The Reservoir Characteristics Of Chang 6 And Chang 8 Oil Group In Western Block Of Mahuangshan

Mahuangshan Region is located in the western part of the middle Tian-huan syncline in the Erdos Basin. It is a bridging transition between the Majiatan area of the Middle West margin belt and the middle Tian-huan syncline in its west area. Under the effect of block faulted anticline belt, the secondary fault block anticline belt played the main role in controlling the migration and accumulation of oil and gas. Anticline, fault block and nosing structures are the local structures. Research focused on target bed in Chang 6 and Chang 8 oil formation with the following characteristics:control area of 120.8km2, total depth between 2400-2900 meters, and over 44 drilled wells having good oil and gas.Lithologic deposit is the chief reservoir in Chang 6, Chang 8 oil group and the sand body of the reservoir mainly is delta type, whose features are deep burial and strong. Chang 6, Chang 8 oil formation in Mahuangshan Region is characterized by low-super-low porosity-permeability reservoirs. With the application of theory and principles of petroleum geology, this article studied the petrology, sedimentary microfacies, physical property, rock formation and heterogeneity of Chang 6 and Chang 8 oil formation in Mahuangshan Region. The research classified and evaluated the reservoir by identifying the preferred and favorable target areas for oil to guide the next exploration of oil and gas.The study employed comprehensive interpretation of core observation and well logging using cycle-thickness contrast, high-resolution sequence formation, principle of contrast by means of patterns and compared different genetic sand body. It further employed ways of handling spatial contact relations to make subdivision and correlation on the small layer of sedimentary period on 44 wells in the area of Chang 6, Chang 8 oil formation. Chang 6 oil formation was divided into 3 sands groups and 9 layers, Chang 8 oil formation was also divided into 2 sands groups and 6 layers and established stratigraphic unit with isochronal stratigraphic framework.Comprehensive analysis was carried out by utilizing factors of the planar characteristics of distribution such as clastic particles components quartz, feldspar, rock debris content, seismic RMS amplitude attribute maps, sandstone thickness and the sand group-sand percentage figure. Sedimentary source studies of research area was conducted by synthetic analysis and found that the sediment source of Chang 6, Chang 8 oil formation was mainly from the northwest direction of study area.Combined with the regional sedimentary background, microfacies was studied as well as sedimentary evolution of the study area deposition through the analysis of well logging, grain size, and contrasted in core observation, description of core wells and knowledge of the coring data. It was found that the main sedimentary environment in Chang 6, Chang 8 oil formation is delta front whose chief sedimentary microfacies comprised distributary channel, mouth bar, distal bar, blanket sand bay.Chang 6 oil formation is mainly composed of the lithic arkose sandstone and feldspathic quartz sandstone, some feldspar lithic sandstone. The lithoclastic quartz sandstone and lithic sandstone are the main types of sandstone in Chang 8 oil formation with a small amount of feldspar sandstone and feldspathic lithic sandstone. The sandstone’s compositional maturity is low, which is from medium to superior and the psephicity is subangularity-subrounded mainly. Porous cementation is the dominant element of the transition type which is followed by pore-base type cementation. Carbonate is the main interstitial material of Chang 6 oil formation, while interstitial material in Chang 6 oil formation is mainly carbonate and clay minerals.The chief pore types in this region are original intergranular pore, inter-grain pore, feldspar dissolved pore, sedimentation rock fragment and microcrack, of which the intergranular pore is the main. The types of throat can be divided into the micro throat, the laminated throat and the curved lamellar throat; with the micro throat and the laminated throat being the major types. The permeability of the reservoir is poor, and the two oil reservoirs all are low-super-low porosity and low-super low permeability reservoirs.Long 6 and long 8 oil unit reservoir in the research area is in the A rock forming stage. It can be seen from a series of lithogenous phase research that the maculosus vugular pore space lithogenous phase, carbonate cementation lithogenous phase, argillaceous cement lithogenous phase and compact lithogenous phase develop in the research area. Using the formation testing data and the correlation of the lithogenous phase of the reservoir, the maculosus vugular pore space lithogenous phase has well corresponding relation with the oil and gas. The constructive rock has better transformation effect on the property of the reservoir and the oil contained in the reservoir is better.The main controlling factors in the reservoir are deposition and diagenesis. The main reason of the densifying is higher content of clastic particles andesine, rock debris and the like. The compact diagenesis and agglutination can reduce the primary porosity greatly which has weak dissolution in the diagenesis period; and the non-developed secondary pore results in compact reservoir.Four parameters such as porosity, sandstone thickness, permeability and mud content are used in setting up the evaluation standards. The reservoir is evaluated by the grey system theory and clustering method. The type of the reservoir in the research area is divided into class-Ⅰ, class-Ⅱ, class-Ⅲ and class-Ⅳ. According to the feature of the reservoir on the plane and the sedimentary facies, class-Ⅰ and class-Ⅱ reservoirs are concentrated in the distributary channel; and the remaining are concentrated in estuary dam and remote sand bank. Class-Ⅱ and class-Ⅲ reservoirs are developed in the remote sand bank. Class-Ⅱ reservoir and little class-Ⅲ reservoirs are developed in the estuary dam. Class-Ⅲ and class-IV reservoirs are developed in remote sand bank.Based on the above analysis, the reservoir development area is predicated by combination of the effective sand thickness, reserve abundance, type and construction part as well as the evaluation standards of oil gas distribution rule and reservoir classification in selecting five beneficial blocks.