Identification of sweet spots in shale-type and siltstone-type “shale oil systems”: A case study of the Chang 7 Member in Ordos Basin

Mid-high maturity shale oil is the most realistic field for the scale breakthrough of terrestrial shale oil production in China. Generally, three deficiencies hinder shale oil development in China: heavy oil density, small sweet spot areas, and poor distribution continuity. Thus, identifying the “sweet spots” in shale oil reservoirs is critical for the efficient exploration and development of terrestrial shale oil. This study targets the siltstone type (Class-II shale oil) and pure shale type (Class-III shale oil) of the Chang 7 Member in the Ordos Basin, and identifies three stratigraphic units, namely the hydrocarbon accumulation unit, hydrocarbon generation unit, and hydrocarbon retention unit, which together constitute the in-source “shale oil system”. The hydrocarbon accumulation unit is mainly siltstone, where the hydrocarbons are migrated from shales. It has favorable pore-throat network connectivity with a pore connectivity ratio of 32–57%, being the siltstone-type sweet spots. The hydrocarbon generation unit is mainly composed of high-TOC mudstone/shale and is the main contributor to in-source hydrocarbon generation and expulsion. This unit has high three-dimensional connectivity (28–30%), as shown by the pore-throat network model, associated with vertical paths for hydrocarbon expulsion. The hydrocarbon retention unit is mainly composed of low-TOC mudstone/shale retaining self-generated and migrated hydrocarbons. The pore connectivity rate is 17–42%, and the pore-throat network connectivity direction is uneven. Light and low-carbon-number hydrocarbons are preferentially trapped or even sealed in small pores of the retention unit, forming the typical mudstone/shale-type sweet spots. In the process called shale oil intra-source migration, the oil migrates in source rocks causing component fractionation, which allows more shale oil to enrich in the hydrocarbon accumulation and retention units to form sweet spots, compared with the hydrocarbon generation unit. The migration paths include the one from mudstone/shale to siltstone interlayers and that from the high-TOC mudstone/shale intervals to the low-TOC intervals. The in-source accumulation of shale oil shows the differentiated enrichment model featuring “high-TOC mudstone/shale generating hydrocarbons, low-TOC mudstone/shale retaining hydrocarbons, siltstone accumulating hydrocarbons and multiple intra-source migration paths”. In the Ordos Basin, the organic-lean (TOC 1–3%) mudstone/shale intervals appear to be the sweet spots of shale oil, where there are abundant medium-short-chain hydrocarbons retained with high flowability. After fracturing stimulation, their production conditions may be even superior to those of siltstones. This proposed idea changes the previous strategy to look for sweet spots in high-TOC intervals derived from the shale gas industry.