关键词: 鄂尔多斯盆地, 非常规石油, 油藏成因, 地质“甜点”, 成藏模式

Abstract: The remaining oil resources in the Member 8 of Yangchang Formation (Chang 8) in Ordos Basin account for 21.5×10⁸t. There are less studies of the accumulation controlling factors and distribution rules of shale oil and tight oil, which restricts the exploration and discovery of unconventional reservoirs in the basin. Based on the data of core, logging, source rock evaluation, and experimental simulation in the study area, deep studies have been conducted on source-reservoir-caprock assemblages, pathway system s, and hydrocarbon accumulation mechanisms. The total amount of hydrocarbon expulsion in the west area of Ganquan is 1.41×10⁸t, and the tectonic evolution dominates the interactive distribution of multiple sets of source rocks and multiple types of sandstone reservoirs. Four favorable source-reservoir-caprock assemblages are developed, including the thin intra-source sandstone interlayers of Chang 8 and of both Member 8 and Member 7 of Yanchang Formation, and the neighboring source and reservoir of the single layers Chang 8₂ and Chang 8₁. The shale reservoirs and tight reservoirs are dominated by type Ⅰ and Ⅱ source-reservoir integrated assemblages and type Ⅲ and Ⅳ neighboring source and reservoir assemblages, respectively. The geologic and recoverable reserves are controlled by reservoir quality and crude oil filling degree, which are the keys to form the geological sweet spots of unconventional oil reservoirs. A multi-sources and multi-reservoirs orderly composite accumulation model is established, and characterized with multi-source hydrocarbon-generating pressurization, driven multi-directional hydrocarbon supply, combined transportation via fracture networks, and differential charging among multi-reservoirs. Shale oil is mainly stored in a free state in thin interlayers of beach bar sandstones with excellent reservoir properties in the source rocks, formed by primary migration and accumulation in the hydrocarbon source rocks. In addition, a small quantity of shale oil is adsorbed in the intra-source silty laminae, or occurred in lamellation fractures and pores in a free state, indicating in-situ accumulation. Tight oil is stored in a free state in sand bodies with excellent reservoir-caprock assemblages outside the source rocks, indicating near-source short-distance secondary migration and accumulation. Moreover, shale oil and tight oil are characterized by spatiotemporally orderly migration and accumulation, continuous composite accumulation, locally enriched sweet spots, vertically superimposed development and regional cluster distribution. Under the guide of the orderly composite accumulation model, a hundred-million-ton large oilfield has been discovered in the western Ganquan area, indicating that Chang 8 in Ordos Basin has huge unconventional petroleum resources. Delta front underwater distributary channel sandstones around the continental depression-type lake basin subsidence center and the alternate deposition areas of lacustrine mud shale and beach bar sandstones are the favorable development areas of shale oil and tight oil.

Key words: Ordos Basin, unconventional oil, reservoir genesis, geological sweet spot, accumulation mode