Diagenesis and Fluid Flow Variability of Structural Heterogeneity Units in Tight Sandstone Carrier Beds of Dibei, Eastern Kuqa Depression

Abstract

Tight sand gas plays an important role in the supply of natural gas production. It has significance for predicting sweet spots to recognize the characteristics and forming of heterogeneity in tight sandstone carrier beds. Heterogeneity responsible for spatial structure, such as the combination and distribution of relatively homogeneous rock layers, is basically established by deposition and eodiagenesis that collectively affect the mesogenesis. We have investigated the structural heterogeneity units by petrofacies in tight sandstone carrier beds of Dibei, eastern Kuqa Depression, according to core, logging, and micropetrology. There are four types of main petrofacies, that is, tight compacted, tight carbonate-cemented, gas-bearing, and water-bearing sandstones. The brine-rock-hydrocarbon diagenesis changes of different heterogeneity structural units have been determined according to the pore bitumen, hydrocarbon inclusions, and quantitative grain fluorescence. Ductile grains or eogenetic calcite cements destroy the reservoir quality of tight compacted or tight carbonate-cemented sandstones. Rigid grains can resist mechanical compaction and oil emplacement before gas charging can inhibit diagenesis to preserve reservoir property of other sandstones. We propose that there is an inheritance relationship between the late gas and early oil migration pathways, which implies that the sweet spots develop in the reservoirs that experienced early oil emplacement.

Small-Scale Diagenetic Heterogeneity Effects on Reservoir Quality of Deep Sandstones: A Case Study from the Lower Jurassic Ahe Formation, Eastern Kuqa Depression

“…Based on the homogenization temperature distribution of aqueous fluid inclusions (converted to age via the thermal history), petroleum charge can be divided into two stages: 25 Ma to 12 Ma and 5 Ma to the present day (Figure 14). These results are consistent with the results of Li et al [30,31] and Shi et al [32]. They indicated that since the deposition of the Miocene Jidike Formation (23 Ma-), the Triassic source rocks began to enter the mature stage; when the Pliocene Kuqa Formation deposited (5 Ma-), the Triassic source rocks reached the peak of gas generation and migration, and the Jurassic source rocks entered the mature stage to generate oil and gas.…”

“…Jidike Formation, the Ahe Formation underwent rapid subsidence and burial and entered the mesodiagenetic stage (Figure 14). In the late Oligocene, the Triassic source rocks entered the threshold of hydrocarbon generation [31,32]. The thermal degradation of organic matter released a large amount of organic acids and CO 2 followed or accompanied by oil generation [38][39][40].…”