Research On Depositional Facies And Characteristics Of The Chang-2 And Chang-6 Reservoirs, Upper Triassic Yanchang Formation, Laoshan Region, Ordos Basin

Stratigraphy, depositional facies and their spatial distributions, petrology, reservoir quality and diagenesis of the sandstones from the Upper Triassic Yanchang Chang-2 and Chang-6 oil-beds are studied in this paper, based on the field geological observation, core description, thin section identification and quantitative statistics. Technical analyses and measurements of light and heave mineral analysis, capillary pressure measurement, X-diffraction analysis, scanning electron microscope observation are also used. Major factors that control and influence the reservoir quality are also discussed in the paper.Research results indicate that the strata spatial distribution patters of the Chang-2 and Chang-6 are almost the same: both are extended in EN-SW or N-S direction and with the thickness increasing from the north to the south or from the northeast to the southwest. The study of depositional facies shows that the Chang-2 is characterized by meandering facies with restricted braided facies. The channel microfacies, alternated with the flood plain microfacies, are distributed in the trend of NE-SW. The depositional patters of the threesub-oil-bearing beds (Chnag-2 , 2 and 2 ) are basically the same, except that the location of channels shifted slightly during different geological times when the Chnag-21, 22 and 23 were deposited. By contrast, the Chang-6 is featured by delta facies with shallow lacustrine facies in the southwestern part of the study area. Among the Chang-6 oil-bearing beds, the Chang-64 is delta-front facies, which is characterized by the connected distribution of subaqueous distributary channels. Whereas the Chang-63 and Chang-62 are composed of delta plain and delta front deposits with the distributary channels and subaqueous distributary channels extended in the trend of NE-SW direction, which are alternated with the subaqueous inter-distributary bay microfacies. The Chang-61, with the NE-SW extension of the distributary channels alternated with the inter-distributary bays, consists of delta plain facies. The provenance might come from the northeast direction.The reservoir quality is controlled mainly by the sedimentary facies. The higher values of porosity and permeability of the reservoir agree with the distribution of the thick sandstone bodies. Moreover, sedimentary facies controlling on reservoir property also expressed on the grain size of the sandstones: reservoir quality of sandstones in the Chang-2 fluvial facies is better than that of the Chang-6 due to the greater grain size of the Chang-2 sandstones than those of the Chang-6. Grain size versus porosity and permeability of the Chang-2 is of positive correlation with correlation coefficient of 0.51and 0.58, respectively; and the correlation coefficient for the Chang-6 is 0.31and 0.12, respectively. Among the fluvial facies, reservoir quality of sandstones in the braided facies is better than that of in the meandering facies. Furthermore, reservoir prosperities are also influenced by petrology and diagenesis of the sandstones. Main petrological components influenced the reservoir quality are plasticfragments (especially mica and mudstones and their alterative products) and content of matrix. The sandstone types of the Chang-2 and Chang-6 are almost the same: primarily arkose and lithic arkose, whereas content of matrix in the Chang2 sandstones (av. 4.6%) is lower than that of in the Chang-6 sandstones (av. 5.2%). The plastic fragments were deformed, contorted and squeezed into intergranular spaces during mechanical compaction, thus reduced part of the primary porosities. Cementation is the other important factor that caused the loss part of the primary porosities. In the cements, carbonate cement and quartz overgrowths are the major factor reduced the reservoir quality. Dissolution is a major approach improving the reservoir quality. The organic acidic fluids derived from organic matters in the source rocks, and inorganic fluids came from rock-water reaction during the late diagenesis, and meteoric waters during the epidiagene...