LITHOLOGICAL CLASSIFICATION AND OIL-BEARING CAPABILITY OF TIGHT RESERVOIR IN PERMIAN LUCAOGOU FORMATION,SATANGHU BASIN,XINJIANG,CHINA

Here we report a set of well-laminated 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 NW-SE trending Santanghu Basin is located north of the Bogda Mountains in NW China, and covers about 23,000 km². 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. Petrographic features of 97 samples show lithology of Lucaogou Formation is complicated, which is characterized by fine laminated black mudstones and interlaminated with or intercalated dolostones, locally silicified. Grains in the mudstones are clay-fine silt sized, angular, poorly sorted, and contain abundant crystal fragments and lack of clay minerals. The lithology of 97 samples is statistically analyzed and roughly 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 deep source,mudstone represents terrestrial sources,and dolomite represents lacustrine chemical precipitation).

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 reservoir among these five lithofacies, followed by tuffaceous mudstone, tuffaceous dolomite and dolomicrite, whereas the pure tuff is the worst.

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 materials from deep resource during the forming period of Lucaogou Formation. Mixed deep and terrigenous materials are the best provenance for tight oil accumulation in an intracontinental rift basin, which suggest that the deep source influences the oil formation.