Geochemical evaluation of produced petroleum from the Middle Permian Lucaogou reservoirs Junggar Basin and its implication for shale oil play

Summary The Lucaogou Formation develops two sweet spots, the upper (P2l22) and lower (P2l12) sweet spots, which are not more than 150 m apart. Interestingly, oils produced from the deep Lucaogou Formation (the lower sweet spots) have higher densities and viscosities than those in the shallow Lucaogou Formation (the upper sweet spots). In this paper, geochemical data from 78 source rock solvent extracts and 17 oil samples from different depths were carried out to clarify unconventional petroleum systems and the cause of high viscosities. Oils produced from the P2l12 interval are characterized by abundant polar compounds, which are derived from the P2l12 and P2l11 intervals source rocks with relatively high marine sources contribution. Oils in the P2l22 interval are generated from the P2l22 interval source rocks with relatively high terrigenous-marine sources contribution. Migration from sources to reservoirs in the Lucaogou shale oil systems is short-distance given thermal maturity considerations. The sources and thermal maturities are the primary reasons for the high viscosities of the crude oils produced from the P2l12 interval. This research also indicates the Lucaogou shale petroleum system could be divided into two subsystems, and different strategies are required for the exploration and development of different unconventional petroleum subsystems.