Discussion on shale oil enrichment pattern in the Ⅲ submember of the third member of Oligocene Hetaoyuan Formation, Biyang Sag, Nanxiang Basin: a case study of cored interval of well YYY1 in the central deep sag zone

Abstract

The third member of Oligocene Hetaoyuan Formation (Eh3) in Biyang Sag of Nanxiang Basin is the first strata in which a breakthrough for continental shale oil exploration is made, but scale development has not been realized due to a variety of factors. In order to reveal the target zone of shale oil exploration and development of the third member of Hetaoyuan Formation in the central deep sag zone of Biyang Sag, a case study on the cored interval of the Ⅲ submember of the third member of Hetaoyuan Formation (Eh33) from shale oil risk exploration well YYY1 is carried out. Systematic studies on the lithofacies types and the characteristics of shale oil enrichment intervals of the cored interval of Eh33 are carried out and the shale oil enrichment pattern and preferred target layers of shale oil exploration and development are defined by taking full advantage of the results of field analysis and combining with previous research results. The findings have shown that seven lithofacies types, i.e. organic laminar mixed shale, organic-rich laminar mixed shale, organic laminar felsic shale, organic-rich laminar felsic shale, organic layered felsic shale and organic massive argillaceous siltstone/siltstone, organic massive fine sandstone, are developed in the middle-lower interval of Eh33 of well YYY1. Eight shale oil enrichment intervals are identified according to two key parameters of free hydrocarbon (S1) and oil saturation index (OSI) and combining with the total porosity of nuclear magnetic logging. The thickness of a single shale oil enrichment interval is 5-20 m and the total thickness is 81 m. The shale oil enrichment intervals developed in the middle interval of Eh33 are all controlled by both in-situ retention hydrocarbon and migration hydrocarbon from the adjacent high TOC layer, with two enrichment patterns: source-reservoir integration and source-reservoir coexistence. The shale oil enrichment intervals developed in the lower interval of Eh33 are all controlled by in-situ retention hydrocarbon, with source-reservoir integration enrichment pattern. The shale oil enrichment interval in the middle interval is defined the preferred target interval for shale oil exploration and development in Eh33 in the central deep sag zone of Biyang Sag. This provides an important basis for shale oil exploration and development in Biyang Sag.

Keywords

• shale oil
• enrichment pattern
• the ⅲ submember of the third member of hetaoyuan formation
• biyang sag
• nanxiang basin
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