Abstract: The Triassic Baikouquan formation in Mahu depression belongs to shallow?water delta deposits under the background of gentle slope, in which gravity?flow lithofacies and fluid?flow lithofacies are developed. There exist six fan dalta systems in peripheral Mahu sag,they are Xiazijie, Huangyangquan, Zhongguai, Karamay, Madong and Xiayan fan dalta systems. According to the ancient slope, provenance supply way, such five types of fan deltas are classified as mountain?pass steep slope, mountain?pass gentle slope, near?mountain gentle slope, near?mountain steep slope and near?alluvial fan steep slope types. The fan?delta front has advantageous hydrocarbon accumulation conditions for the following reasons. It is adjacent to the Mahu sag; its sand bodies are widely distributed and extended to a large distance, and superposed in covering the mid?lower slope areas of the sag; there exist faults connecting underlying oil source rocks; the fan?delta plain facies tight belts on the sides and updip directions can shape effective shielding layers and the lateral and top?bottom lacustrine mudstone is developed as barriers, etc. All these can control the distribution and enrichment of oil and gas in vertical and areal directions.Hence the Mahu slope area possesses a macroscopic geological background for large?scale hydrocarbon accumulation, and the above research results can provide guidance for the lithologic reservoir exploration in Mahu sag