Analyzing major controlling factors of shale oil 'sweet spots' in the Chang-7 member of the Triassic Yanchang Formation, Ordos Basin

Abstract

The Chang-7 Member of the Upper-Triassic Yanchang Formation has favorable geological conditions (e.g., extensive distribution of source rock, broad distribution of fine-grained sand bodies, stable structure, etc.) for developing a large shale oil pool. Initial assessments have estimated that the Chang-7 has shale oil resources of 3.0 × 109 t approximately. The discovered Xin'anbian giant shale oil field has an estimated reserve of one billion tons. The shale oil in the Chang-7 Member has been developed by volumetric fracturing in long horizontal wells since 2011. By the end of 2021, more than 500 horizontal wells had been drilled, with average initial production of 9.6 t/d per well. Some large-scale effective development pilot zones (e.g., X233, Z183, A83, etc.) have been constructed. The source rock and reservoir in the Chang-7 Member are heterogeneous. Under an extensive distribution background of shale oil layers, there are some local 'sweet spots'. In order to reveal controlling factors for the productivity of shale oil, research involving the comparison of development effects, formation of geologic conditions, and technologies of the three typical pilot zones, has been conducted. The results showed that organic abundance in the source rock is the primary controlling factor for “sweet spot” distribution of the shale oil. The average TOC value in the black shale source rock in the Chang-7 Member is about 10% higher than that in the dark gray mudstone. The average hydrocarbon generation intensity is about 2.0 × 103 t/km2, the average hydrocarbon expulsion rate is 34.6%, and the production rate of gaseous hydrocarbons is 14.65–39.46 m3/t. In the vicinity of black shale with a high TOC value, the oil filling intensity in the shale oil reservoirs is high, with oil saturation of up to about 70%, and a gas-to-oil ratio >90 m3/t. Secondly, the 'sweet spots' with such petrophysical properties control the enrichment of reservoirs. Owing to conditions of low-porosityand low-permeability, the shale oil reservoirs in the Chang-7 Member of the Ordos Basin still developed 'sweet spots',with porosity larger than 9% and permeability larger than 0.08 × 10−3 μm2 . These reservoir sweet spots have a pyrolysis yield of hydrocarbons >9 mg/g and the development wells have high initial and total production, a slow decline rate, and low water cut. Moreover, the dissolved gas in shale oil in the Chang-7 Member of the Ordos Basin is an oil-type gas that was generated during the primary pyrolysis stage of sapropel kerogen after entering a mature period. The gas-to-oil ratio in crude oil is controlled by several factors, such as organic abundance and maturity in source rock, thickness, petrophysical properties, and fractures in reservoirs. The overlap of black shale with high TOC, source rock thickness >10 m, thermal maturity Ro > 0.8%, and type I and II high-quality reservoir beds are the pre-requisites for the development of sweet spots with high and stable production of shale oil. The produced gas-to-oil ratio in the development wells is stable (generally 19.68 m3/t). Some wells have produced oil continuously and stably at over 14 t/d for 5 years. This paper delineates the distribution patterns of shale oil sweet spots in the Chang-7 Member of the Ordos Basin and confirms that profitable and efficient development of shale oil can be achieved in this basin.