Research On Sedimentary Facies And Reservoir Of Shanxi Formation In Yanchang Gas Field

Yanchang exploration area is located in mid-eastern the Xiabei slope of the Ordos Basin. The tectonic fractures are poorly developed, the main is the monoclonal structure and local small nose shaped uplift. The gas Field is mainly located in the Zizhou, Qianjian, Zichang and Yan’an area of10755km2. The exploration in recent years shows that the upper paleoioic of area is of great exploration potential. The Shanxi Formation is main gas-bearing formation in the area, and award of high productivity wells in the exploration. Meanwhile, the uncertainty distribution of reservoir sandstones and oil—gas abundance law restrict the following exploration. It is urgent to enhance the reservoir and sedimentary facies study. Aiming at the recent problems, on the basis of previous studies according to the available area data, Based on the sedimentology, sequence stratigraphy, reservoir geology and other theories, To research in the stratigraphic classification, sedimentary facies analysis, reservoir study and analysis of favorable exploration target, which provided scientific basis for later exploration and well spacing. The conclusions are as follows,On the basis of stratigraphic classification of the upper Paleozoic in Ordos Basin, combined with key bed, lithologic character and electric property character, cycle of sedimentation, etc. Shanxi Formation is divided into six sand formations, Shan23, Shan22’Shan21, Shanl3, Shanl2, Shanll. The characteristic of strata are discussed, meanwhile in the base-level cyclic sequences method of cross for major guiding ideology, using the method of stratum thickness, this paper divided Shanxi formation of study area into two long-term. Six middle-term datum plane cycles.The two long-term cycles correspond to the first and the second member of Shanxi Formation respectively. The six middle-term cycles grossly correspond to six sand sets of Shan23～Shani1, this is the basis for the next sedimentary facies study.On the basis of the background of areal geology, combined with clastic rock type, geochemical characteristics and the distributing of diverse sedimentary characteristics, the origin of clast composing of sandstone are analysed when Upper Paleozoic are deposited, the result shows that they are origined in northeast and south of the Ordos Basin and Shanxi Formation sediments are origined in low grade metamorphic crystal fundus in north of the Ordos Basin. Shanxi Formation rock types in the study area mainly in clastic rocks, there are sandstone, siltstone, mudstone, carbonaceous mudstone and coal. The sandstone in this area are widely distributed and most developed in the shan23in the bottom of the Shanxi Formation, and containing a lot of unequal grain sandstone, gritstone, medium sandstone. The coal seam distributed in the second member of Shanxi Formation. The composition of sandstone dominated by quartz and cuttings, a few feldspar, rich in mica and kaolinite cement, heterobase-suppored, pore cementitious, sorting of grains are mid-good, grinding assume hypo-edge angle, with high maturity. Particle size analysis show that is the three-step, and main is jump particles, it’s the typical characteristics of the channel deposit, common parallel bedding, staggered bedding, lenticular bedding and erosion surface, and other sedimentary structures. There have a lot of carbonization fossil plants and biological trace fossils sometimes. According to sedimentological marker, alaeontologic marker, geophysical marker and geochemical marker, with logging facies analysis, Shanxi Formation of the study areas are delta front subfacies. Delta front can be devided into four microfacies, distributary channel underwater, interdistributary, riedel swamp and sand sheet. The features during Shanxi Formation depositional period were shallow water, flat terrain, relatively weak reworking formation of lake waves. Underwater distributary channel and bay are the main sedimentary microfacies and types of the delta front in which the typical mouth bar developed poorly. The energy of underwater distributary channel was strong, the bottom washout structures are very developed, and the river sandstone usually clip mud bricks, sandstone sorts are mid-fine grained sandstone of quartz sandstone with good sorting and grinding which is less than5m in thickness, big thick layer of sandstone is overlaied by multi-stages channels cutting. Sandstone had a poor horizontal continuity and lens shaped distribution in the section, and shaped distribution in lobes in the plane, which exhibited in certain effected by the river, and they spreaded from north to south banding distribution. Therefore, the sandy body of subaqueous distributary channel, not only the framework sandstone of delta front subfacies, but also the main sandstone for gas accumulation in study area.Besides, depositional model of delta system arc constructed and depositional evolvement are analysed by the author. Compared with every gas-bearing formation, the water gradually deepened during Shanxi Formation depositional process, that means, the character of lake expanding and river atrophying subsisted from Shan23to Shan11, the shore-lacuatrine system and subsidence center both moved toward north, the northern delta system moved backward and southern delta system northward.The gas reservoir type of Upper Paleozoic in the study area is lithologic gas reservoir. So the source rock, reservoir spreading controlled by favorable facies belt, and beneficial tectonic locations are the main factors of gas reservoir-forming. Based on all of these, Shanxi Formation reservoir characteristics to analyze, predict favorable reservoir distribution in the study area.According to abundant groundwork, the Peortlogy fature, regulation of sandstone reservoir characteristic are researched by mass modern testing means, such as microscope flake, EMS, high pressure mercury intrusion, thin section and so on. Results indicate that reservoir is mainly quartz sandstone which has high component and texture maturity, the secondly are lithic quartz sandstone and lithic sandstone, primary intergranular porosity, intergranular dissolution porosity, intragranular dissolution porosity, intercrystal porosity and microcrack are most common in Shanxi Formation of which remnant intergranular and corrosion porosity are main reservoir space in the area. Low porosity and permeability is the main characteristic of Shanxi Formation reservoir, porosity is lower12%, mainly concentrated in the2to8percent permeability is less than1.0×10-3μm2. The reservoir quality is controlled mainly by the sedimentary facies and diagenesis. In the area, the most high quality reservoir should be in underwater distributary channel of delta front the thick sandstone body, which has the higher values of porosity and permeability and good separation of grain size. The reservoir is classified as Ⅰ、Ⅱ、Ⅲ、Ⅳ types by comprehensive consideration of the relations among reservoir quality, sandstone distribution and gas bearing center. The four types of reservoir are abundant in Shanxi Formation. And Ⅰ type is mainly distributing in shan23, Ⅱ type is mainly distributing in sha22, Ⅲ type is mainly distributing in shan13.The Ⅳ type reservoir quality is poor, mainly distributing in shan12and shan11.