| The study area, located in the northeastern Sichuan Basin, is the main gas source area of "Sichuan-to-East gas transmission project" because of its abundant gas source. After the successful exploration in marine strata of Sichuan Basin, China Petroleum and Sinopec start to do more and more researches in shallow continental facies strata in recent years. As the main gas-bearing strata, breakthroughs have been made in the basic geological study (such as stratigraphic division, stratigraphic characteristics and distribution, sedimentary environment, source rock characteristics, hydrocarbon genesis and reservoir petrology characteristics and so on) of Xujiahe formation. During the early stage of oil and gas exploration, exploration is mainly concentrated in the western and central area of Sichuan Basin. The research is relatively lagged in northeastern area of Sichuan Basin.A lot of predecessors’ research results show that the main continental facies strata of Puguang area has a good preservation condition, and it is a favorable gas-bearing area, whose oil-gas show is active. This paper will base on the first-hand data of one-week on-site core observations, combined with the predecessors’ researches, the actual production data and the analysis of test data, etc.. Focusing on the key anatomical well-PL2, we also have done comprehensive analysis of adjacent area and neighboring well. This paper adopts many research methods, such as core facies analysis and log facies analysis, to study the sedimentary system and the division of sedimentary facies, sedimentary microfacies characteristics of Xujiahe formation, Ziliujing formation, Qianfoya formation of the study area. Then describe and characterize planar distribution and evolution law of sedimentary microfacies based on the analysis of single well sedimentary facieses and profile facieses. Finally, focus on studying the favorable sedimentary facies belt and then define favorable zone.Through approximately one year study, the main conclusions can be drawn as follows:(1) Refer to the predecessors’ achievements, this paper has distinguished and divided the sequence boundary of the study area. In total, four three-grade sequence interface have been established including SB1, SB2, SB3 and SB4. Among them, SB1 is at the bottom of Xujiahe formation, SB2 lies between the third and the fourth stage of Xujiahe formation, SB3 located in the top of Xu-6 member of Xujiahe formation, SB4 lies between Ziliujing formation and Qianfoya formation.(2) During the second and the third sedimentary stage of the Xujiahe formation (T3x1-2), the source direction mainly came from Dabashan orogenic belt, Kaijing-Liangping ancient uplift and Longmenshan orogenic belt are the second source direction. During the fourth to the sixth sedimentary stage of the Xujiahe formation (T3X4-6), clastic material came from Dabashan orogenic belt and Longmenshan orogenic belt. During the sedimentary stage of Ziliujing formation and Qianfoya formation, the clastic material mainly came from Dabashan orogenic belt.(3) Sedimentary evolution of Xujiahe formation is mainly controlled by the tectonic evolution. On the influence of An’xian movement, Basin-forming period of Xujiahe formation can be divided into "the upper basin-forming period of T3X2-T3X3 member" and "the lower basin-forming period of T3X4-T3X6 member". Xujiahe formation has experienced the two stages of tectonic-sedimentary evolution. The lower basin-forming period is characterized by low-amplitude subsiding of the depression, while the the foreland basin rapid subside during the upper basin-forming period. A long-time hiatus occurred between Xu-3 and Xu-4 member.(4) Rock types in the target strata of the study area can be divided into four categories:conglomerate, sandstone, mudstone, and limestone. During Xu-2, Xu-4 and Xu-6 member of Xujiahe formation sedimentary period, the tectonic activities of basin margin were active. Clastic material, mainly composed of fine sandstone, was supplied adequately. Among them, it widely develop thin beds conglomerate and glutenite or pebbled sandstone at the bottom of Xu-4 member of Xujiahe formation. The lake rised and source shriveled during the sedimentary period of Xu-3 and Xu-5 member of Xujiahe formation, resulting in the deposition mudstone and carbon mudstone. Ziliujing and Qianfoya formation mainly composed of mudstone, siltstone and argillaceous siltstone. In addition Zi-4 member of Ziliujing formation has developed coquinite.(5) Sandstone reservoir of Xujiahe, Ziliujing and Qianfoya formation of the study area has a low compositional maturity and high textural maturity. Among the rock detrital components in sandstone reservoir, the content of quartz has a relatively high proportion, especially in Xu-2 member. The content of feldspar is from 9.80% to 20.55%. Among the cuttings components, metamorphic debris has a higher proportion, the highest content of it can achieve 21.28%, the followed is volcanic debris, with the average content of 16.71%. The content of sediment lithic fragments is lowest whose highest do not exceed 9%. According to the statistical data, matrix has a low content and cement’s content is higher than it. Cement mainly contains calcite, clay mineral and siliceous. The particle size of detrital grain in the study area is fine and mainly fine siltstone.(6) The types of sedimentary facies in the study area mainly includes three kinds of subfacies (braided river delta plain, braided delta front, lakeshore-shallow lake subfacies) and nine kinds of microfacies (channel fill deposit, braided channel, plain swamp, underwater distributary channel, interdistributary bay, shore or shallow lake sand bars, shore or shallow lake muds, coastal marsh, shell beach). Xujiahe formation develop braided river delta deposition with a large plain. Xu-2, Xu-4 and Xu-6 member mainly develop river deposition, Xu-3 and Xu-5 member develop plain swamp and channel fill deposition, mainly including fine dark mudstone. Ziliujing and Qianfoya formation mainly develop lakeshore-shallow lake deposition(7) Through the comparison and analysis of phase plane distribution and evolution laws of Xujiahe formation in the study area, we have found that the deposition in different members of Xujiahe formation is inheritable, witch presents a process of water oscillation. Braided channel sand of Xu-2 member, with better continuity and overlay thickness, is much more developed than Xu-4 and Xu-6 member of Xujiahe formation. |
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