Hyperpycnal flow deposits of the Permian Lucaogou Formation in the Jimusaer Sag and its peripheries, Junggar Basin

The sedimentary origin of the Permian Lucaogou Formation reservoirs in the Jimusaer Sag, Junggar Basin has long been a controversial topic. Based on data from outcrop and core observations, logging, and seismic surveys, as well as lab tests of the Lucaogou Formation, we conduct sedimentary analysis, obtaining the following results: (1) The reservoirs of the Permian Lucaogou Formation in the study area are of siltstone/fine-grained sandstone and argillaceous siltstone. They feature well-developed paired graded bedding sequences with internal erosion surfaces, massive bedding, and terrestrial plant fragments, which can be an interpretation of a sedimentary origin of lacustrine hyperpycnal flows; (2) The channel subfacies of hyperpycnal flow deposits consists primarily of massive- and cross-bedding siltstone/fine-grained sandstone, with a single-layer thickness of 0.8~2.8 m, averaging 1.6 m and NMR-derived porosity of 6%~12%, averaging 9.5%. In contrast, the lobe subfacies of hyperpycnal flow deposits predominantly comprises siltstones showing graded bedding and climbing-ripple bedding, with a single-layer thickness of 0.5~1.4 m, averaging 0.9 m and NMR-derived porosity of 3.5%~7.8%, averaging 5.2%; (3) The channel subfacies showing seismic reflections featuring lensoid filling presents a stacking pattern of aggradation with thick laminated accretionary fillings that are in the shape of bands or tongues on the plane, with a lateral width of 1~3 km. In contrast, the lobe subfacies displays draping seismic reflections, the sand bodies of which are of thinly laminated lateral-accretion and progradational bedding in the shape of fan on the plane, with both length and width exceeding 10 km; (4) During the deposition of the Lucaogou Formation, terrigenous clastics and organic matter, carried by frequent flood-induced hyperpycnal flows in the mountainous area of the southern Jimusaer Sag, were rapidly deposited in the sag, forming sandstone reservoirs and high-quality source rocks, and leading to the extensive contact and frequent alternation between the sand bodies of channel and lobe subfacies and the source rocks of hyperpycnal flow deposits of marginal and deep lacustrine facies. Such a sedimentary pattern of lacustrine hyperpycnal flows characterized by the coexistence of coarse-and fine-grained sandstones and paragenetic source rock-reservoir sequences, provides a novel sedimentary interpretation on the origin of high-quality shale oil reservoirs in the Jimusaer Sag.