Formation and preservation of ultra-deep high-quality dolomite reservoirs under the coupling of sedimentation and diagenesis in the central Tarim Basin, NW China

Abstract

The formation mechanism of high-quality reservoirs is a key issue for ultra-deep (>6000 m) oil and gas exploration. The Lower Cambrian Xiaoerbrak dolomite with a burial depth of more than 7000 m in the Tazhong area of the Tarim Basin still keeps a maximum porosity of up to 20%, which is rare in the world. Based on the identification of meter-scale sedimentary cycles in cored intervals, the main controlling factors on the formation of high-quality reservoirs were discussed. The upper part of the sedimentary cycle is mainly composed of stromatolitic dolomite and oolitic dolomite. Comparatively, quartz-bearing dolomite and algal-bonded dolomite, as the product of low energy depositional environment are dominant in the lower part of each sedimentary cycle. Vugs formed by meteoric dissolution widely exist in the middle-upper part of the sedimentary cycle due to brief to moderate long-term subaerial exposure, whereas the lower part shows low porosity with a few intercrystalline pores. It indicates that subaerial exposure dissolution caused by periodic sea level decline is the main source of pores. The stylolite density and thickness of insoluble residue decrease rapidly from the lower to upper part of each sedimentary cycle. The content of aluminosilicate minerals in dolomite is positively related to the stylolite density, indicating that the aluminosilicate minerals play a promoting role in the pressure solution process. Pure dolomites with the least aluminosilicate minerals content in the upper of the sedimentary cycles have the strongest resistance to pressure solution, which is beneficial to ultra-deep high-quality reservoir preservation. Coupling of sedimentation and diagenesis to illustrate the origin of ultra-deep high-quality dolomite reservoirs is proposed.