The deeply buried reservoirs of Wenchang Formation in the Lufeng Depression, Pearl River Mouth Basin, display strong heterogeneity, and the major controls for the development of high-quality reservoirs remain unclear. To address these issues, we conducted a series of experiment analyses, including petrographic microscope, scanning electron microscopy, and X-ray diffraction, and analyzed the impacts of sedimentation and diagenesis on the quality of deeply buried reservoirs. The results demonstrate that the sandbodies of subaqueous distributary channel and mouth bar deposited in lowstand systems tract (LST) and highstand systems tract (HST), as compared to the beach-bar and subaqueous fan sandstones deposited in transgressive systems tract (TST), have coarser grain size, higher quartz content, and lower muddy matrix content, which induced stronger anti-compaction capability, higher preservation of intergranular pore spaces, and thus better reservoir qualities. The reservoir types developed in subaqueous distributary channel and mouth bar are mainly types I, II, and III with medium-low porosity and low-ultra low permeability, while beach-bar and subaqueous fan mainly developed type III reservoir with low-porosity and ultra-low permeability. The reservoirs developed in E2w of the study area have undergone strong compaction, intense dissolution, but weak cementation. The subaqueous distributary channel and mouth bar reservoirs in LST are adjacent to Ew4 source rock in spatial distribution, resulting in strong organic acid dissolution, and developed numerous dissolved pores. The charging of hydrocarbons before deep burial further inhibited the later compaction and cementation and protects the preservation of residual primary intergranular pores and secondary dissolved pores. The combination of these factors leads to the development of the abnormally high porosity and high-quality reservoirs in LST. The results of this study reveal the genetic mechanism of deep, high-quality reservoirs in the rift basin and guide the selection of high-quality reservoirs in the later stage.
Chlorite rims have been of interest to petroleum geologists over the past several decades, as a significant number of abnormal high-porosity sandstone reservoirs at great depths have been related to chlorite rims. To clarify the contribution of chlorite rims to porosity preservation in sandstones, the relationship between chlorite rims and porosity evolution in Chang 7 sandstones of the Upper Triassic Yanchang Formation in the Ordos Basin in north-central China was investigated using thin sections, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction analyses. Based on the detrital composition and diagenetic evolution pathway analyses, it was concluded that chlorite rims inhibited the formation of quartz cementation. However, in addition to quartz cementation, compaction and other cements (commonly carbonates and clays) also control the porosity evolution of sandstones. In sandstones in which porosity reduction is largely controlled by compaction or other cements, chlorite rims may have a limited effect on porosity preservation. Thus, chlorite rims can play an important role in porosity preservation only in sandstones in which quartz cementation is the main process of porosity destruction. The results of this study indicate that the development of high-porosity sandstones is often controlled by many factors, and the effects of chlorite rims on porosity preservation are not always significant.
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