| Active natural gas appearance during the drilling of carbonate reservoir shows a potential prospect in continental strata of Xuejiahe formation in Puguang area, but it is difficult to explore the natural gas in this area because of formation compaction and poor reservoir property. The Studies on the continental strata in Northeast Sichuan area are very few and data are also limited for a long time, so the study of continental beds is relatively dropped behind.The thesis is a further study of the sedimentary strata diagenesis and reservoir diagenesis of the Upper Triassic Xuejiahe formation in Puguang area. Using sedimentology and reservoir geology theory, the reservoir was characterized basing on the new data obtained core and laboratory data, combined with all mud logging, logging and gas testing data.Physical properties and the factors influence them were discussed. After establishing the reservoir classification standard, evaluation and distribution of better reservoir were analyzed which give a clue for the further exploration in this area.Study shows that Xujiahe group can be further divided into six members (e.g.T3x1-T3x6). From south to north and from west to east, the strata thickness has a trend of gradually thinning in Xujiahe formation in this study area. Based on the previous study of Xujiahe reservoir in Sichuan Basin, combing with comprehensive research on macroscopic core data, mud logging data, microscopic analysis data and petrologic characteristics, sedimentary structure and sedimentary rhythm, through logging data and logging response analysis, the environments of the reservoir were discussed. Xujiahe formation of study area was deposited in braided delta environment, composing delta plain, delta front, shallow lake environment,which can be further divided into several facies associations include distrivutary channel, interdistributary bay, swamp micro-facies; underwater distributary channel, mouth bar,subaerial distributary bay,nearshore sand bar and lake bay.The rsevroir of Xujiahe is well developed in T3X2, T3X4, T3X6, which is composed of a set of terrigenous clastic rock and characterized by lower composition maturity but higher texture maturity. The rock is high in structural maturity and relatively low in compositional maturity. Low compositional maturity is related to low content of quartz and higher contents of feldspar and lithic with an average of maturity index(quartz/feldspar+lithic) of1.27,while Grains fairly are well sorted and grinded caused the matrix content is low. The Xu2is mainly consisting of feldspar detritus quartz sandstone and feldspthic lithic sandstone, while the Xu4is mainly composing feldspthic lithic sandstone and lithic sandstone and Xu6is mainly made of lithic sandstone. FromTsx2to T3X6, the content of quartz and feldspar decreases gradually, but lithic fragment increases gradually with a decrease in the compositional maturity. The grain of Xujiahe sandstone is mostly fine and medium grain, respectively37.3%and32%, followed by the silt accounted for21.5%.The research shows that the diagenesis of study area is strong. Compaction and cementation reduced the porosity, while dissolution and tectonic fracture increased porosity. Through a lot of core observation, chip appraisal, scanning electron microscope, we believe the reservoir is in high diagenetic stage.It is in the stage B of the middle diagenetic phase gernerally and late diagenetic stage locally. The main pore type is secondary pore including dissolved pores between grains, dissolved pores in grains and intercrystalline pore but few primary pores were found. The pore texture types of the reservoir is included big pore max throat, medium pore medium throat, fine pore fine throat and minimal pore minimal throat, and the latter two types play a main role in the reservoir. The reservoir is poor in pore throat structure overall, in which T3X4is the best, followed by the T3x2. According to the pore structure parameters such as mercury injection saturation, mercury with drawal efficiency, displacement pressure, the reservoir can be divided into two classes:I (better reservoirs) and Ⅱ(worse reservoir). Class I reservoir has a larger into mercury, higher mercury withdrawal efficiency, lower displacement pressure and pore throat coarse, while class Ⅱ shows a contrary,each class also can be divided into two categories. The development of cracks have greatly improved the tight reservoir percolation capacity which can not only increase the permeability of the reservoir, but also can form the main reservoir space. Fractures are mainly structural fractures and occurred in low angles. Five stages of fracture filling were recoginized and most fulfilled or partially filled by carbonaceous and calcite which is developed in conglomerate and sandstone. It is significantly higher in T3X2than other sections.Though physical property is highly heterogeneous, better reservoir with relatively high porosity and permability is locally developed, the reservoir is tight to super tight reservoir which is low-porosity low-permeability and especially low-porosity and low-permeability. The physical property of reservoir is poor in general. Porosity is between0.1-6.65%, with an average of2.28%, while permeability is between0.004-4.5856×10-3μm2, with an average of0.114×10-3μm2in the cores. There is a great difference in reservoir physical among different members and T3X2is the best reservoir.Combining with testing gas data, the reservoir is evaluated. It is concluded that porosity lower limit is3%and permeability limits is0.08×10-3μm2.Therefore, the reservoir is classified into four types. The most main types of sandstone reservoir in Xujiahe formation is II type (the better reservoir)and III type (the worse reservoir). With poor ability of permeability, structural fractures play a very important role in the reservoir. The I and II types of reservoir are the better reservoirs in study area. Longitudnially, the better rsevroir of Xujiahe formation principally located in T3x2and T3x4.The reservoir thickness of I type are thin, generally less than5m, while25meters with average of II type. The prediction of highquality reservoir in Xujiahe formiaiton includes the area between the well PG2-PG105-PG4-PG106and PG304-PG204-PG8.Two main factors influencing reservoir property are sedimentation and diagenesis. Sedimentary enviroment of reservoir controls the reservoir distributions in macro-scope and rock type and structure component features which made a further impact on the latter rock diagenesis. Reservoir formed in underwater distributary channel and distributary channel envirionment, always has coarse grain size, nice separation, good textural maturity and component maturity, which will give a good condition for reservoir properties. Diagenesis especially compaction, carbonate cementation and siliceous cementation controlled the evolution and pore throat structure characteristics of reservoir space which ultimately influenced physical properties and the distribution of the present reservoir. |
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