Lithofacies and organic geochemistry of the Middle Permian Lucaogou Formation in the Jimusar Sag of the Junggar Basin, NW China

• The Lucaogou Formation was divided into shale, siltstone and dolomite lithofacies.
• The Lucaogou Formation contained three thansgressive and two regressive sequences.
• Organic geochemical analysis indicates that the shales are high-quality source rocks.
• The regressive sequences are the exploration and development targets of tight oil.

The Middle Permian Lucaogou Formation in the Jimusar Sag, Eastern Junggar Basin, is one of the highlights for unconventional petroleum exploration and development in China. Approximately 580 m-long cores from 18 wells in the Sag were carefully examined, and 500 core samples were collected. These samples were analysed by combined methods including X-Ray diffraction (XRD), thin-section observation and organic geochemical analyses.Based on texture, fabric and mineralogy, the Lucaogou Formation is mainly classified into three lithofacies: (1) shale; (2) siltstone; and (3) dolostone. They are further grouped into three lithofacies associations (LFA): (1) shale LFA; (2) shale interbedded with siltstone LFA; and (3) siltstone intercalated with shale LFA. Compared with North American Barnett shale, the XRD data of the Lucaogou Formation reflects that the concentrations of the plagioclase minerals are generally high. In addition, these rocks commonly contain high contents of brittle minerals (quartz+feldspar+carbonate+pyrite) (avg. 88%) and low contents of clay minerals (avg. 10.97%). The poorly sorted and subangular mineral grains reflect low textural maturity. The organic geochemical data of the shales reflect that the shales have high contents of total organic carbon (TOC), mainly sapropel type (II1) kerogen and matured organic matter. Thus, the shales are expected to be good-quality source rock. The transgressive–regressive (T–R) sequences analysis shows that the Lucaogou Formation can be divided into three transgressive intervals and two regressive intervals. These transgressive intervals are in favour of the formation of excellent source rocks for their low-energy, deep to semi-deep lacustrine environments. In contrast, the regressive intervals contain more rocks with brittle minerals for their relatively high energy of the sedimentation hydrodynamic conditions. The regressive intervals of the Lucaogou Formation as a petroleum reservoir are favourable for fracturing and the production of tight oil.