Features of Magmatic-Hydrothermal Exhalative Sedimentary Rocks and Its Impact on Oil Formation in Permian Lucaogou Formation, Santanghu Basin, Xinjiang, China

Here we report a set of volcanic -hydrothermal exhalative sedimentary rocks in Permian Lucaogou Formation in Santanghu Basin which have been regarded as typical "organic-rich shale" deposited in a profundal rift lake. The basin became an intracontinental rift since late Carboniferous, where fluvial-lacustrine sedimentation started since early Permian. The basin has five depressions, and Malang Depression is the center of Santanghu Basin as the focus of this study.

Samples from Well L1 in the Malang Depression were subject to petrographic, elemental chemical, and organic geochemistry. Lithology of Lucaogou Formation is characterized by fine laminated black mudstones and interlaminated with or intercalated dolostones. It is statistically analyzed and divided into five lithofacies, including dolomicrite, tuff, tuffaceous mudstone, tuffaceous dolomite and mixed lithology rock (dolomitic tuffaceous mudstone), which means there are three provenance in Lucaogou forming period (Tuff represents volcanic-hydrothermal activity from deep source, mudstone represents terrestrial sources, and dolomite represents lacustrine chemical precipitation).Grains are clay-fine silt sized, angular, poorly sorted, and contain abundant crystal fragments and lack of clay minerals.

The δ²⁶Mg (‰) of 37 samples ranges from -0.08 to 0.27‰, -0.19‰ on average. ⁸⁷Sr/⁸⁶Sr ranges from 0.7049 to 0.7065, 0.7054 on average. These indicate the provenance is strongly affected by volcanic-hydrothermal activity from deep source during the forming period of Lucaogou Formation.

The TOC (Total Organic Carbon) data of these five lithofacies are 3.76%, 0.59%, 5.10%, 3.86%, 6.55% respectively, which implied that mixed lithology rock is the best rock type as source rock among these five lithofacies, followed by tuffaceous mudstone, tuffaceous dolomite and dolomicrite, whereas the pure tuff is the worst.

We assume that mixed deep source and terrigenous materials are the best provenance of an intracontinental rift basin, which suggest that volcanic-hydrothermal activity influences the oil formation. Perhaps this theory also applies to other rift basins for oil and gas exploration.