Lithofacies Evaluation of Paleogene Yingxiongling Shale Oil, Qaidam Basin

Abstract

Abundant lacustrine shale oil resources have been discovered in the Eocene saline lake system, Yingxiongling Sag, Qaidam Basin. Compared with the Cretaceous Qingshankou Formation in the Songliao Basin and Triassic Chang 7 Member in the Ordos Basin, the lithofacies of Yingxiongling shale are more complex, featuring thinner single-layer and faster vertical variations. How to establish a reasonable lithofacies classification and evaluate the effectiveness of different lithofacies is the key to unlocking the giant resources of Yingxiongling shale oil. Based on the total organic carbon analysis, rock pyrolysis analysis, fluorescence thin section observation, oil saturation analysis, helium porosity measurement, high pressure mercury intrusion by scanning electron microscopy, focused ion beam-scanning electron microscopy, and nitrogen adsorption analysis, the organic geochemistry, reservoir property, and oil-bearing property were comprehensively analyzed. This paper proposed a lithofacies classification scheme of Yingxiongling shale oil and systematically evaluated the quality of representative lithofacies. The key findings were as follows: (1) Eight types of lithofacies were identified based on the rock structure and mineral composition, namely, thin-bedded/laminated dolomitic limestone, thin-bedded/laminated limy dolostone, thin-bedded/laminated mixed rock, thin-bedded sandstone, and laminated shale. (2) The types and combinations of lamination varied greatly among different lithofacies. Dolomite primarily formed through calcite shrinkage during the dolomitization process, resulting in the formation of abundant intercrystalline pores. (3) More organic matter was observed in the calcite laminae, which was deposited as a result of small calcite particle flocculation. (4) The combination of laminated dolomitic limestone and thin-bedded limy dolostone formed a favorable source–reservoir assemblage, which was the pay zone for horizontal wells. These understandings could further be helpful in understanding shale oil enrichment in the Qaidam Basin and could provide scientific guidance and technical support for the exploration and development of global plateau shale oil.