| Sichuan Basin, including numerous gas bearing series, rich in natural gas resources. After2000, China Sinopec re-analyzed the gas resource potential in northeastern Sichuan,and thought that there were four sets of high-quality source rocks in northeastern Sichuan,including Lower Cambrian, Upper Ordovician to Lower Silurian, Lower Permian and Upper Permian.Also,there were several high quality reservoirs in Carboniferous Huanglong Formation, Upper Permian Changxing Formation, Lower Triassic Feixianguan Formation and Jialingjiang Formation and a thick and high-quality set of gypsum rock as caprock in Jialingjiang Formation.These unique reservoir conditions laid a solid material foundation for riching large-scale natural gas and preferred the northeastern Sichuan to exploring oil and gas first. As one of most exploration potential strata, the Upper Triassic Xujiahe Formation formed in the Late Triassic when Sichuan Basin transformed from marine cratonic basins to the continental basins,and it is a set of land face beds with coal-bearing clastic in it.I have detailed study in Xujiahe formation Xu1-3members on the basis of the description of field crop, cores of Huizhu1well and Pulu1well, combined with the region’s geology, well logging and analysis of laboratory data. First,based on core datas and logging datas,I fulill stratigraphic division and correlation strata of Xujiahe Foormation,and then I divided it into six members, but the papers only study Xu1-3members.Because of the Indosinian movement in Late Triassic, the Sichuan Basin become continental facies from marine facies. Tethys seawater completely withdraw from the Yangtze platform.The basin mainly accepted continental facies clastic rock as sedimentary rocks.The entire Xujiahe Formation generally have achieved468m-1000m Sediment thickness, mainly consists of a set of gray fine-grained sandstone, sandstone and pebbly sandstone and gray mudstone interbedded in unequal thickness, which clip the black coal seam and carbonaceous mudstone.T3x1developed relatively in Yuanba area,composing by dark gray shale, gray and black shale, carbonaceous shale, cipping aleurolite and fine sandstone; T3x2mainly developed thick layer of gray sandstone,with dark gray siltstone, gray and black mudstone, carbonaceous shale and coal line upper.Tongnan area lost the T3X1and T3X2because of high Palaeogeomorphology. T3x3developed black carbonaceous mudstone at the top, gray siltstone upper, then the lower part is dark gray silty mudstone, black carbonaceous mudstone with gray muddy siltstone clipping. From lithology,Xujiahe Formation with thick massive sandstones and mudstones is different from the underlying marine carbonatite. From electrical, the GR curves in Xujiahe Formation mainly show box-shape, a few are finger-shape, the resistivity curve shows hump-shape high value,being different from the underlying Leikoupo Formation with teeth-shape GR curves.Then,I obtain that the sedimentary facies of T3X1to T3X3conclude delta and lake.And they can be diveded into several subfacies,such as deltaic plain、deltaic front and bend including swamp、tributary bay and unde-water distributary channel microfacies etc. The depositional model is the river-Delta-lake mode.In the section,I choose two main lines to research,one is southeast direction and the other is southwest direction.The southeast section is located in the heart of the study area. From the vertical point of view,it shows delta plain、delta front and delta plain from bottom to top;The southwest section has fewer sedimentary facies because it is along the source direction,and the sedimentary facies changed a little.On the plane, T3X1mainly developed swamp microface in delta plain,there are shallow lake sediments in the northwest corner and delta front sediments in the central area. T3x2mainly developed delta plain and delta front subfacies respectively in northeast and central parts of the study area,and we can find shallow lake sediments west. From northeast to southwest, T3X3developed delta plain, delta front and shallow lake subfacies, including distributary channel, swamps, underwater distributary rivers and bays microfacies.Combined with regional tectonic evolution, well Sedimentary facies and flat sedimentary facies analysis,I know that the deposition process is going first and coming then of water on vertical and plane.The T3X1and lower T3x2developed delta plain subfacies when water going down,and the upper T3x2and T3x3developed delta front and delta plain subfacies when water coming up.Then,we find a large number of cracks in the Xujiahe Formation by observing the cores.and they are almostly filled by calcite,quartz, carbonaceous, argillaceous,few of them are filled by gray matter filled with cement. Comprehensive analysis of internal factors affecting the development of fractures, we find the relationship between sedimentary facies and fracture development.Internal factors affecting the development of fractures include rock composition、the thickness of the rock formations and rock combination.Generally,rocks containing the high brittleness ingredients have more cracks than low brittle rocks;Coarse particles have more cracks,especially with the brittle ingredients. Under the same stress, rocks of high porosity have low toughness,then to have more cracks.Rocks with different thicknesses are significantly different in mechanical properties,when,under the same positions,thin beds will have more cracks than thick bads. The overall development of crack is also related to the combination of rocks.Finally, with the study of sedimentary facies, fracture development in this area and the relationship between fractures and sedimentary facies,we find that in the study area T3x1and lower T3x2which developed delta plain subfacies have less fractures than T3X3and upper T3x2which mainly developed delta front subfacies.In those microfacies,under-water distributary channel is very conducive to the development of fractures.,and swamps and bays almost have no fracture in them. |
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