Reservoir characteristics of the Silurian Kepingtage Formation sandstones in the southern Tahe region, Tarim Basin

Porosities of the Kepingtage Formation sandstones at depths of 5311 to 6231 m (17,424 to 20,443 ft) range from 0.9% to 17.1%, averaging 9.6%, and their corresponding permeabilities range from 0.01 to 786 md, averaging 12.2 md in the southern Tahe region of the Tarim Basin. The good reservoir quality of the Kepingtage sandstones results from the combination of favorable sedimentary facies, inhibition of mechanical compaction by eogenetic calcite cementation, inhibition of quartz cementation by chlorite grain coatings, and late dissolution by acidic components derived from kerogen maturation. The Kepingtage sandstones were located at depths from 1000 to 2000 m (3280 to 6560 ft) for approximately 270 m.y. and were in the eogenesis stage by the end of the Jurassic. Cenozoic deposition forced the Kepingtage sandstones into the mesogenesis stage of a deeper burial environment, with a low geothermal gradient of 23 degrees C to 25 degrees C/km. The medium- and finegrained litharenites from the fluvial and delta distributary channel facies were cemented with calcite before thorough compaction and restrained late compaction, and intergranular pores were preserved. Chlorite grain coatings around the quartz grains of the foreshore quartzarenite and sublitharenite restricted quartz overgrowth and resisted compaction, thus preserving intergranular pores. Organic acids from Cambrian-Ordovician source rocks migrated through faults and unconformities and dissolved lithic fragments, feldspar, and calcite cement to form secondary pores during mesogenesis. The sandstones of the fluvial and deltadistributary channel and foreshore facies provided the material basis for the development of the good reservoir quality of the Kepingtage sandstones.