Reservoir Property Diversity of Different Lithofacies in Sandy Conglomerate during Oil-gas Injection Period and Reservoir-formation Significance: A Case from Baikouquan Formation in west slope of Mahu depression, Junggar Basin

Key words: 
• west slope of Mahu Depression /
• Baikouquan Formation /
• lithofacies /
• reservoir performance /
• sealing blocks

Abstract: The reservoir-forming situations of source rocks, reservoir rocks, cap rocks and migration condition in Baikouquan Formation lithologic reservoir, west slope of Mahu Depression, are favorable.The valuable trap sealing condition in lateral and upward direction is the key to reservoir-forming. On the basis of making clear the fact that the mud content is the key factor to the reservoir performance of the sand conglomerate reservoir in this area, this paper divided the sand conglomerate reservoir into three lithofacies: poor-mud conglomerate (mud content<5%), mud-bearing conglomerate (mud content 5%~8%) and mud-rich conglomerate (mud content>8%).During the period of early oil-gas injection in early Jurassic, the three lithofacies above mentioned are all the valuable reservoirs. During the period of main oil-gas injection in early Cretaceous, the reservoir performance of the mud-rich conglomerate is obviously worse, and then the mud-rich conglomerate evolved into the sealing block finally, whereas the poor-mud conglomerate and the mud- bearing conglomerate are still acted as valuable reservoir, with the reservoir performance of the former is better than the latter. Fan delta front sub-facies (traction flow) deposition are mainly corresponding to the poor-mud conglomerate, and acted as valuable reservoir during the period of oil-gas injection, and then acted as the main reservoir in T₁b lithologic reservoir, west slope of Mahu Depression. The fan delta plain (traction flow) deposition and sandy debris flow (gravity flow) deposition are mainly corresponding to mud-rich conglomerate, mainly acted as the sealing blocks in the bottom and the lateral direction of the large lithologic reservoir with the poor reservoir performance. The two sealing blocks mentioned above, configured with the valuable reservoirs in fan delta front sub-facies (traction flow), jointly constitute the large lithologic reservoir in T₁b, west slope of Mahu Depression.