Abstract:

The Lower Cretaceous sandstone reservoirs in the Kuqa depression of the Tarim basin have an overall burial depth of 4 000-6 500 m. Despite their great depth and abundant oil and gas resources, significant differences in sandstone tightness and pore types exist between the northern and southern structural belts of the depression, resulting in a lack of systematic research on the pore preservation mechanisms of the Lower Cretaceous sandstone reservoirs. This study systematically analyzes the petrological, petrophysical, and diagenetic characteristics of the Lower Cretaceous deep-buried sandstones in the southern slope belt by integrating analytical data from petrographic thin sections, SEM, petrophysical measurements, and XRD, along with diagenetic process reconstruction and porosity restoration techniques. The aim is to elucidate the pore formation and preservation mechanisms and to identify the controlling factors and distribution patterns of high-quality reservoirs. The results show that the Lower Cretaceous sandstones are dominated by medium-fine grained feldspathic lithic sandstones and lithic feldspathic sandstones, with moderate to good compositional and textural maturity. Intergranular spaces are filled with calcareous and zeolitic authigenic minerals, with predominantly pore-contact cementation. The main reservoir space consists of residual primary pores, followed by uneven intergranular and intragranular dissolution pores, with an average porosity of 13.73% and an average permeability of 324.05×10-3 μm2, classifying the reservoir as medium-high porosity and medium-high permeability. The sandstone reservoirs have mainly undergone compaction, cementation, and dissolution. Among these, compaction is overall weak, cementation is widely developed, and dissolution occurs locally, mostly involving the dissolution of feldspar and lithic grains. The preservation of residual primary pores in the Lower Cretaceous sandstone reservoirs of the southern slope belt is controlled by three factors. The sand bodies of subaqueous distributary channels in braided river delta fronts, formed under high-energy hydrodynamic conditions, have good sorting and low matrix content, providing favorable initial material conditions. Weak compaction, widespread early carbonate cements, and high quartz content collectively enhance the rock's resistance to compaction, while the special diagenetic environment inhibits more intense cementation from filling the residual pores. The unique burial pattern of slow shallow burial in the early stage followed by rapid deep burial in the late stage is a distinctive and crucial factor for the preservation of residual primary pores in these sandstone reservoirs.

Key words: pore preservation mechanism, primary pores, deep reservoir, Lower Cretaceous, Kuqa southern slope zone, Tarim basin