages from research papers published prior to the widespread adoption of LLMs. Our dataset includes both hallucinated and non-hallucinated paragraph instances, constructed through controlled perturbations at varying levels of noise and validated by human annotators. A rationale is paired with each instance, explaining the nature of the modification. SciHallu covers multiple academic fields, such as Computer Science, Health Sciences, and Humanities and Social Sciences. It is built using a model-guided annotation pipeline, followed by expert human validation. We evaluate state-of-the-art LLMs on both binary and fine-grained classification tasks, revealing challenges in detecting subtle hallucinations. SciHallu supports the development of context-aware systems for more trustworthy scientific content generation.